A R T I C L E S

Neurofibromatosis Type 1: A Model Condition forthe Study of the Molecular Basis of VariableExpressivity in Human DisordersJOHN C. CAREY* AND DAVID H. VISKOCHIL

Neurofibromatosis type 1 (NF1) is a pleiotropic autosomal dominant disorder with marked variability ofclinical expression. As in other heritable disorders, the mapping and cloning of the gene responsible for NF1have increased our understanding of the pathogenesis of the condition. In particular, the phenotypicvariability and variable expressivity can be studied using molecular techniques. In this article we summarizethe current knowledge of genotype/phenotype correlation in NF1 and examine the potential molecularbasis for variable expressivity. Am. J. Med. Genet. (Semin. Med. Genet.) 89:7–13, 1999.Q 1999 Wiley-Liss, Inc.

KEY WORDS: neurofibromatosis type 1; expressivity; genotype/phenotype correlation; heterogeneity

INTRODUCTION

Neurofibromatosis type 1 (NF1) is themost common autosomal dominant dis-order of humans appearing in child-hood; it occurs with a frequency ofabout one in 4,000 individuals [Fried-man, 1999]. NF1 comprises cafe-au-laitmacules, dermal neurofibromas, andLisch nodules and predisposes to othermanifestations, including learning dis-abilities, congenital dysplasias, and ma-lignancies. Patients with NF1 exhibit amarked degree of variability of clinicalfindings, even in the same family. Seg-regation ratios are 50%, indicating full

penetrance [Friedman, 1999]. The geneis highly pleiotropic, in that there is adiverse set of manifestations involvingvarious systems and types of abnormali-ties that are due to the gene mutation.Thus, the disorder is a syndrome in thetruest sense of the word as used by ge-neticists [Opitz, 1994].

The NF1 gene was mapped to thepericentromeric region of 17q in 1987and was identified by two independentresearch groups in 1990 [reviewed inMarchuk and Collins, 1994]. The genespans more than 300 kb of genomicDNA and contains 60 exons [Upadhy-aya and Cooper, 1998]. The open read-ing frame predicts a protein of 2,818amino acids, neurofibromin, which hashomology to the catalytic domain ofGTPase-activating proteins (GAPs).Three genes are embedded in an intron,and these are transcribed from the op-posite strand of the NF1 gene. Thelinkage and the characterization of theNF1 gene led to improved diagnostictesting and to the prospect of a betterunderstanding of the pathogenesis ofthis biologically intriguing condition.Advances in knowledge of the NF1gene and its product, neurofibromin,have paralleled an increase in knowl-edge of the phenotypic aspects andnatural history of NF1. Here we reviewthe present understanding of NF1 inthe context of the basic principles ofhuman genetics, particularly pheno-

typic variability. We summarize thecurrent knowledge of genotype/phe-notype correlation in NF1 and the syn-drome picture of the neurofibromatoses.

HETEROGENEITY

The neurofibromatoses are a heteroge-neous group of conditions exhibitingclinical manifestations commonly seenin the classic forms of NF, that is, NF1and NF2. Riccardi [1982] initially pro-posed a classification of the differenttypes of NF. Viskochil and Carey[1992, 1994] offered a different ap-proach to classification that was influ-enced by the molecular advances in thecontext of newly recognized clinicalphenotypes. Table I summarizes an up-dated and current version of this classi-fication of the neurofibromatoses basedon more recent information. Alternateforms of NF1 and NF2 are defined asconditions showing some of the classicfeatures of either condition, but not in atypical manner. The segmental form ofNF1 exemplifies this grouping. Relatedforms of NF1 and NF2 represent a setof variant conditions with clinical char-acteristics of NF1 (or NF2), but withadditional manifestations not typicallyseen in the classic neurofibromatoses. Inthis sense the related forms of NF mayor may not satisfy the clinical criteria fordiagnosis of either NF1 or NF2, but, by

John Carey is a professor in the Depart-ment of Pediatrics at the University ofUtah Health Science Center in Salt LakeCity. He has had a career-long interest inneurofibromatosis (NF) type 1 and hasbeen the clinician collaborating with mo-lecular biologists at the University ofUtah on NF genomics efforts. He co-directs the NF clinic with Dr. Viskochil.David Viskochil is an associate professorin the Department of Pediatrics at theUniversity of Utah Health Science Centerin Salt Lake City. He was a member ofone of the teams that identified and ini-tially characterized the NF1 gene. Henow staffs the NF clinic and serves as aco-vice chairman on the Clinical CareCommittee of the National Neurofibro-matosis Foundation.

*Correspondence to: Departmentof Pediatrics, Division of Medical Genet-ics, University of Utah Health ScienceCenter, Salt Lake City, UT 84132. E-mail:john.carey@hsc.utah.edu

AMERICAN JOURNAL OF MEDICAL GENETICS (SEMIN. MED. GENET.) 89:7–13 (1999)

© 1999 Wiley-Liss, Inc.

definition, they must have distinctivefindings that go beyond the usual clini-cal spectrum of these conditions. Weaddress recent developments in knowl-edge of segmental NF, spinal NF, fa-milial cafe-au-lait spots, and neu-rofibromatosis-Noonan syndrome(NFNS).

Segmental NF

Individuals with NF1 manifestationslimited to one or more areas of thebody are often designated as having seg-mental NF. Riccardi classified this con-dition as NF type V, and the topic wasreviewed thoroughly by Viskochil andCarey [1994]. More recently, Hager etal. [1997] summarized the thinkingabout this apparently distinct entity inthe dermatology literature. More than80 patients have been described sinceCrow et al. [1956] originally identifiedthe condition as sectorial NF. Some pa-tients with segmental NF have only lo-calized cafe-au-lait spots. Most have asegmental distribution of dermal neuro-fibromas. Internal NF manifestationsare uncommon in such patients.

Most cases are sporadic, but therehave been five reports of parents withsegmental distribution of NF1 whohave had offspring with classic NF1[Moss and Green, 1994]. These casesand the more localized distribution ofNF1 signs suggest that segmental NFmay represent somatic mosaicism of anNF1 mutation. Somatic mosaicism has

been described in three patients withNF1, but none had the localized distri-bution characteristic of so-called seg-mental NF [Colman et al., 1996; Ains-worth et al., 1997; Wu et al., 1997a](see Expressivity). The hypothesis thatthe localized findings are due to mosa-icism is tenable and fits informationfrom other recent reports of mosaicismin heritable disorders [Zlotogora,1998]. Mosaicism in NF1 is further ex-emplified by the observation of germ-line mosaicism in a family with twochildren who have the same intragenic

deletion seen in 10% of the father’ssperm [Lazaro et al., 1995]. Investiga-tors are attempting to further test thehypothesis that patients with segmentalNF have mosaicism of an NF1 muta-tion [Wallace et al., 1997].

Familial Spinal Neurofibromatosis

Familial spinal NF has been consideredan alternate form of NF. Three multi-generational families have been de-scribed since the original article of Pulst

et al. [Pulst et al., 1991; Poyhonen etal., 1997; Ars et al., 1998]. Patients inthese families have multiple neurofibro-mas along the spinal cord, occasionallyinvolving the cervical, thoracic, andlumbar regions in their entirety. Thepatients have multiple cafe-au-lait spotsbut generally lack dermal neurofibro-mas and Lisch nodules. Since Lischnodules are a consistent finding in ado-lescents and adults with NF1, and sincegeneralized spinal NF is relatively un-common in classic NF1, this conditionis considered to be a distinct clinical en-tity. Like the Watson syndrome andNFNS, the phenotype runs true withinfamilies.

Pulst et al. [1991] demonstratedlinkage in one of their two families tomarkers near the NF1 gene. Poyhonenet al. [1997] also mapped the gene in athree-generation kindred to the NF1region on 17q. Moreover, linkage tothe NF2 locus was excluded in thesefamilies. Ars et al. [1998] identified aunique frameshift mutation in exon 46of the NF1 gene in a family with spinalNF. This mutation results in a truncatedNF1 protein, and the author suggestedthat this particular mutation was an ex-planation for the clinical uniformity inthis family. Further work on well-documented families with familial spi-nal NF will be required to determine atrue phenotype/genotype correlation.However, the detection of this muta-tion in a family with this distinct phe-notype suggests that allelic heterogene-ity of the NF1 gene can explain somephenotypic findings (see Expressivity).

Familial Cafe-au-Lait Spots

Riccardi [1982] initially recognizedmultigenerational families of individualswith cafe-au-lait spots and no othersigns of NF1. He suggested that thisclinical picture be called NF VI in hisoriginal proposed classification. Despiteextensive discussion of this apparent en-tity, very few well-documented multi-generational families with autosomaldominant cafe-au-lait macules havebeen reported. The absence of neuro-fibromas, axillary freckling, and Lischnodules indicates that these families seg-regate a trait phenotypically distinctfrom classic NF1. Charrow et al. [1993]

TABLE I. The Neurofibromatoses

Neurofibromatosis type 1 (NF1)Whole-gene-deletion phenotypeAlternate forms of NF1 (conditions with incomplete/atypical features)

Mixed NFLocalized NF

Segmental NFGastrointestinal NFFamilial spinal NFFamilial cafe-au-lait spots

Related forms of NF1 (conditions with additional features)NF/Noonan syndromeWatson syndrome

Neurofibromatosis type 2 (NF2)Alternate form of NF2

Schwannomatosis

The hypothesis that thelocalized findings are due to

mosaicism is tenable andfits knowledge of other

recent reports of mosaicismin heritable disorders.

8 AMERICAN JOURNAL OF MEDICAL GENETICS (SEMIN. MED. GENET.) ARTICLE

and Brunner et al. [1993] both ex-cluded linkage of this trait to the NF1locus in two families. These investiga-tors suggested that familial cafe-au-laitspots represented a dominant gene ge-netically separate from NF1. On theother hand, Abeliovich et al. [1995] es-tablished close linkage between familialcafe-au-lait spots and the NF1 locus inthree generations of a family and con-cluded that their family had a trait allelicto NF1. The Watson syndrome is alsothought to be allelic to NF1 [Allansonet al., 1991; Viskochil and Carey,1994]. Thus, two conditions with find-ings that are either not typical of classicNF1 (familial cafe-au-lait spots) or withfindings beyond the typical features ofNF1 (Watson syndrome) both map tothe NF1 locus. Further molecular work

will be needed to confirm that both ofthese conditions are truly allelic toNF1. Disease-causing mutations infamilies with familial cafe-au-lait spotsor Watson syndrome would settle theissue with certainty.

Neurofibromatosis-Noonan Syndrome

NFNS was defined by Allanson et al.[1985] and exemplifies a related form ofNF1: patients may or may not meet cri-teria for NF1, but they have features ofNoonan syndrome. Carey [1998] re-viewed the topic of NFNS and pro-posed several theories to explain thecombination of NF and the Noonansyndrome. Most intriguing, however, isthe report of Carey et al. [1998] of atwo-generation family with NFNS anda 3-bp deletion in exon 17 of the NF1gene. This presumably disease-causingmutation deletes a methionine residuefrom the neurofibromin peptide. If oneassumes that this mutation is the basis

for the NFNS phenotype, then this par-ticular family represents another ex-ample of phenotype/genotype correla-tion in NF1. Recent reviews suggestthat there is no known genotype/phe-notype correlation in NF1 other thanthe distinct phenotype of an entire genedeletion [Upadhyaya and Cooper,1998]. However, the family withNFNS, the family with familial spinalneurofibroma, and the linkage of famil-ial cafe-au-lait spots and Watson syn-drome to the NF1 locus all suggest thatthere may be allelic heterogeneity inNF1.

Further support for this notioncomes from the interesting family stud-ied by Tassabehji et al. [1993]. Theseinvestigators reported on a three-generation family with cafe-au-laitspots, low intelligence/global develop-mental delay, and minor anomalies. Inthis family, Lisch nodules and neurofi-bromas were not present, and pulmonicvalvular stenosis was found in one pa-tient. The authors indicated that pa-tients in this family had features of bothWatson and Noonan syndromes. Wehave had the opportunity to review de-tailed photographs of these patients(courtesy of Dr. Dian Donnai): Thesepatients do not appear to have typicalNoonan syndrome and thus represent adistinct entity. The NF1 mutation inthis family is unique in that they havean in-frame, 42-bp tandem duplicationin exon 28 of the NF1 gene. Individualsin this family, like the one reported byCarey et al. [1998], do not meet thecriteria for NF1 and yet have character-istics that go beyond NF1 (pulmonicstenosis); in addition, distinctive muta-tions of the NF1 gene are present, withrelated phenotypes.

Other Forms

An alternate form of NF2 is schwanno-matosis. This entity will not be re-viewed, since the theme of this series isNF1. Clinical aspects of schwannoma-tosis were described thoroughly byMacCollin et al. [1996]. Moreover,Gorlin and Koutlas [1998] reported ona multigenerational family with thecombination of multiple schwannomas,multiple nevi, and multiple vaginalleiomyomas. Further molecular investi-

gation of this interesting family may in-dicate whether this condition is a re-lated form of NF2. Another form oflocalized NF, gastrointestinal NF, ischaracterized by neurofibromas alongthe gastrointestinal tract without otherfindings of NF1. This condition willnot be reviewed here, since it was cov-ered by Viskochil and Carey [1994] intheir review and no new data are avail-able.

PLEIOTROPY

NF1 involves not only the nervous sys-tem but also the vascular and skeletalsystems. It includes a characteristicgrowth pattern of relative macro-cephaly and short stature. Thus it rep-resents the prototype of a pleiotropiccongenital multiple dysplasia syndrome.The pleiotropic effects of the NF1 mu-tation also include a predilection forhamartomas, dysplasias, and malignan-cies. However, the pathogenetic basisfor these diverse effects is not well un-derstood. While the inactivation of theGAP-related domain of the gene prod-uct is suggestive of a tumor-suppressorparadigm of cellular proliferation, veryfew of the other manifestations of NF1can be explained by molecular knowl-edge of neurofibromin. In particular,the vascular and osseous dysplasias areintriguing and elude understanding.Any all-encompassing explanation ofthe pathogenesis of NF1 must explainthe learning disability, the dysplasias,and the tumor-suppressor effects. Fu-ture work on the role of neurofibrominin development might provide insightsabout the pathogenetic sequence of ef-

TABLE II. Mechanisms toExplain Variable Expressivity inNeurofibromatosis Type 1

Allelic heterogeneityTwo-hit hypothesis/tumor

suppressor geneSomatic mosaicismContiguous gene deletionsModifying genes (epistasis)Epigenetic factorsEnvironmental factorsStochastic factors

Two conditions withfindings that are either nottypical of classical NF1 or

with findings beyondtypical NF1, both map to

the NF1 locus.

ARTICLE AMERICAN JOURNAL OF MEDICAL GENETICS (SEMIN. MED. GENET.) 9

fects related to haploinsufficiency of theNF1 gene.

EXPRESSIVITYVariable expressivity is one of the hall-marks of NF1. Within the same family,patients have marked differences inphenotype. There is both intrafamilialand interfamilial variability [Carey etal., 1979; Easton et al., 1993]. Familiesin which individuals with the sameNF1 mutation have the full spectrum ofdisease—from a mild clinical form to aserious, medically significant one—illustrate the intrafamilial variability.Table II lists a number of proposedmechanisms of action to explain thewide range of manifestations in NF1.This discussion updates a review byRiccardi [1993] on potential explana-tions for variable expressivity in NF1.

Two monographs summarize themanifestations of NF1 [Riccardi, 1992;Huson and Hughes, 1994]. Over theyears, Riccardi [1992] has proposed ascheme for grading clinical severity.Other measures of the range of severityof NF1 include survival tables and list-ings of the incidence of manifestationsin large series. To date, attempts toquantify variable expressivity in NF1(and other heritable conditions) havebeen tentative at best and are fraughtwith selection bias and small numbers inmost series. Nevertheless, understand-ing the variability of NF1 is a challeng-ing biological question. The list ofmechanisms of action in Table II is pro-visional, and, as better understanding ofmolecular pathogenesis arises, the cat-egories will be expanded and modified.

Locus heterogeneity, as found in

other hamartomatous conditions, suchas tuberous sclerosis and hereditarymultiple exostoses, is not apparent inNF1 and can be eliminated as an expla-nation. Allelic heterogeneity was men-tioned earlier in connection with theconditions listed in Table I. As indi-cated, a number of discrete and consis-tent phenotypes, sometimes referred toas subtypes, appear either to map to theNF1 region or to exhibit mutations ofthe NF1 gene. The alternate NF formof spinal NF and the related form ofNFNS are examples of conditions inwhich unique mutations of the NF1gene have been described in familieswith the respective phenotypes. In bothof these examples, the mutation wasproposed to cause the disease.

Another explanation for the vari-ability of clinical manifestations in pa-

tients with NF1 is the somatic “secondhit” of the normal NF1 allele. Thisconcept is well established and relates tothe notion that NF1 acts as a tumor-suppressor gene [Marchuk and Collins,1994]. Loss of heterozygosity and docu-mented inactivating second hits in theother allele have now been establishedin a number of neoplasms, includingdermal neurofibromas, pheochromocy-tomas, and myeloid dysplasia disorders[Xu et al., 1992; Shannon et al., 1994;Sawada et al., 1996]. More work will benecessary to delineate the additional ge-netic alterations that underlie the devel-opment of malignant nerve sheath tu-mors in patients with NF1. A secondhit of the normal NF1 allele does notreadily explain many NF manifesta-tions, including the learning disabilitiesand osseous dysplasias.

The concept of somatic mosaicism

in NF1 was mentioned earlier in ourdiscussion of segmental NF. Colman etal. [1996] described the case of a patientwith somatic mosaicism of the NF1gene who was not diagnosed with NF1until age 27, when a neurofibroma wasdiscovered. Cafe-au-lait spots and axil-lary freckling had been noted when thepatient was 7 years old, and multipleneurofibromas developed after shereached age 26. She had bilateral Lischnodules. In this case, age at diagnosisand the appearance of cafe-au-lait spotswere later than most typical patientswho meet the National Institutes ofHealth criteria for NF1. The authorssuggested that her relatively late-onsetsymptoms were related to the mosa-icism of the large deletion. The largedeletion encompassed more than 100kb and extended from at least exon 4 tointron 39. The case of the first patientwith documented mosaicism was re-ported by Lazaro et al. [1995] in a manwith germline mosaicism. This patientwas clinically normal and had two chil-dren with NF1. Two other patientswere found to have somatic mosaicismof variable-size deletions of the NF1gene [Ainsworth et al., 1997; Wu et al.,1997a]. One of them had a deletion ofthe entire NF1 gene. The patient citedby Ainsworth also had symptoms ofrelatively late onset, a mild phenotype,and a deletion similar in size to that ofthe patient of Colman et al. [1996].Thus, somatic mosaicism occurs in pa-tients with NF1 and may, in some cases,account for a later onset or a milderphenotype. Riccardi and Lewis [1988]proposed this idea more than 10 yearsago, when they noted that the prede-cessors of multigenerational families hada milder phenotype, suggesting to themthat some progenitors have somaticmosaicism that accounts for the pheno-typic difference.

Another explanation of variation inexpression of NF1 is the hypothesis ofmodifying genes. Easton et al. [1993]showed that four NF1 traits, includingseizures, optic gliomas, learning dis-abilities, and scoliosis, had significant fa-milial clustering, exemplifying segrega-tion of epistatic genes unlinked to theNF1 locus. Recent observations in pa-tients with NF1 support this genetic

Families in whichindividuals with the sameNF1 mutation will have

the full spectrum of disease,from a mild clinical

presentation to a serious,medically significant one,illustrate the intrafamilial

variability.

The alternate NF form ofspinal NF and the related

NF form of NFNS areexamples of conditions inwhich unique mutation ofthe NF1 gene have beendescribed in families withthe respective phenotypes.

10 AMERICAN JOURNAL OF MEDICAL GENETICS (SEMIN. MED. GENET.) ARTICLE

mechanism. Stevenson et al. [1999]found striking male predominance inpatients with NF1 who have pseudoar-throsis of the tibia. Genes involved inmale sex-limited traits might play a rolein this osseous dysplasia. There is also amale predominance in children withNF1 who have myelodysplasia syn-drome [Miles et al., 1996]. In addition,optic gliomas seem to occur with muchless frequency in NF1 patients of Afri-can-American descent than in other pa-tients with NF1 [Saal et al., 1995;Pletcher et al., 1996].

Regarding environmental factors,Riccardi has suggested that mechanicaltrauma may play a role in the develop-ment of dermal neurofibromas [Ric-cardi, 1993]. Other environmental fac-tors, such as use of oral contraceptivesand the state of pregnancy, have beenproposed as determinants in develop-ment of manifestations, but the evi-dence is lacking [Dugoff and Sujansky,1996]. Riccardi [1993] and others havealso proposed that stochastic factorsmodify expressivity. Certainly, any cli-nician who has looked at the apparentlyrandom distribution of cafe-au-laitspots and dermal neurofibromas in pa-tients with NF1 will think of chanceand randomness.

The idea that contiguous genes atthe NF1 locus could modify pheno-typic expression emerged as a potentialexplanation of variable expressivity inthe report of Kayes et al. [1992]. Thispatient had a large de novo deletion ofthe NF1 gene, mental retardation, andminor anomalies. Later, these research-ers described the cases of four additionalpatients with a deletion of more than700 kb [Kayes et al., 1994]. The dele-tions spanned the NF1 gene and in-volved DNA material both telomericand centromeric of the gene. This phe-notype has been described in severalother patients, and the notion that thereis a discrete phenotype due to deletionof the entire NF1 gene, includingflanking DNA, has been established[Wu et al., 1995; Wu et al., 1997b;Cnossen et al., 1997; Leppig et al.,1997; Tonsgard et al., 1997; Valero etal., 1997; Upadhyaya et al., 1998]. Thissyndrome is characterized by early on-set of many cutaneous neurofibromas,

minor anomalies, and learning or devel-opmental disabilities beyond the usualdegree seen in classic NF1. Kayes et al.[1994] and Wu et al. [1995, 1997b]published facial photographs. The cra-niofacial anomalies in the infants andyounger children are not as striking orcharacteristic as in the older individuals.Affected patients have a prominentforehead; hypertelorism; a broad, thickprominent nasal tip; and a “coarse” fa-cial appearance with thick lips. Kaplanand Rosenblot [1985] described thecases of two families with what theyconsidered a “distinctive facial appear-ance.” The first family in this report wasrestudied and found to have a deletionof the NF1 gene and flanking contigu-ous sequences. In retrospect, this familywas the first report of the whole-gene-deletion phenotype. The particular mi-crodeletion can be detected by fluores-cence in situ hybridization (FISH),

which could provide a simple and sen-sitive approach to the molecular diag-nosis of large NF1 deletions [Leppig etal., 1996].

Recent articles on this topic sug-gest that this discrete phenotype repre-sents the only clear-cut example of ge-notype/phenotype correlation. In aroutine screening protocol, Rasmussenet al. [1998] detected large NF1 dele-tions in five of 67 NF1 patients. Theauthors make the point that their caseswere not preselected for any anomaliesand did not appear to have the distinc-tive phenotype described earlier in in-dividuals with the complete NF1 dele-tion. However, two of their patientshad somatic mosaicism of the large genedeletion, which may have accountedfor the differences in phenotype. Thecases of these patients, and the cases thatthese researchers cite from the litera-

ture, indicate that 4–7% of NF1 patientshave large NF1 deletions spanning mostof the gene.. Tonsgard et al. [1997]found that four of 35 available patientswith NF1 who had some of the featuresof this phenotype had the whole genedeletion, as detected by FISH analysis.This suggests that at the mild end of thespectrum, this phenotype may not beeasy to pick out in a series of patientswith NF1. In its more typical manifes-tation, as illustrated by the patients ofLeppig et al. [1996] and Wu et al.[1997b], the phenotype can be recog-nized easily.

Two other patients with NF1 havebeen found to have microscopically de-tectable 17q11.2 deletions by standardcytogenetics [Riva et al., 1996; Up-adhyaya et al., 1996]. Both had devel-opmental disabilities beyond what is ex-pected in NF1. The patient of Upadhy-aya et al. had craniofacial characteristicseven more distinctive than those of theyoung children with the whole-genedeletion. The cases of these patients,like the others, suggest that contiguousgenes would modify the typical NF1phenotype and explain some cases withmore serious manifestations. Analysisusing FISH could be performed on pa-tients with these signs in the clinical set-ting as a testing procedure to detect amicrodeletion. Further work will be re-quired to determine if the diagnosis of awhole-gene deletion in patients withNF1 affects treatment. Given that lossof contiguous genes likely plays a rolein this more distinctive phenotype, ef-forts to identify adjacent genes seemswarranted. To date, only the three NF1embedded genes are known to be de-leted in whole-gene deletion patients.Other candidate genes have not beenidentified at or near the NF1 locus.

MUTATIONThe known NF1 mutations were re-viewed by Korf [1998] and Upadhyayaand Cooper [1998]. Several mutationtypes have been described in these re-views, but no correlation with pheno-type was documented. Most of the fullycharacterized NF1 mutations are eithernonsense or frameshift mutations,which presumably lead to a prematuretruncation of neurofibromin. Large de-

Given that loss ofcontiguous genes likely

plays a role in this moredistinctive phenotype,

efforts to identify adjacentgenes seems warranted.

ARTICLE AMERICAN JOURNAL OF MEDICAL GENETICS (SEMIN. MED. GENET.) 11

letions of the NF1 gene, which accountfor about 4–7% of cases, are predomi-nantly of maternal origin (80%). Singlebase pair substitutions are mostly of pa-ternal origin [Upadhyaya et al., 1998].

NF1 mutations have been detectedin two patients with phenotypes notrepresentative of NF1. Legius et al.[1995] reported on a patient with en-cephalocranio-cutaneous lipomatosis(ECCL) who had a de novo nonsensemutation in exon 29 of the NF1 gene.The patient has cafe-au-lait spots, buthe did not fulfill the National Institutesof Health criteria of NF1, and he hadmany signs of ECCL. Another patientof Legius et al. had multiple lentiginesand a de novo mutation of the NF1gene [Wu et al., 1996]. These cases mayrepresent other examples of allelic het-erogeneity and again bring up the issueof modifying genes in clinical expres-sivity.

SUMMARY

Advances in the understanding of themolecular biology of the NF1 gene andits encoded polypeptide are providinginitial insights into phenotypic variabil-ity. Some of the previously describeddiscrete phenotypes, that is, alternateand related forms of NF1, have poten-tial explanations at the molecular level.Mosaicism has been confirmed in NF1and may explain segmental NF. Con-tiguous genes surrounding the NF1gene and random somatic inactivationof the normal NF1 allele have been of-fered as explanations for phenotypicvariability. Intrafamilial variability maybe due to modifying genes at other loci,but it could be explained by somaticmosaicism or a second hit. While thewhole-gene-deletion phenotype is usu-ally thought to be the only example ofgenotype/phenotype correlation, thereare now other proposed genetic expla-nations for some NF1-related pheno-types, spinal NF, autosomal dominantcafe-au-lait spots, and NFNS. Futurework in the areas of the specific con-tiguous genes and the molecular fea-tures of patients with NFNS and furtherstudy of patients with segmental NF willexpand our knowledge of these topics.

ACKNOWLEDGMENTS

The authors thank the many familieswith NF1, whose questions havehelped stimulate our work and ourthinking. The authors also express theirappreciation to David Brock for clericaland administrative assistance.

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