—  SPECIALTY CONFERENCE  —

Pediatric Pathology

Case 1 - Osteopetrosis

Raj P. Kapur
Children's Hospital and Medical Center
Seattle, WA


Click on each slide thumbnail image for an enlarged view
Clinical History
The patient was admitted to our institution at age 3 months with conjunctivitis, a bulging anterior fontanelle, hepatosplenomegaly, low-grade fever (99.8°C), and nasal congestion. A clinical and laboratory investigation, which included a complete blood count and chest radiograph, was initiated to exclude pneumonia and/or sepsis. His platelet count was 45,000/mm3, hematocrit was 31.1%, and white blood cell count was 19,800/mm3 (22% neturophils). The peripheral smear revealed many immature hematopoietic cells, including 6% blasts.

This infant represented the first pregnancy for a non-consanguinous couple. The mother received good prenatal care. He was delivered at term and weighed 6lbs, 13oz. At 2.5 months, while on vacation with his family, the baby developed a fever and upper respiratory symptoms and was seen in an emergency department, where a "sepsis work-up" was negative. He was sent home and his symptoms improved transiently, but then recurred. Ocular erythema and discharge appeared one day prior to admission.


Case 1 - Figure 1 - An iliac crest biopsy demonstrates relatively uniform persistence of primitive woven bone with intervening fibrous marrow. Ineffective remodeling of bone is evident from the paucity of laminated osteoid and foci of mineralized matrix that resemble calcified cartilage. The density of hematopoietic cells in the marrow is markedly reduced.
Case 1 - Figure 2 - At high magnification, numerous osteoclasts are easily identified along the surfaces of bony trabeculae. The surfaces of bone facing these multinucleate cells are flat, as opposed to the deep Howship lacunae that characterize normal osteoclast-mediated bone resorption.
Case 1 - Figure 3 - A low magnification image of the core biopsy demonstrates the diffuse uniform nature of the underlying defect in osteoid resorption, reduced hematopoiesis, and marrow fibrosis.

Differential Diagnosis
In this case, the clinical differential diagnosis included leukemia and lysosomal storage disorders. A bone marrow biopsy, performed to rule out leukemia, was being processed at the same time that the final interpretation of a chest radiograph was posted by the radiologist. The latter read:

"The bones are dense and demonstrate diffuse periosteal reaction.

Impression:
1. Dense bones with diffuse periosteal reaction. A metabolic bone disease, such as osteopetrosis, should be considered."
Additional laboratory studies demonstrated an elevated alkaline phosphatase (1,179 U/L; normal 95-380 U/L), low serum phosphate (3.6 mg/dL; normal = 3.9-6.5 mg/dL), and low serum calcium (8.2 mg/dL; normal = 8.7-10.7 mg/dL). A skeletal survey demonstrated findings consistent with osteopetrosis including generalized increase in bone density, diffuse periosteal reaction, and "metaphyseal widening with 'frayed' appearance and irregular lucency".

Pathological Diagnosis: Osteopetrosis

The histological features of osteopetrosis are evident in this biopsy [1, 2] . Most obvious is a disproportionate amount of osteoid and fibrous tissue with a marked paucity of hematopoietic marrow. The osteoid has an immature woven morphology and is organized into a reticular network, which varies little from the cortical surface toward the deeper aspect of the core. Small deposits of residual calcified cartilaginous matrix ("enchondral trabeculae") are present within the osteoid, but such collections are not as prominent as in many published examples of malignant osteopetrosis. These trabeculae resemble "primary trabeculae" that are found normally near the growth plates during enchondral ossification. They lack the lamellar appearance of mature remodelled bone. They are lined by osteoblasts and osteoclasts, with increased densities of osteoclasts in many areas. Cytologically, the osteoclasts are unremarkable, but associated Howship's lacunae are difficult to find and appear much shallower than expected. The portions of the biopsy that are not occupied by osteoid contain excessive fibrous tissue.

Discussion
Osteopetrosis is a defect in bone metabolism that results from deficient osteoclast activity [3]. It is a genetically heterogeneous disorder that is generally divided into three clinical subtypes based on age of presentation, severity, and mode of transmission (Table 1).

Table 1: Clinical Types of Osteopetrosis
  Malignant IntermediateDelayed-Onseta
Presentation Infancy Childhood Teen-to-adult
Common reasons forpresentation Visual disturbances
Failure-to-thrive
Recurrent infection
Seizures
Fractures
Anemia 		Fractures
Optic nerve atrophy
Anemia
Recurrent infections
Dental abscesses 		Fractures
Facial palsy
Anemia
Dental abscesses
Associated disordersb Renal tubular acidosis
Cerebral calcificiations and mental retardation 		   
Inheritance pattern Autosomal recessive Autosomal recessive Autosomal dominant
OMIM No.c 259700, 259730 259710 166600

aTwo forms of delayed onset osteopetrosis (OPTA) have been described. The table refers to OPTA type II. OPTA type I is more limited to the skull base, not associated with fractures, and associated with mutations in the low density lipoprotein receptor-related protein 5 gene.

bPresence of these findings strongly suggests mutations in specific genes, as discussed in text.

cEntry number in Online Mendelian Inheritance of Man (http://www3.ncbi.nlm.nih.gov/entrez/)

Autosomal recessive "malignant" osteopetrosis is most severe. Patients characteristically present as infants, usually within the first 3 months [4]. Parental concerns about their baby's vision is the most common mode of presentation, but other reasons include recurrent infections and failure to thrive, hypocalcemia and seizures, fractures, excessive bruising, nasal congestion, and occasionally dysmorphic facies [5]. Hepatosplenomegaly is invariably present.

Even though the presenting symptoms are non-specific, the diagnosis of malignant osteopetrosis can usually be established based on clinical and radiological criteria alone. Bone biopsy is not usually required. However, the condition is so rare (incidence = 1:200,000) that it is often not considered unless the characteristic sclerotic bone changes are noted in a radiograph, which was obtained for other reasons. When suspected, an appropriate work-up includes complete blood count, serum electrolytes, Ca2+, PO42-, and urea, blood gas studies, a thorough skeletal survey, ophthalmological and neurological evaluation, and tissue typing of child and family for possible bone marrow transplantation [5]. The natural history of malignant osteopetrosis is progressive bone marrow failure, recurrent infections, and death, usually by two years of age. Bone marrow transplantation is the only treatment that significantly alters the natural course of the disease for a subset of patients [4].

Most of the clinical and pathological features of malignant osteopetrosis can be attributed to defective or deficient osteoclasts. Osteoclast-mediated bone resorbtion is required for normal bone remodeling [6]. Osteoclasts remove calcified cartilage and woven bone in the immature skeleton, which permit deposition of mature lamellar bone and allow for adequate hematopoiesis in the intervening marrow. Despite the sclerotic appearance of the bones in osteopetrosis, they are brittle and vulnerable to pathological fractures. Anemia with or without granulocytopenia or thrombocytopenia is generally attributed to deficent marrow space or secondary changes in the marrow microenvironment, although the underlying molecular defects that cause osteopetrosis may directly affect hematopoietic cell lines, as well (see below). Leukocyte superoxide production is usually impaired in these patients, which compounds their risk for infection. Hepatosplenomegaly is due to compensatory hematopoiesis. As bone resorbtion is necessary for proportionate enlargement of skeletal ostia, visual disturbances (failure to achieve normal milestones, abnormal eye movements, squinting), nasolacrimal duct stenosis, and other neurological findings result from compression on the cranial nerves by narrow cranial foramina.

Much progress has been made towards understanding the molecular genetics and cell biology of osteopetrosis, in part because of murine models for the disorder [7]. All of the mutations associated known to be associated with this condition in humans or mice, affect gene products that regulate the genesis or function of osteoclasts [8]. The genetic defects are divided into two classes, based on whether they arrest osteoclastogenesis or osteoclast function (Table 2). The two classes are easily distinguished by bone marrow biopsy because osteoclasts are severely reduced in number or absent in osteoclastogenic defects, but are normal or increased in disorders of osteoclast function. Genetic disorders that produce each type of defect have been identified in mice. In contrast, to date, only defects in genes that affect osteoclast function, not osteoclastogenesis, have been identified in humans with malignant osteopetrosis.

Table 2: Genetic defects associated with osteopetrosis in mice or humans
  Gene Symbol Species Reference
Osteoclastogenesis:
(reduced or absent osteoclast number)		 mi Mouse [9]
c-fos Mouse [10]
PU.1 Mouse [11]
opgl (same as RankL) Mouse [12, 13]
rank Mouse [13, 14]
op Mouse [15]
   
Osteoclast Function:
(normal or increased osteoclast number)		 cathepsin-K Mousea [16]
traf6 Mouse [17]
c-src Mouse [18]
ATP6i (same as TCIRG1) Mouse and Human [19]
CA2 Mouse, Human and Others [20]
ClC7 Mouse and Human [21]
grey lethal (same as OSTM1) Mouse and Human [22]

aHumans with Cathepsin-K-gene mutations have pycnodysostosis, a recessive disorder characterized by short stature, abnormal craniofacial features, and osteosclerosis, which is clinically milder than malignant osteopetrosis.

At present, mutations in four different genes, ATP6i, carbonic anhydrase type II (CA2), ClC7, and Grey Lethal (GL) have been reported in patients with malignant osteopetrosis. Certain phenotypic features can help distinguish between these three genetic etiologies. Awareness of these findings may help guide mutational analysis, which is useful for genetic counseling and prenatal diagnosis.

ATP6i (also referred to as TCIRG1): Mutations in ATP6i account for approximately 50% of cases [23, 24, 25] . ATP6i encodes the osteoclast-specific a3 subunit of the vacuolar-type translocating ATPase (V-ATPase), which is localized to the ruffled border of the plasma membrane where bone resorbtion takes place. V-ATPase translocates protons into Howship's lacuna to counter the high buffering capacity of phosphates and solubilize hydroxyapetite crystals. Although some bone resorbtion is mediated by osteoclasts from patients with ATP6i mutations, it is much less efficient than normal [26]. Malignant osteopetrosis is associated with a wide variety of recessive mutations in the ATP6i gene.

CA2: Approximately 50 cases of malignant osteopetrosis and carbonic anhydrase type II deficiency have been described to date [26]. These patients also exhibit renal tubular acidosis and cerebral calcifications ("marbled brain") with mental retardation of variable severity [20]. Very rare patients have been described with renal tubular acidosis and malignant osteopetrosis, but no detectable CA2 mutation [27].

ClC7: ClC7 encodes a protein that is structurally homologous to chloride channels, but which lacks demonstrable channel activity when expressed in Xenopus oocytes [6]. The function of the ClC7 gene product is not established, but it may regulate one or more chloride channel needed to balance proton transport across the ruffled border. In any case, homozygous mutations of this gene are associated with malignant osteopetrosis and retinal degeneration, independent of optic nerve compression [21]. In addition, apparent dominant negative mutations that putatively reduce, but do not eliminate, ClC7 activity are responsible for delayed-onset osteopetrosis (see Table 1) [28].

GL (also referred to as OSTM1): Grey lethal is named after the murine homologue, which was identified in mice with a spontaneous mutation that conferred autosomal recessive osteopetrosis and a grey coat color [22]. The product of this gene is an intracellular transmembrane protein that is required for normal maturation of osteoclasts and melanocytes. At present, only one patient has been described with homozygous GL mutations [22]. He died at nine days of age with malignant osteopetrosis, hepatic fibrosis, and a poorly defined pattern of white matter pathology [29].

Bone marrow transplantation may be the only potentially effective therapy available for patients with malignant osteopetrosis. In humans, most of the mutations that have been described to date affect genes that are expressed by osteoclasts. Resulting defects in osteoclast-maturation or bone resorbtion are thought to be cell autonomous and potentially reversible if the defective osteoclasts are replaced. Unfortunately, some of the secondary effects of impaired bone resorbtion (e.g., cranial nerve compression) do not resolve even if the skeletal anomalies improve. Furthermore, non-osteoclast-mediated defects (e.g., renal tubular acidosis and cerebral deficits in CAII-deficiency) will not be corrected by bone marrow transplantation.

References

  1. Helfrich MH, Aronson DC, Everts V, Mieremet RHP, Gerritsen G, Eckhardt PG, et al. Morphologic features of bone in human osteopetrosis. Bone 1991;12:411-19.
  2. Shapiro F, Glimcher MJ, Holtrop ME, Tashjian AH, Brickley-Parsons D, Kenzora JE. Human osteopetrosis: a histological, ultrastructural, and biochemical study. J Bone Joint Surg 1980;62A:384-99.
  3. Tolar J, Teitelbaum SL, Orchard PJ. Osteopetrosis. New Engl J Med 2004;351:2839-49.
  4. Gerritsen G, Vossen JM, van Loo IHG, Hermans J, Helfrich MH, Griscelli C, et al. Autosomal recessive osteopetrosis: variability of findings at diagnosis and during the natural course. Pediatrics 1994;93:247-53.
  5. Wilson CJ, Vellodi A. Autosomal recessive osteopetrosis: diagnosis, management, and outcome. Arch Dis Child 2000;83:449-52.
  6. Blair HC, Athanasou NA. Recent advances in osteoclast biology and pathological bone resorbtion. Histol Histopathol 2004;19:189-99.
  7. Teitelbaum SL, Ross FP. Genetic regulation of osteoclast development and function. Nat Rev Genet 2003;4:638-49.
  8. Boyle WJ, Simonet WS, Lacey DL. Osteoclast differentiation and activation. Nature 2003;423:337-42.
  9. Packer SO. The eye and skeletal defects of two mutant alleles at the microphthalmia locus of Mus musculus. J Exp Zool 1957;165:21-45.
  10. Wang Z-Q, Ovitt C, Grigoriadis AE, Mohle-Steinlein U, Ruther U, Wagner EF. Bone and hematopoietic defects in mice lacking c-fos. Nature 1992;360:741-5.
  11. Tondravi M, McKercher SR, Anderson K, Erdmann JM, Quiroz M, Maki R, et al. Osteopetrosis in mice lacking haematopoietic transcription factor PU.1. Nature 1997;386:81-4.
  12. Kong Y-Y, Yoshida H, Sarosi I, Tan H-L, Timms E, Capparelli C, et al. OPGL is a key regulator of osteoclastogenesis, lymphocyte development, and lymph-node organogenesis. Nature 1999;397:315-23.
  13. Fata JE, Kong Y-Y, Li J, Sasaki T, Irie-Sasaki J, Moorehead RA, et al. The osteoclast differentiation factor osteoprotegrin-ligand is essential for mammary gland development. Cell 2000;103:41-50.
  14. Li J, Sarosi I, Yan X-Q, Morony S, Capparelli C, Tan H-L, et al. RANK is the intrinsic hematopoietic cell surface receptor that controls osteoclastogenesis and regulation of bone mass and calcium metabolism. Proc Natl Acad Sci USA 2000;97:1566-71.
  15. Naito M, Hayashi S-i, Yoshida H, Nishikawa S-i, Shultz LD, Takahashi K. Abnormal differentiation of tissue macrophage populations in 'osteopetrosis' (op) mice defective in the production of macrophage colony-stimulating factor. Am J Pathol 1991;139:657-67.
  16. Saftig P, Hunziker E, Wehmeyer O, Jones S, Boyde A, Rommerskirch W, et al. Impaired osteoclastic bone resorbtion leads to osteopetrosis in cathepsin-K-deficient mice. Proc Natl Acad Sci USA 1998;95:13453-8.
  17. Lomago MA, Yeh W-C, Sarosi I, Duncan GS, Furlonger C, Ho A, et al. TRAF6 deficiency results in osteopetrosis and defective interleukin-1, CD40, and LPS signaling. Genes Dev 1999;13:1015-24.
  18. Soriano P, Montgomery C, Geske R, Bradley A. Targeted disruption of the c-src proto-oncogene leads to osteopetrosis in mice. Cell 1991;64:693-702.
  19. Yi Y-H, Chen W, Liang Y, Li E, Stashenko P. Atp6i-deficient mice exhibit severe osteopetrosis due to loss of osteoclast-mediated extracellular acidification. Nat Genet 1999;23:447-51.
  20. Shah GN, Bonapace G, Hu PY, Strisciuglio P, Sly WS. Carbonic anhydrase II deficiency syndrome (osteopetrosis with renal tubular acidosis and brain calcification): novel mutations in CA2 identified by direct sequencing expand the opportunity for genotype-phenotype correlation. Hum Mutat 2004;Online #737.
  21. Kornak U, Kasper D, Bosl MR, Kaiser E, Schweizer M, Schulz A, et al. Loss of the ClC-7 chlorid channel leads to osteopetrosis in mice and man. Cell 2001;104:205-15.
  22. Chalhoub N, Behachenhou N, Rajapurohitam V, Pata M, Ferron M, Frattini A, et al. Grey-lethal mutation induces severe malignant autosomal recessive osteopetrosis in mouse and human. Nat Med 2003;9:399-406.
  23. Frattini A, Orchard PJ, Sobacchi C, Giliani S, Abinun M, Mattsson JP, et al. Defects in TCIRG1 subunit of the vacuolar proton pump are responsible for a subset of human autosomal recessive osteopetrosis. Nat Genet 2000;25:343-6.
  24. Kornak U, Schulz A, Friedrich W, Uhlhaas S, Kremens B, Voit T, et al. Mutations in the A3 subunit of the vacuolar H(+)-ATPase cause infantile malignant osteopetrosis. Hum Mol Genet 2000;9:2059-63.
  25. Sobacchi C, Frattini A, Orchard P, Porras O, Tezcan I, Andolina M, et al. The mutational spectrum of human malignant autosomal recessive osteopetrosis. Hum Mol Genet 2001;10:1767-73.
  26. Taranta A, Migliaccio S, Recchia I, Caniglia M, Luciani M, De Rossi G, et al. Genotype-phenotype relationship in human ATP6i-dependent autosomal recessive osteopetrosis. Am J Pathol 2003;162:57-68.
  27. Borthwick KJ, Kandemir N, Topaloglu R, Kornak U, Bakkaloglu A, Yordam N, et al. A phenocopy of CAII deficiency: a novel genetic explanation for inherited infantile osteopetrosis with distal renal tubular acidosis. J Med Genet 2003;40:115-21.
  28. Cleiren E, Benichou O, Van Hul E, Gram J, Bollersley J, Singer FR, et al. Albers-Schonberg disease (autosomal dominant osteopetrosis, type II) results from mutations in the ClCN7 chloride channel gene. Hum Mol Genet 2001;10:2861-7.
  29. Quarello P, Forni M, Barberis L, Defilippi C, Campagnoli F, Silverstro L, et al. Severe malignant osteopetrosis caused by a GL gene mutation. J Bone Miner Res 2004;19:1194-9.