Bone Mineral Density and Laboratory Evaluation ofa Type II Autosomal DominantOsteopetrosis Carrier

Istvan Takacs,1 Heather Cooper,2 David D. Weaver,2 and Michael J. Econs1,2*1Department of Medicine, Indiana University School of Medicine, Indianapolis, Indiana2Department of Medical and Molecular Genetics, Indiana University School of Medicine, Indianapolis, Indiana

Type II autosomal dominant osteopetrosis(ADO2) is an inherited disorder character-ized by increased skeletal mass and charac-teristic abnormalities evident on radiogra-phy. Although previous investigators havedescribed nonpenetrant individuals (carri-ers), it is not known whether carriers mani-fest subtle abnormalities. We hypothesizedthat ADO2 carriers would have an abnor-mality of osteoclast function that wouldlead to changes in bone mineral density(BMD), in serum tartrate-resistant acidphosphatase (TRAP), or in creatine kinaseisoenzyme BB (CK-BB) levels that wouldpermit carrier recognition. We identified afemale carrier in a well-established ADO2family and measured BMD, serum TRAP,and CK-BB concentrations. She had normalBMD, serum TRAP, and CK-BB concentra-tions. Thus, these measurements cannot beused to exclude carrier status in individualswho are seen for genetic counseling. How-ever, measurements in other asymptoticcarriers are necessary before concludingthat these measurements are normal in allor most nonpenetrant individuals. Am. J.Med. Genet. 85:9–12, 1999.© 1999 Wiley-Liss, Inc.

KEY WORDS: autosomal dominant osteope-trosis type II (ADO2); pen-etrance; carrier; creatine ki-nase isoenzyme BB (CK-BB);

tartrate-resistant acid phos-phatase (TRAP); bone min-eral density (BMD)

INTRODUCTION

Type II autosomal dominant osteopetrosis (ADO2) isan inherited disorder with an incidence of 1:20,000 anda penetrance of approximately 75% [Johnston et al.,1968; Bollerslev, 1987]. It is characterized on radiogra-phy by osteosclerosis and a concomitant high bone min-eral density [Grodum et al., 1995]. Patients with ADO2have end-plate thickening of the vertebrae (Rugger-Jersey spine) and end-bone thickening in the pelvisand long bones. In addition to the radiographic hall-marks of the disease, ADO2 patients frequently haveincreased concentrations of serum tartrate-resistantacid phosphatase (TRAP) and creatine kinase isoen-zyme BB (CK-BB) [Johnston et al., 1968; Bollerslev,1995; Felix et al., 1996; Whyte et al., 1996]. Althoughthe disorder may be asymptomatic and is often referredto as the ”benign” form of osteopetrosis, to distinguishit from the “malignant” recessive form, which is usuallylethal without treatment, ADO2 patients may sufferfrom many complications of their disease. These in-clude nerve entrapment syndromes and anemia withextramedullary hematopoiesis, osteomyelitis, and frac-ture [Johnston et al., 1968; Bollerslev and Mosekilde,1993; Bollerslev and Andersen 1989]. Although the dis-order results in increased bone mineral density (BMD),the bone is of poor quality and prone to fracture. In-deed, these fractures frequently result in deformity.Additionally, hip fractures, which often occur duringyoung adulthood, may lead to loss of mobility.

Although previous investigators [Johnston et al.,1968; Bollerslev, 1987] have documented incompletepenetrance of the disease (i.e., asymptotic ADO2 carri-ers exist), it is unknown whether carriers manifestsubtle abnormalities. Since ADO2 is an osteoscleroticdisorder that results from abnormal osteoclast activity[Felix et al., 1996], we hypothesized that ADO2 carri-ers would have a subtle abnormality of osteoclast func-tion that would lead to changes in BMD, TRAP, or CK-BB levels. Detection of these changes would permit car-

Presented in part at the joint meeting of the American Societyfor Bone and Mineral Research and the International Bone andMineral Society, December 1–6, 1998.

Contract grant sponsor: Charles E. Culpeper Foundation; Con-tract grant sponsor: NIH; Contract grant numbers: AR42228,AG05793, MO1RR00750.

*Correspondence to: Michael J. Econs, M.D., Indiana Univer-sity School of Medicine, 975 W. Walnut Street, IB 445, Indianapo-lis, IN 46202. E-mail: MEcons@iupui.edu

Received 22 September 1998; Accepted 19 February 1999

American Journal of Medical Genetics 85:9–12 (1999)

© 1999 Wiley-Liss, Inc.

rier testing. Identification of carriers would not onlyfacilitate positional cloning studies that are designed toidentify the disease gene but would also markedly fa-cilitate genetic counseling of asymptomatic children ofaffected individuals, who may be carriers for the dis-ease and are at risk of having affected children. Wetherefore measured BMD, TRAP, and CK-BB in a well-documented carrier from an ADO2 family.

MATERIALS AND METHODS

Serum calcium, phosphorus, alkaline phosphatase,and creatinine concentrations were measured by stan-dard automated techniques (Cobas Mira, Roche Diag-

nostic System, Branchburg, NJ). TRAP was measuredas originally described by Lau et al. [1987]. The totalCK activity in serum was measured at 37°C on a Vitros750 XRC analyzer (Johnson and Johnson Diagnostic,Rochester, NY). CK activity was expressed as interna-tional units per liter (IU/L), with a reference limit inadults of 180 IU/L based on the manufacturer’s recom-mendation. CK-BB was quantitated after electropho-retic separation on agarose gels (REP CK-16 Isoen-zyme KIT, Helena Laboratories, Beaumont, TX) andincubation with enzyme substrate. The isoenzymebands were visualized by photographing the gel underultraviolet light (340 nm).

Fig. 1. Pedigree and radiographs from the four individuals reported. Roman numerals denote generations, squares represent male family members,and circles indicate female family members. Solid symbols indicate affected individuals, and the open symbol denotes the obligate carrier. Radiographsof the pelvis of individual 1 demonstrate end-bone thickening and multiple fractures. Radiographs of the spine and femur from individual 2 show normalresults. Plain films of the spine and femur for two of her sons are also shown. They demonstrate the classic Rugger-Jersey spine, which is typical of ADO2.

10 Takacs et al.

BMDs of the total body, lumbar spine, and right fe-mur were measured by dual-energy X-ray absorptiom-etry (DEXA) utilizing either a Lunar DPXL (case 1) orDPXIQ (case 2) densitometer (Lunar Corp., Madison,WI). The results are expressed in grams per squarecentimeter and Z scores, which compare the BMD mea-surements to an age- and gender-matched normalpopulation. The study was approved by the IndianaUniversity Medical Center Institutional Review Board,and all subjects or their parents gave written informedconsent before participating in research studies.

CLINICAL HISTORIESIndividual 1

The grandmother is a 57-year-old Caucasian woman,a member of a previously reported family; her motherand sister have ADO2 [Johnston et al., 1968]. She wasdiagnosed with ADO2 at age 10 during a family screen-ing. She suffered her first fracture at age 12. Since thenshe has had multiple hip (Fig. 1), humerus, metatarsal,and tibial fractures. She has been wheelchair boundsince age 35. In addition to the fractures, her coursehas been complicated by osteomyelitis of the mandible,which has been refractory to treatment. Her bone den-sity is markedly elevated (Table I).

Individual 2

The mother is a 34-year-old woman who has nosymptoms of bone disease and is phenotypically nor-mal. There is no history of any major medical disorder.A metabolic bone radiographic survey failed to detectany abnormalities. Representative radiographs fromthe lumbar spine and femur are shown in Figure 1. HerBMD results are in the normal range (Table I). Serumcalcium, phosphorus, alkaline phosphatase, and creat-inine concentrations were all within normal limits. Se-rum TRAP was 7.4 U/L (normal range 3.3–10 U/L).Total CK activity was 119 U/L, and serum CK-BB wasundetectable (normal <2%). She has three children, oneof whom is unaffected (asymptomatic with normalmetabolic bone radiographs). The cases of the other twochildren are presented here.

Individuals 3 and 4

Individual 3 was diagnosed at age 3 and individual 4at age 5 after radiographs were obtained during a fam-ily screening. Both children are asymptomatic and

have never had a fracture. Radiographs from both boysdemonstrate classic signs of ADO2 (Fig. 1).

DISCUSSION

ADO2 is an osteosclerotic disorder that results fromdefective osteoclast-mediated bone resorption; how-ever, the number of osteoclasts is increased [Felix etal., 1996]. We hypothesized that individual 2, who is anasymptomatic “carrier,” would have a subtle abnormal-ity of osteoclast function that would lead to an increasein bone mineral density, which would be detectable byDEXA scanning. Additionally, it was possible that thisindividual might have a subtle increase in osteoclastnumber, which could result in an increased serumTRAP concentration [Ballanti et al., 1997]. Finally, wemeasured the serum CK-BB concentration, since thisenzyme has been reported to be present in the serum ofADO2 patients [Yoneyama et al., 1989; Whyte et al,1996].

Our results in a carrier from a well-established fam-ily with ADO2 indicate that this individual has normalBMD and serum TRAP and CK-BB concentrations.Thus, these measurements cannot be used to excludecarrier status in individuals who are seen for geneticcounseling. However, measurements in other asymp-tomatic carriers are necessary before concluding thatthese measurements are normal in all or most carriers.

Previous investigators have described four obligatecarriers [Johnston et al., 1968; Bollerslev, 1987]. Theseindividuals had normal radiographs [Johnston et al.,1968; Bollerslev, 1987] and bone biopsies [Johnston etal., 1968]. The mechanism by which one individual be-comes a carrier while another member of the same fam-ily with the same mutation has severe disease is cur-rently unknown. It is possible that environmental fac-tors may influence development of the disease.Alternatively, other interacting genes may compensatefor mutations in the ADO2 gene. Theoretically, im-printing could result in nonpenetrance. However, thisdoes not explain the findings in our kindred, since in-dividuals 1, 2, 3, and 4 all inherited the mutant genefrom their mothers. In any event, a better understand-ing of the mechanism of nonpenetrance in ADO2 maylead to new therapies for the disease and may providea better understanding of osteoclast regulation and/orfunction.

In summary, we sought to identify subtle, but clini-cally detectable abnormalities in an obligate ADO2 car-rier, to facilitate positional cloning studies and geneticcounseling of asymptomatic offspring of ADO2 pa-tients. Our results indicate that BMD, TRAP, and CK-BB measurements do not exclude the carrier statusand that advances in genetic counseling of these indi-viduals may be dependent upon efforts to identify thedisease gene.

REFERENCES

Ballanti P, Minisola S, Pacitti MT, Scarnecchia L, Rosso R, Mazzuoli GF,Bonucci E. 1997. Tartrate-resistant acid phosphate activity as osteo-clastic marker: sensitivity of cytochemical assessment and serum assay

TABLE I. BMD Measurements in Cases 1 and 2*

Individual 1 Individual 2

Total body BMDg/cm2 1.778 1.111Z score +8.8 −0.2

Lumbar spine (L2–4)g/cm2 2.383 1.156Z score +10.9 −0.4

Femur neck (right)g/cm2 N.M. 0.975Z score N.M. 0.0

*BMD, bone mineral density; N.M., not measurable.

Normal BMD in an ADO2 Carrier 11

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