J Neurol (2004) 251 : 1173–1182DOI 10.1007/s00415-004-0590-1 REVIEW

Giovanni MeolaRichard T. Moxley III

Myotonic dystrophy type 2 and related myotonic disorders

Historical background for the definition of myotonic dystrophy type 2 (DM2)

Myotonic dystrophies (DM) are genetically heteroge-neous neuromuscular disorders characterized by auto-somal dominant inheritance, weakness, wasting, myo-tonia, posterior capsular iridescent cataract andmultisystem complications, often involving the heart,brain and endocrine system [12, 25, 40, 58].

Each of these disorders has variable phenotypes.However, despite the variability and the similar charac-teristics, it is usually possible to identify clinical and lab-oratory features that establish the diagnosis.

Myotonic dystrophy type 1 (DM1) results from anunstable (CTG)n expansion in 3’UTR of the DM proteinkinase gene (DMPK) at 19q13.3. Clinically distinct dis-orders with some similarity to DM1 but not associated

with the DM1 (CTG)n expansion and unlinked to19q13.3 have been described in the past decade.

In 1994, Thornton et al. [59] and Ricker et al. [41] re-ported several unusual myotonic dystrophy patientswithout the abnormal expansion of CTG repeats onchromosome 19q13.3 diagnostic for DM1. Because ofthe preferential weakness of proximal leg muscles thisnew disorder became known as proximal myotonic my-opathy (PROMM-[MIM6000109] [2, 17–19, 27, 28, 35, 37,42, 43, 56, 59, 66]).

With more clinical studies, three disorders were de-scribed, each sharing the core features of dominant in-heritance, myotonia, weakness, multi-system disorders,and a normal size of the CTG repeat in the DM1 gene.These three disorders were: proximal myotonic myopa-thy (PROMM), proximal myotonic dystrophy (PDM)[61], and type 2 myotonic dystrophy (DM2 [6]). Theywere characterized by weakness that was either pre-

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Received: 7 April 2004Accepted: 22 June 2004

Dr. Giovanni Meola (�)Department of NeurologyUniversity of MilanIstituto Policlinico San DonatoVia Morandi 3020097 San Donato MilaneseMilan, ItalyTel.: +39-02/52774480Fax: +39-02/5274717E-Mail: giovanni.meola@unimi.it

R. T. Moxley III, MDDepartment of NeurologyUniversity of RochesterStrong Memorial HospitalRochester, NY, USA

■ Abstract The myotonic dystro-phies are a group of dominantly in-herited disorders characterized bymuscle wasting, myotonia,cataracts, hypogonadism and othersystem manifestations. Myotonicdystrophy type 1 (DM1) resultsfrom an unstable expansion of aCTG repeat in 3’ UTR of the DMprotein kinase (DMPK) gene onchromosome 19q 13.3. Myotonicdystrophy type 2 (DM2) is causedby an unstable expansion of aCCTG tetraplet repeat in intron 1 ofthe zinc finger 9 (ZFN9 gene) onchromosome 3q 21.3. However, theclinical diagnosis of DM2 is morecomplex than that of DM1, and

conventional molecular geneticmethods used for diagnosis ofDM1 are not helpful for DM2. Wehere describe the detailed clinical,laboratory and biomolecular teststo identify DM2 and related myo-tonic disorders. At present, foci ofaccumulated noncoding CCTG re-peat RNA (ribonuclear inclusions)in the cell nuclei are thought to in-terfere with the regulation and ex-pression of several genes at the ba-sis of multisystemic aspects ofmyotonic dystrophy type 2.

■ Key words myotonic dystrophytype 2 · myotonic dystrophy type 1· DM1 · DM2 · DMPK · ZFN9

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dominantly proximal (PROMM and PDM) or distal andproximal (DM2).

The mutation underlying DM2 and a subset of pa-tients with PROMM was mapped to 3q21.3 [38, 43] andwas recently identified as an unstable (CCTG)n expan-sion in intron 1 of ZNF9 [20]. Reports have also de-scribed atypical cases of PROMM in which the DM2 mu-tation analysis was not performed [4, 9, 13, 44, 46, 50, 52,55, 64]. DM2 represents a spectrum of clinical manifes-tations.PROMM and PDM are part of this spectrum,onemild and the other a severe phenotype. Patients have acommon mutation at the 3q21 locus of DM2.

In summary, only two loci are currently assigned(DM1 and DM2), associated with myotonic dystrophytype 1 (DM1) and myotonic dystrophy type 2 (DM2) re-spectively. This nomenclature dose not preclude the useof clinical terms such as proximal myotonic myopathy(PROMM) and proximal myotonic dystrophy (PDM)but should help to avoid confusion, and allow logical ex-tension to naming newly identified multisystemic my-otonic disorders and allelic variants [58].

In this review we highlight the clinical and laboratoryfeatures in DM2 families with confirmed mutations andwe also discuss progressive myotonic myopathies withmultisystem symptoms that have not had DNA testing toestablish the diagnosis of DM2.

Myotonic dystrophy type 2 (DM2)

DM2 typically presents in adult life, usually with myoto-nia, stiffness of muscles, or weakness (proximal lowerlimbs or deep finger flexors).

■ Neuromuscular involvement

Table 1 summarizes the neuromuscular symptoms,signs and system involvement in DM1 and DM2. A milddegree of weakness of the orbicularis oculi and of the fa-cial muscles is often present, being more apparent in thelater stages of the disease. Facial weakness is seldom se-vere enough to cause indistinct speech or trouble withfacial expression in contrast to patients with advancedDM1. Wasting of the temporalis muscles, which is typi-cal in DM1, is only rarely present in DM2. Weakness ofthe sternocleidomastoid muscles, especially neck flex-ion, is common in patients with DM2, but it is not as se-vere or frequent as it is in patients with DM1.

Weakness of neck extension typically develops overtime in DM1 and may also be present in DM2. It is lesssevere,however, than in DM1,and a “dropped head”pos-ture is rarely observed.No ophthalmoplegia is present inDM2. Ptosis, typical in DM1, is rare in DM2, but it maydevelop.

Weakness of the thigh and hip flexor and extensor

muscles is the most frequently observed limb weakness.Patients with DM2 have difficulty arising from a squat,climbing stairs or arising from a chair. Weakness of thethigh is not a typical initial complaint or finding in DM1.Proximal weakness, primarily of the knee extensors, oc-curs in DM1, but it develops later as the disease worsens.Distal weakness may also be present early in the clinicalcourse of DM2, and it involves the flexor bigitorum pro-fundus muscle, as occurs in DM1. Distal weakness of thefingers can occur as the first and only sign in DM2, butit is usually accompanied by weakness of thigh musclesand hip flexors. Muscle atrophy is usually mild in DM2and wasting of the forearm and intrinsic hand musclesis very uncommon.In contrast to patients with DM1,pa-tients with DM2 do not lose their grip or percussion my-

Table 1 Comparison of neuromuscular and systemic involvement in DM1 vs DM2

DM1 DM2

Localization of muscle weaknessPredominantly proximal at onset – +Predominantly distal at onset + –Neck flexors ++ +Facial muscles ++ ±Jaw muscles + ±Extraocular muscles – –Ptosis + ±

Localization of muscle atrophyPredominantly proximal at onset – +Predominantly distal at onset ++ –Sternomastoids ++ ±Temporalis muscles ++ ±Facial muscles ++ +

HypertrophyCalf muscles – +

MyotoniaGrip ++ +Orbicularis oculi + ±Tongue ++ +Jaw muscles + +Limb muscles – +Fluctuating + ++

Cataracts + +

Cardiac conduction arrhythmias ++ ±

BrainCognitive dysfunction ++ +Visual-spatial deficits ++ +Behavioral abnormalities ++ +Hypersomnia ++ +Mental retardation + –

EndocrineThyroid dysfunction + ±Diabetes mellitus/insulin resistance + +Hypogonadism + +

+ = present; ++ = present and very prominent; ± = present but only to a minordegree; – = not present

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otonia as muscle weakness proceeds. This probably re-lates to the maintenance of their muscle bulk since evenin the late stages, typical DM2 patients have relativelymild muscle wasting. In fact, muscle hypertrophy occa-sionally occurs in DM2, and patients develop significantenlargement of their calves.

Deep tendon reflexes in DM2 patients are usuallynormal or brisk in contrast to DM1 in which tendon re-flexes are hypoactive or absent.

Myotonia is more readily observed in patients withDM1 than in DM2. Myotonia of grip and percussion(thenar and forearm wrist extensor muscles) myotoniaare prominent in DM1, especially in the early stages, andthe myotonia is usually seen in the muscles of the face,tongue, jaw and hands. Myotonia in DM1 is usually ob-vious to the patients and their clinicians, and it seemsrelatively constant in severity. It worsens with cold, withstress, and typically shows a warm-up phenomenon.

In patients with DM2 myotonia is often less apparentto the patients and their care providers, and it is typi-cally less severe than in DM1.The myotonia varies more,and patients will report days or even weeks duringwhich they remain free of myotonia. When the patientsdo complain of stiffness, often it is in the thigh muscles.The clinical appearance of the myotonia in DM2 oftendiffers from that observed in DM1. It is also more diffi-cult to detect myotonia on electromyographic investiga-tion in DM2. There may only be increased insertionalactivity with bizarre high frequency discharges in somemuscles.A determined search for myotonia is often nec-essary. There is no standardized guide to the selection ofmuscles for electromyographic sampling in patientswith DM2. However, investigators typically sample thefollowing muscles: first dorsal interosseous, extensors ofthe wrist, knee extensors, foot dorsiflexors, andparaspinal muscles.

Pain is often a major complaint and managementconcern in patients with DM2. This pain occurs inde-pendently of the severity of myotonia and is usually notprecipitated by exercise. It varies and can disappear forweeks. Occasionally the muscles become painful totouch or deep palpation. Chest pain may also occur inDM2, and in some instances has led to urgent investiga-tions for coronary artery disease. More investigation isneeded to delineate the pathophysiology that underliesthe pain in DM2.

The one group of patients with the DM2 mutationhave marked muscle wasting, proximal myotonic dys-trophy (PDM),described by Udd and colleagues [61].Se-vere wasting of the thigh and shoulder girdle musclesdevelops in the late stages of PDM, but the bulk of thedistal muscles remains relatively well preserved, a cleardifference from the pattern of wasting observed in thelater stages of DM1. Patients with PDM have no clinicalmyotonia throughout their illness and also have hypo-gonadism and cataracts. It is interesting to note that one

kindred described recently has one member manifest-ing the clinical features of PROMM and another mem-ber shows the phenotype of PDM [51]. The clinical andlaboratory data in these families show overlapping fea-tures with those of other multisystemic myotonic my-opathies and suggest several questions,which need to beanswered with the investigation of more families: 1)Why does the pattern and the severity of muscle wastingand weakness vary between kindreds with PROMM andPDM that link to the same chromosome 3q 21 locus?Why does the phenotype vary within the same kindred,such as in this German family? 2) Why is clinical myoto-nia lacking in PDM and why does it seem to fluctuatemore in DM2 than in DM1?

■ Systemic involvement

Cataracts

Cataracts develop before 50 years of age and appear asiridescent, posterior capsular opacities on slit-lamp inpatients with DM2. The cataracts in DM2 have an ap-pearance identical to that observed in DM1.

Heart

Cardiac problems are a less frequent complication inDM2 [11, 31] than in patients with DM1. The alterationsin cardiac conduction in DM2 are primarily limited tofirst degree atrio-ventricular and bundle branch block.Cardiac problems are a more benign complication inDM2 although there have been anecdotal reports of sud-den death, pacemaker implantation, and severe cardiacarrhythmias in small numbers of patients [33]. Annualmonitoring of the ECG is recommended for patientswith DM2. If symptoms of cardiac involvement appear(palpitations, fatigue, dyspnea), more thorough evalua-tion is indicated. One of the major clinical differencesbetween DM1 and DM2 that influences the prognosis isthe lack of significant respiratory deficiency in DM2.This preservation of pulmonary function lessens thetendency for right heart strain that occurs with pul-monary failure in DM1.

Brain

Cognitive abnormalities, especially abnormalities in vi-sual-spatial and frontal lobe function, occur in patientswith DM2 [29], and occasionally there are alterations inthe white matter of the brain. There are similarities insome of the brain manifestations of DM2 to those inDM1. However, there are significant differences. Mentalretardation is a predominant feature of the congenitaland childhood forms of DM1 and is often associatedwith generalized atrophy on brain imaging. Mental re-

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tardation has been reported in one patient with DM2,but that may have been coincidental [42] and was notevident in a review of 234 DM2 cases [7].

Meola et al. [29] described a reduction in cerebralblood flow in the frontal and temporal poles in patientswith DM2, suggesting that this alteration may con-tribute to the abnormalities in visual spatial and frontallobe function on neuropsychological testing. The pa-tients in these studies had only minimal changes inwhite matter on MRI of the brain and it is difficult torelate the white matter changes to their decrease inbrain function [29].

The type of cognitive impairment that occurs in DM2is similar to but less severe than that of DM1.A recent in-vestigation compared a large cohort of DM2 patients toa group of similarly aged patients with DM1 [32]. Bothgroups displayed a specific type of “avoidant” personal-ity and had significant impairment in frontal lobe func-tion, especially limited ability to perform executivefunctions.The abnormalities were milder in the patientswith DM2, but were sufficiently prominent to indicatethat this disorder, like DM1, has significant brain mani-festations [32]. Recently the extent of brain atrophy wasinvestigated in 10 DM1 and 9 DM2 patients in compari-son with age-matched controls. As a quantitativemarker, the ratio of brain parenchymal to intracranialvolume, called brain parenchymal fraction (BPF), wascalculated from 3 dimensional MRI. Compared withcontrols the BPF in DM1 was significantly decreased,while in DM2 it was only slightly decreased. This study[16] also demonstrates structural involvement of theCNS in DM2.

Endocrine

DM2 affects the function of the testes [6]. Occasionallymen will present with primary hypogonadism [59]. Tes-ticular dysfunction and hypogonadism appear to bemore common in DM1, but assessment of gonadal func-tion in DM2 is an important part of the ongoing care ofpatients. Similarly, the frequency of insulin resistanceand glucose intolerance seems lower in DM2 than inDM1 [48, 49]. These impressions require further investi-gation. No carefully controlled studies of the incidenceof thyroid disease have yet been published. Hypothy-roidism can worsen or precipitate the manifestations ofDM2 [47], and patients with hypothyroidism who havepersistent weakness and excessive stiffness should be in-vestigated for covert DM2. Pregnancy and menses mayalso exacerbate muscle pain, myotonia and musclecramps [35] in DM2.

Hyperhidrosis

Increased sweating is a somewhat unique complaint inDM2 and is especially prominent in the hands and trunk

in most affected individuals. It occurs throughout thecourse of the disease. Its etiology is unknown.

■ Laboratory investigations

Blood tests

There are no pathognomonic findings on routine labo-ratory testing in DM2. Creatine kinase levels may be el-evated (up to ten times normal), and there may be a 2–3fold elevation of gamma-glutamyl transferase. A de-crease in gamma globulin concentration on serum elec-trophoresis may be present, similar to that observed inDM1. The significance and importance of these abnor-malities in both DM2 and DM1 is unclear.

Electromyography

Electromyography with detailed examination in search-ing for myotonia is useful when suspecting the diagno-sis of DM2, although it is noteworthy that myotonia isdemonstrable in 75 % of patients on physical examina-tion and 90 % by EMG [7] rather less than would be thefigures for adult DM1.

Muscle biopsy

Characteristic findings in muscle biopsy specimens inDM2 have been difficult to detect. Previous reports bythe ENMC Study Group on DM2 have described thepathological findings in skeletal muscle, as showingnonspecific alterations, occasionally resembling DM1[33]. In contrast, muscle biopsies in patients with DM1frequently have revealed abnormal findings, includingfiber size variability, ring-binden fibers, increased num-ber of central nuclei within the cells, with nuclearchains, the sarcoplasmic masses and in the very latestages some muscles have replacement of muscle tissuewith fibrotic and fatty tissue. Type I atrophy is typical inDM1.

Less characteristic abnormalities appear in musclebiopsy specimens from patients with DM2 [2]. In par-ticular, there is no preferential type 1 atrophy. Recently,a collaborative study [Finnish (vastus lateralis), Italian(biceps brachii) and French (deltoid) biopsies] exam-ined muscle samples of different muscles from patientswith the DM2 and identified characteristic abnormali-ties [63]. The staining of muscle not only included rou-tine histochemical staining but also staining with anti-bodies to slow and fast myosin heavy chains. Theimmunostaining of myosin heavy chains demonstratedvery small fibers with insufficient quantities of sarco-meric protein, i. e. myosins, detected with the ATPasereaction. All nuclear clump fibers were type 2. The in-vestigators found similar abnormalities in the three

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different muscle groups biopsied. There were threecharacteristic abnormalities: 1) scattered, very smallatrophic type 2 fibers; 2) nuclear clumps containing type2 fibers, and, 3) the early appearance of type 2 nuclearclump fibers in clinically strong proximal muscles, pref-erentially vastus lateralis [63]. The findings of type 2fibre atrophy were recently demonstrated in musclebiopsies from 57 patients with genetically confirmedmyotonic dystrophy type 2 [53]. Demonstration of thesecharacteristics in muscle biopsy specimens can be usedas a screening tool to select patients for molecular diag-nostic testing for the DM2 mutation [62].

Table 2 summarizes the clinical and laboratory find-ings in 60 patients with DM2 confirmed mutation fromItalian and American families.

Progressive myotonic myopathies with multiorgandysfunction and without DNA testing of DM2mutation

This section of the article reviews the peculiar-border-line cases from families with progressive myotonic my-opathies in which the DM2 mutation analysis was notperformed.

These cases emphasize the wide spectrum of findingsthat can occur in the DM2 related syndromes (Table 3).Involvement of the CNS is a feature in some of theseatypical cases. Early reports describe four German fam-ilies [9, 13] with leukoencephalopathy and the clinicalmanifestations included stroke-like episodes, seizures,hypersomnia and parkinsonism. While sleep distur-bances are not typical in DM2, one report describes two

Table 2 Summary of core complaintsa and findings in 60 patientsb with DM2 con-firmed mutation from 5 Italian and 3 American families

Percentage Age at first coreof patients symptoms

(years; n = 60)

Age of appearance of first core symptomsWeakness 73 20–60Myotonia 60 20–50Cataracts 20 31–66

Clinical and laboratory featuresElectrical myotonia 97Weakness hip flexors 80Cataracts 60Elevation of creatine kinase 60Elevation of γ-glutamil transferase 58Muscle pain 46Sensorineural deafness 40CNS: cognitive, visuospatial, behavioural

abnormalities 34Electrocardiographic abnormalities 20

a Myotonia, weakness, cataract; b 34 women, 26 men Tabl

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sisters with the PROMM phenotype and unusual long-standing insomnia [46]. They had difficulty in initiatingsleep and had daytime hypersomnolence [46]. Majorpsychiatric problems are not common in DM2 but a re-cent report describes medication reactions in a PROMMpatient with schizophrenia [52]. The patient was intoler-ant to narcoleptics and was susceptible to malignant hy-perthermia.

An unrecognized cause of camptocormia (forwardflexion of the trunk both in stance and gait) is describedin 3 French patients with PROMM [44, 55] and an atyp-ical case of PROMM with parkinsonism has been re-cently reported [4].

Involvement of cardiac conduction may also occur inatypical cases [64] as well atypical cases with PROMMand primary hyperparathyroidism have been reported[50].

Current clinical approach to identify DM2

Demonstration of the DM2 mutation with DNA analysisis the “gold standard” for establishing the diagnosis.However, it is important to keep in mind that the DM2mutation may cause a wider spectrum of manifestationsand a broader phenotype. The recent observation thatDM2 patients may display extremely small type 2 fibersand nuclear clump fibers in muscle samples [63], sug-gests that muscle biopsy analysis may provide an addi-tional indication for considering DM2 and mutationanalysis [62].

The diagnosis of DM2 is based on the presence of anabnormal CCTG repeat expansion in the 3q21.3 ZNF9locus for DM2.

Conventional Southern-Blot (SB) protocols, that havebeen routinely used to identify the abnormally ex-panded alleles containing the CTGn repeats in patientswith DM1, have detected the DM2 mutation in only 80 %of individuals known to have the disease [7]. A moresensitive method is available to identify the unstableDM2 mutation, and it is possible to demonstrate thepresence of the enlarged repeats in those individualswho had a negative result on DNA testing using the con-ventional SB protocol. There is a three-step diagnosticprocedure. 1) In step 1 PCR analysis of the CL3N58 mi-crosatellite marker occurs and screens for homozygousindividuals and carriers of the mutations. The large re-peat in the mutated allele prevents PCR amplificationand gives a “blank” for the involved allele. The appear-ance is the same as that of true homozygotes,who do notcarry the DM2 mutation. Individuals who have two sep-arate alleles identified on PCR screening with theCL3N58 marker are excluded from having the DM2 mu-tation. 2) In step 2 Southern analysis is performed onthose individuals who appear to have a homozygouspattern on PCR screening.

Approximately 12 % of the population is normallyhomozygous and those individuals need to be distin-guished from affected individuals carrying the DM2mutation. Almost 20 % of affected individuals had in-conclusive results on Southern analysis. Evaluation ofthese cases requires step 3. Repeat Assay (RA). By usinga PCR primer that primes from multiple sites within theelongated CCTG repeat tract, RA can be used to detectDM2 expansions by the presence of a smear of productswith molecular weights higher than in control lanes. Al-though the RA identifies the presence or absence ofDM2 expansions overcoming the sensitivity problem,this method does not provide any information on the ex-pansion size. To insure specificity, RA PCR products aretransferred to a nylon membrane and probed with an in-ternal oligonucleotide probe.

The three step analysis developed by Dr. Ranum andher group provides > 99 % sensitivity and specificity toidentify known DM2 mutations [7].

Recently in 94 DM2 patients (57 females and 37males) from 79 independent families (92 patients wereof German and 2 of Slovenian origin) improvement ofthe diagnostic DNA testing has been reported. In thispaper, the combination of pulse-field gel electrophoresisand semiquantitative Southern blot analysis with anovel hybridization probe, led to unequivocal results inabout 98 % of cases with clinical diagnosis of DM2 [14].

In the large study of 139 families there was no signif-icant correlation between age of onset and the size of theCCTGn repeat. However, there was intergenerational in-stability and there was a tendency for contraction of therepeat expansion. The most consistent correlation withrepeat size was the age of the patient at the time theblood sample was taken, indicating a tendency for thesize of CCTGn repeat in circulating leucocytes to in-crease over time [7].

A large proportion of the families with DM2 have asuggested preferentially German-Eastern European an-cestry [7], and a recent genetic study of 17 unrelatedfamilies of European descent from different countries(France, Finland, Germany, Italy, Spain, Switzerland, UKand USA) suggests that there may be a single foundermutation for DM2 in these families [1].The investigatorsin this study used a modified Southern blot analysiswith pulsed field/inversion gel electrophoresis (FIGE) toidentify the DM2 mutation. This methodology identi-fied CCTG expansions in alleles isolated from affectedfamily members that ranged in size from 4 to 27Kb.Highresolution haplotype analysis with 5 microsatellite and22 single nucleotide polymorphisms (SNP) around theDM2 mutation showed linkage disequilibrium (LD) anda single shared haplotype of at least 132Kb among pa-tients from these different regions of the world. Thesedata suggest that there is a single founding mutation inDM2 patients of European origin, a situation reminis-cent of that seen in DM1 [1].

1179

A companion paper suggests that the predominantNorthern European ancestry of families with DM2 re-sults from a common founder and that the loss of inter-ruptions within the CCTG portion of the repeat tractmay predispose alleles to further expansion. The com-plex repeat motif and flanking sequences within intron1 are conserved among humans, chimpanzees, gorillas,mice and rats [21].

Very recently a new approach for DM2 mutation de-tection has been reported using chromogenic in situ hy-bridization technique (CISH) on muscle biopsy mater-ial. In this study frozen muscle biopsy specimens fromnine mutation verified DM2 patients were tested with la-belled sense (CCTG)8 and antisense (CAGG)8 oligonu-cleotide probes using CISH. Accumulated mutant RNAas ribonuclear inclusions in all DM2 samples were eas-ily detected. In a second series the CISH method was ap-plied to 52 patients with undetermined neuromusculardisorders and proved to correctly identify the DM2 mu-tation in all 16 previously undiagnosed patients. Thisnew finding indicates that CISH on muscle sections issufficiently sensitive and specific for molecular diagno-sis of DM2 [45].

■ Diagnostic evaluation of patients with clinicalmyotonia and muscle weakness

The major problem in making the diagnosis of patientswith muscle weakness, clinical myotonia without cleardiagnostic findings on examination, is to consider ifthese patients have or have not multisystemic involve-ment.

Common complaints, such as trouble in seeing, gaitinstability, dropping things, muscle stiffness, musclepain, or trouble in swallowing may be symptoms of oneof the myotonic dystrophies.

Patients with DM1 and DM2 may also present withone or more of the multisystem manifestations of thediseases. Cardiac arrhythmias and early cataracts (typi-cally, iridescent posterior capsular cataracts) are exam-ples. In contrast, patients with other hereditary my-otonic disorders, such as chloride or sodium channelmyotonias, do not have cardiac arrhythmias orcataracts.

Although various abnormalities have been identifiedin DM1 and DM2 none are specific and it must be em-phasized that confirmation of the diagnosis depends en-tirely upon DNA analysis. However, a number of consis-tent abnormalities were noted in DM2 that helped toestablish it as a specific entity and to target the biomol-ecular diagnosis (i. e. the muscle biopsy with a relativelyspecific finding of type 2 fibre atrophy with pyknotic nu-clear clumps [63] and presence of ribonuclear inclusions[26, 45]).

Pathomechanisms and future studies

The similarities that exist between the different myo-tonic dystrophies (dominant inheritance, distinctivecataracts, myotonia, muscle weakness and multisystemdisorders affecting the heart, brain and endocrine sys-tems) [30, 34] suggest common pathomechanisms thatconnect DM1 and DM2 [39].

How does the expansion of noncoding CTG or CCTGrepeats result in dominantly inherited neuromusculardisease? The clinical and pathological features of DMpromoted the possibility that the DM mutation could actin trans at the RNA level in a dominant-negative manner(transdominant RNA gain-of-function [65]). This hy-pothesis suggests that the CUG repeat in DMPK andCCUG in ZNF9 mRNAs exert a toxic effect on cellular me-tabolism. Support for this hypothesis comes from trans-genic mice model expressing an expanded CUG repeat ina transcript unrelated to DM1 and DM2 loci. Those micedeveloped myotonia and myopathy, demonstrating thatthe CUG repeats are toxic to muscle cells and sufficient toproduce the most characteristic features of DM [22].

Both DM1 and DM2 mutant transcripts accumulatein foci within muscle nuclei [5, 57]. Subsequently, a pro-tein was identified that binds to RNAs whose sequencecontains a CUG-BP triplet repeat [60].

It has been shown that CUG-BP has an importantfunction in the processing of many mRNAs in a stepcalled “splicing”, in which parts of the RNA transcriptsunnecessary for the translation step are removed tomake the mature RNA. Specifically, it was shown thatsplicing of the cardiac troponin T (cTNT) RNA is alteredin DM1 muscle [36].Whether these defects in cTNT pro-cessing are associated with cardiac conduction defectsin DM1 remains unclear.However, it has been found thatthe splicing of the insulin receptor mRNA is also regu-lated by CUG-BP and this regulation is altered in my-otonic dystrophy types 1 and 2, and appears to accountdirectly for the insulin resistance that DM1 [48] andDM2 [49] patients develop.

Similarly, CUG-BP also regulates the splicing of themuscle chloride channel 1 (CLC-1) gene and patient spe-cific defects in CLC-1 splicing correlate directly withmyotonia, the most characteristic symptom of myotonicdystrophy [24].

Similarly, myotubularin-related 1 (MTMR1), an im-portant gene in skeletal muscle development,has alteredsplicing patterns in DM1 cells [3], as does TAU, a proteinthat is involved in many neurodegenerative diseases andalso forms abnormal protein aggregates in DM1 brains[54].

More recently, an alteration in the availability oramount of a critical nuclear regulatory protein, such asmuscleblind (MBNL), is a specific example of how ab-normally expanded mRNA transcripts might lead tomalregulation of cellular function [15, 23, 25].

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These studies confirm the key prediction of MBNLprotein sequestration for DM pathogenesis. Loss of spe-cific MBNL isoforms that associate with expanded(CUG)n and (CCUG)n RNAs is sufficient to cause my-otonia, cataracts and RNA splicing, defects that are sim-ilar to those seen in DM. Some aspects of the DM phe-notype may not result from loss of MBNL1 functionalone. Additional muscle blind proteins (MBNL2 andMBNL3) are also recruited to nuclear foci [10], so theirsequestration may be required to fully replicate the mul-tisystemic DM phenotype.

Alternatively transcription factors leaching fromchromatin by mutant RNA provides a potentially unify-ing pathomechanistic explanation for both DM1 andDM2 [8].

Future research is necessary to determine if there arefamilies with individuals having the clinical phenotypeof PROMM but lacking the gene mutations for bothDM1 and DM2 (DM3/PROMM?). Studies of potentialpathophysiological mechanisms for the different clini-

cal manifestations and therapeutic trials in patients withDM2 are needed. Development of a representative ani-mal model of DM2 (and of DM1 as well) will help to elu-cidate the pathophysiology that underlies the differentdisease manifestations of myotonic dystrophies (DM1,DM2). And at a fundamental level we need to under-stand the cause for the repeat instability in both DM1and DM2. This discovery will offer a major opportunityto clarify pathophysiology and to pioneer new ap-proaches to treatment.

■ Acknowledgements The authors acknowledge the generous sup-port of the University of Milan, Italy (MIUR 60 % and ex 40 %) givento G. Meola.

RTM has received generous support from National Institutes ofHealth (NIH) grants: RO1 AR44069 and RO1 AR49077 of the NationalInstitute of Arthritis and Musculoskeletal and Skin Diseases [NI-AMS]; from the NO1 AR02250 contract provided by NIAMS; fromMO1 RR00044 General Clinical Research Center grant provided byNational Center for Research Resources. Dr. Moxley also acknowl-edges additional generous support from a grant provided by the Foodand Drug Administration, FD-R-001662.

References

1. Bachinski LL, Udd B, Meola G, SansoneV, Bassez G, Eymard B, Thornton CA,Moxley RT, Harper PS, Rogers MT,Jurkat-Rott K, Lehmann-Horn F,Wieser T, Gamez J, Navarro C, BottaniA, Kohler A, Shriver MD, Sallinen R,Wessman M, Zhang S, Wright FA,Krahe R (2003) Confirmation of thetype 2 myotonic dystrophy (CCTG)nexpansion mutation in patients withproximal myotonic myopathy/proxi-mal myotonic dystrophy of differentEuropean origins: a single shared hap-lotype indicates an ancestral foundereffect. Am J Hum Gen 73:835–848

2. Bassez G, Attarian S, Laforet P, AzulayJP, Rouche A, Ferrer X, Urtizberea JA,Pellissier JF, Duboc D, Fardeau M,Pouget J, Eymard B (2001) Myopathieproximale avec myotonie (PROMM):étude clinique et histologique. RevNeurol (Paris)2:209–218

3. Buj-Bello A, Furling D, Tronchère H,Laporte J, Lerouge T, Butler-BrowneGS, Mandel JL (2002) Muscle-specificalternative splicing of myotubularin-related 1 gene is impaired in DM1muscle cells. Hum Mol Genet 11:2297–2307

4. Chu K, Cho J-W, Song E-C, Jeon BS(2002) A patient with proximal myo-tonic myopathy and parkinsonism.Can J Neurol Sci 29:188–190

5. Davis BM, McCurrach ME, Taneja KL,Singer RH, Housman DE (1997) Ex-pansion of a CUG trinucleotide repeatin the 3’ untranslated region of myo-tonic dystrophy protein kinase tran-scripts results in nuclear retention oftranscripts. Proc Natl Acad Sci USA94:7388–7393

6. Day JW, Roelofs R, Leroy B, Pech I,Penzow K, Ranum LP (1999) Clinicaland genetic characteristics of a five-generation family with a novel form ofmyotonic dystrophy (DM2). Neuro-muscul Disord 9:19–27

7. Day JW, Ricker K, Jacobsen JF,l Ras-mussen LJ, Dick KA, Kress W, Schnei-der C, Koch MC, Beilman GJ, HarrisonAR, Dalton JC, Ranum LP (2003)Myotonic dystrophy type 2: molecular,diagnostic and clinical spectrum.Neurology 60:657–664

8. Ebralidze A, Wang Y, Petkova V,Ebralidse K, Junghans RP (2004) RNAleaching of transcription factors dis-rupts transcription in myotonic dys-trophy. Science 303:383–387

9. Eger K, Schulte-Mattler WJ, Zierz S(1997) Proximal myotonic myopathy(PROMM). Nervenarzt 68:839–844

10. Fardaei M, Larkin K, Brook D,Hamshere MG (2001) In vivo co-locali-sation of MBNL protein with DMPKexpanded-repeat transcripts. NucleiAcid Res 29:2766–2771

11. Flachenecker P, Schneider C, CursiefenS, Ricker K, Toyka KV, Reiners K,(2003) Assessment of cardiovascularautonomic function in myotonic dys-trophy type 2 (DM2/PROMM). Neuro-muscul Disord 3:289–293

12. Harper PS (2001) Myotonic dystrophy.Major problems in neurology. WBSaunders Co (3rd ed), London, vol. 37

13. Hund E, Jansen O, Koch MC, Ricker K,Fogel W, Niedermaier N, Otto M, KuhnE Meinck HM (1997) Proximal myo-tonic myopathy with MRI white matterabnormalities of the brain. Neurology48:33–37

14. Jakubiczka S, Vielhaber S, Kress W,Kupferling P, Reuner U, Kunath B,Wieacker P (2004) Improvement of thediagnostic procedure in proximal my-otonic myopathy/myotonic dystrophytype 2. Neurogenetics 5:55–59

15. Kanadia RN, Johnstone KA, MankodiA, Lungu C, Thornton CA, Esson D,Timmers AM, Hauswirth WW, Swan-son MS (2003) A muscleblind knock-out model for myotonic dystrophy.Science 302:1978–1980

16. Kassubek J, Juengling FD, Hoffmann S,Rosenbohm A, Kurt A, Jurkat-Rott K,Steinbach P, Wolf M, Ludolph AC,Lehmann-Horn F, Lerche H, Weber YG(2003) Quantification of brain atrophyin patients with myotonic dystrophyand proximal myotonic myopathy: acontrolled 3-dimensional magneticresonance imaging study. NeurosciLett 348:73–76

1181

17. Kohler A, Burkhard P, Hefft S, BottaniA, Pizzolato GP, Magistris MR (2000)Proximal myotonic myopathy: clinical,electrophysiological and pathologicalfindings in a family. Eur Neurol 43:50–53

18. Kress W, Mueller-Myhsok B, Ricker K,Schneider C, Koch MC, Toyka KV,Mueller CR, Grimm T (2000) Proof ofgenetic heterogeneity in the proximalmyotonic myopathy syndrome(PROMM) and its relationship tomyotonic dystrophy type 2 (DM2).Neuromuscul Disord 10:478–480

19. Lamont PJ, Jacob RL, Mastaglia FL,Laing NG (2004) An expansion in theZNF9 gene causes PROMM in a previ-ously described family with an inci-dental CLCN1 mutation. J Neurol Neu-rosurg Psychiatry 75:343

20. Liquori C, Ricker K, Moseley M, Jacob-sen JF, Kress W, Naylor SL, Day JW,Ranum LP (2001) Myotonic dystrophytype 2 caused by a CCTG expansion inintron 1 of ZNF9. Science 293:864–867

21. Liquori CL, Ikeda Y, Weatherspoon M,Ricker K, Schoser BG, Dalton JC, DayJW, Ranum LP (2003) Myotonic dys-trophy type 2: human founder haplo-type and evolutionary conservation ofthe repeat tract. Am J Hum Gen 73:849–862

22. Mankodi A, Logigian E, Callahan L,McClain C, White R, Henderson D,Krym M, Thornton CA (2000) Myo-tonic dystrophy in transgenic miceexpressing an expanded CUG repeat.Science 289:1769–1773

23. Mankodi A, Urbinati CR, Yuan QP,Moxley RT, Sansone V, Krym M, Hen-derson D, Schalling M, Swanson MS,Thornton CA (2001) Muscleblind lo-calizes to nuclear foci of aberrant RNAin myotonic dystrophy types 1 and 2.Hum Mol Genet 10:2165–2170

24. Mankodi A, Takahashi MP, Jiang H,Beck CL, Bowers WJ, Moxley RT, Can-non SC, Thornton CA (2002) Ex-panded CUG repeats trigger aberrantsplicing of ClC-1 chloride channel pre-mRNA and hyperexcitability of skele-tal muscle in myotonic dystrophy. MolCell 10:35–44

25. Mankodi A, Thornton CA (2002) My-otonic syndromes. Curr Opin Neurol15:545–552

26. Mankodi A, Teng-Umnuay P, Krym M,Henderson D, Swanson M, ThorntonCA (2003) Ribonuclear inclusions inskeletal muscle in myotonic dystrophytypes 1 and 2. Ann Neurol 54:760–768

27. Mastaglia FL, Harker N, Philips BA,Day TJ, Hankey GJ, Laing NG, Fabian V,Kakulas BA (1998) Dominantly inher-ited proximal myotonic myopathy andleukoencephalopathy in a family withan incidental CLCN1 mutation. J Neu-rol Neurosurg Psychiatry 64:543–547

28. Meola G, Sansone V, Radice S, SkradskiS, Ptacek L (1996) A family with an un-usual myotonic and myopathic pheno-type and no CTG expansion (proximalmyotonic myopathy syndrome): achallenge for future molecular studies.Neuromuscul Disord 6:143–150

29. Meola G, Sansone V, Perani D, Collelu-ori A, Cappa S, Cotelli M, Fazio F,Thornton CA, Moxley RT (1999) Re-duced cerebral blood flow and im-paired visual-spatial function in proxi-mal myotonic myopathy. Neurology53:1042–1050

30. Meola G (2000) Clinical and geneticheterogeneity in myotonic dystrophies.Muscle Nerve 23:1789–1799

31. Meola G, Sansone V, Marinou K, CotelliM, Moxley RT 3rd, Thornton CA, DeAmbroggi L (2002) Proximal myotonicmyopathy: a syndrome with afavourable prognosis? J Neurol Sci193:89–96

32. Meola G, Sansone V, Perani D, ScaroneS, Cappa S, Dragoni C, Cattaneo E,Cotelli M, Gobbo C, Fazio F, SicilianoG, Mancuso M, Vitelli E, Zhang S,Krahe R, Moxley RT (2003) Executivedysfunction and avoidant personalitytrait in myotonic dystrophy type 1(DM1) and in proximal myotonic my-opathy (DM2/PROMM). NeuromusculDisord 13:813–821

33. Moxley RT, Meola G, Udd B, Ricker K(2002) 84th ENMC International Work-shop: PROMM (Proximal MyotonicMyopathy) and other Proximal Myo-tonic Syndromes – Workshop Report:Neuromuscul Disord 12:306–317

34. Moxley RT, Meola G (2003) The myo-tonic dystrophies. In Rosenberg RN,Prusiner SB, Di Mauro S, Barchi RL,Nestler EJ (ed) The molecular and ge-netic basis of neurologic and psychi-atric disease. Butterworth-Heinemann,Philadelphia, chapter 47, pp 511–518

35. Newman B, Meola G, O’Donovan DG,Schapira AHV, Kingston H (1999)Proximal myotonic myopathy(PROMM) presenting as myotoniaduring pregnancy. Neuromuscul Dis-ord 9:144–149

36. Philips AV, Timchenko LT, Cooper TA(1998) Disruption of splicing regulatedby a CUG-binding protein in myotonicdystrophy. Science 280:737–741

37. Phillips MF, Rogers MT, Barnetson R,Braun C, Harley HG, Myring J, StevensD, Wiles CM, Harper PS (1998)PROMM: the expanding phenotype. Afamily with a proximal myopathy, my-otonia and deafness. NeuromusculDisord 8:439–446

38. Ranum LPW, Rasmussen PF, BenzowKA, Koob MD, Day JW (1998) Geneticmapping of a second myotonic dystro-phy locus. Nat Genet 19:196–198

39. Ranum LPW, Day JW (2002) Myotonicdystrophy: clinical and molecular par-allels between myotonic dystrophytype 1 and type 2. Curr Neurol Neu-rosci Rep 2:465–470

40. Ranum LPW, Day JW (2002) Domi-nantly inherited non-coding micro-satellite expansion disorders. CurrOpin Gen & Dev 12:266–271

41. Ricker K, Koch MC, Lehmann-Horn F,Pongratz D, Otto M Heine R, MoxleyRT 3rd. (1994) Proximal myotonic my-opathy: a new dominant disorder withmyotonia, muscle weakness, andcataracts. Neurology 44:1448–1452

42. Ricker K, Koch MC, Lehmann-Horn F,Pongratz D, Speich N, Reiners K,Schneider C, Moxley RT 3rd (1995)Proximal myotonic myopathy. Clinicalfeatures of a multisystem disordersimilar to myotonic dystrophy. Arch.Neurol 52:25–31

43. Ricker K, Grimm T, Koch MC, Schnei-der C, Kress W, Reimers CD, Schulte-Mattler W, Mueller-Myhsok B, ToykaKV, Mueller CR (1999) Linkage ofproximal myotonic myopathy to chro-mosome 3q. Neurology 52:170–171

44. Rimbaux S, Pellieux S, Bergemer AM,Saïkali I, Gherardi R, Fouquet B (2003)Camptocormie rèvèlatrice d’une my-opathie myotonique proximale. RevNeurol (Paris) 159:678–680

45. Sallinen R, Vihola A, Bachinski LL,Huoponen K, Haapasalo H, HackmanP, Zhang S, Sirito M, Kalimo H, MeolaG, Horelli-Kuitunen N, Wessmann M,Krahe R, Udd B (2004) New methodsfor molecular diagnosis and demon-stration of the (CCTG)n mutation inmyotonic dystrophy type 2 (DM2).Neuromuscul Disord 14:274–283

46. Sander HW, Tavoulareas GP,Chokroverty S (1996) Heat-sensitivemyotonia in proximal myotonic my-opathy. Neurology 47:956–962

47. Sansone V, Griggs RC, Moxley RT(2000) Hypothyroidism unmaskingproximal myotonic myopathy. Neuro-muscul Disord 10:165–172

48. Savkur RS, Philips AV, Cooper TA(2001) Aberrant regulation of insulinreceptor alternative splicing is associ-ated with insulin resistance in myo-tonic dystrophy. Nature Genet 29:40–47

49. Savkur RS, Philips AV, Cooper TA, Dal-ton JC, Moseley ML, Ranum LP, DayJW (2004) Insulin receptor splicing al-teration in myotonic dystrophy type 2.Am J Hum Genet 74:1309–1313

50. Schneider C, Grimm T, Kress W, Som-mer C, Müller CR (2000) Hyper-parathyroidism in a patient with prox-imal myotonic myopathy (PROMM).Neuromuscul Disord 10:481–483

1182

51. Schneider C, Wessig C, Müller CR,Brechtelsbauer D, Grimm T (2001)Proximal myotonic myopathy andproximal myotonic dystrophy: Twodifferent entities? The phenotypicvariability of proximal myotonic syn-dromes. Neuromuscul Disord 11:485–488

52. Schneider C, Pedrosa Gil F, SchneiderM, Anetseder M, Kress W, Müller CR(2002) Intolerance to neuroleptics andsusceptibility for malignant hyper-termia in a patient with proximalmyotonic myopathy (PROMM) andschizophrenia. Neuromuscul Disord12:31–35

53. Schoser BGH, Schneider-Gold C, KressW, Goebel HH, Reilich P, Koch MC,Pongratz DE, Toyka KV, Lochmüller H,Ricker K (2004) Muscle pathology in57 patients with myotonic dystrophytype 2. Muscle Nerve 29:275–281

54. Sergeant N, Sablonniere B, Schraen-Maschke S, Ghestem A, Maurage CA,Wattez A, Vermersch P, Delacourte A(2001) Dysregulation of human brainmicrotubule-associated tau mRNAmaturation in myotonic dystrophytype 1. Hum Mol Genet 10:2143–2155

55. Serratrice J, Weiller PJ, Pouget J, Serra-trice G (2000) An unrecognized causeof camptocormia: proximal myotonicmyopathy. Press Med 29:1121–1123

56. Sun C, Henriksen OA, Tranebjaerg L(1999) Proximal myotonic myopathy:clinical and molecular investigation ofa Norvegian family with PROMM. ClinGenet 56:457–461

57. Taneja KL, McCurrach ME, SchallingM, Housman D, Singer RH (1995) Fociof trinucleotide repeat transcripts innuclei of myotonic dystrophy cells andtissues. J Cell Biol 128:995–1002

58. The International Myotonic DystrophyConsortium (IDMC) (2000) Newnomenclature and DNA testing guide-lines for myotonic dystrophy type 1.Neurology 54:1218–1221

59. Thornton CA, Griggs RC, Moxley RT(1994) Myotonic dystrophy with notrinucleotide repeat expansion. AnnNeurol 35:269–272

60. Timchenko NA, Cai ZJ, Welm AL,Reddy S, Ashizawa T, Timchenko LT(2001) RNA CUG repeats sequesterCUGBP1 and alter protein levels andactivity of CUGPB1. J Biol Chem276:7820–7826

61. Udd B, Krahe R, Wallgren-Petterson C,Falck B, Kalimo H (1997) Proximalmyotonic dystrophy – a family withautosomal dominant muscular dystro-phy, cataracts, hearing loss, and hypo-gonadism: heterogeneity of proximalmyotonic disorders. NeuromusculDisord 7:217–228

62. Udd B, Meola G, Krahe R, Thornton C,Ranum L, Day J, Bassez G, Ricker K(2003) Report of the 115th ENMCworkshop: DM2/PROMM and othermyotonic dystrophies. 3rd Workshop,14–16 February 2003, Naarden, TheNetherlands. Neuromuscul Disord13:589–596

63. Vihola A, Bassez G, Meola G, Zhang S,Haapasalo H, Paetau A, Mancinelli E,Rouche A, Hogrel JY, Laforet P,Maisonobe T, Pellissier JF, Krahe R,Eymard B, Udd B (2003) Histopatho-logical differences of myotonic dystro-phy type 1 (DM1) and PROMM/DM2.Neurology 60:1854–1857

64. von zur Muhlen F, Klass C, Kreuzer H,Mall G, Giese A, Reimer CD (1998)Cardiac involvement in proximalmyotonic myopathy. Heart 79:619–621

65. Wang J, Pegoraro E, Menegazzo E,Giannarelli M, Hoop RC, Angelini C,Hoffman EP (1995) Myotonic Dystro-phy: evidence for a possibile domi-nant-negative RNA mutation. HumMol Genet 4:599–606

66. Wieser T, Bönsch D, Eger K, Schulte-Mattler W, Zierz S (2000) A family withPROMM not linked to the recentlymapped PROMM locus DM2. Neuro-muscul Disord 10:141–143