Transient infantile osteopetrosis

Transient Generalized Osteosclerosis in a Newborn Mimicking Congenital Osteopetrosis with Negative Comprehensive Genetic Workup: A Case Report

We present a newborn with transient generalized osteosclerosis and negative genetic workup. The etiology of this condition is unknown. Given overlapping radiologic signs with severe forms of osteopetrosis, familiarity with this condition is crucial for correct diagnosis and management.

Chondrocytic ephrin B2 promotes cartilage destruction by osteoclasts in endochondral ossification

The majority of the skeleton arises by endochondral ossification, whereby cartilaginous templates expand and are resorbed by osteoclasts then replaced by osteoblastic bone formation. Ephrin B2 is a receptor tyrosine kinase expressed by osteoblasts and growth plate chondrocytes that promotes osteoblast differentiation and inhibits osteoclast formation. We investigated the role of ephrin B2 in endochondral ossification using Osx1Cre-targeted gene deletion. Neonatal Osx1Cre.Efnb2(Δ/Δ) mice exhibited a transient osteopetrosis demonstrated by increased trabecular bone volume with a high content of growth plate cartilage remnants and increased cortical thickness, but normal osteoclast numbers within the primary spongiosa. Osteoclasts at the growth plate had an abnormal morphology and expressed low levels of tartrate-resistant acid phosphatase; this was not observed in more mature bone. Electron microscopy revealed a lack of sealing zones and poor attachment of Osx1Cre.Efnb2(Δ/Δ) osteoclasts to growth plate cartilage. Osteoblasts at the growth plate were also poorly attached and impaired in their ability to deposit osteoid. By 6 months of age, trabecular bone mass, osteoclast morphology and osteoid deposition by Osx1Cre.Efnb2(Δ/Δ) osteoblasts were normal. Cultured chondrocytes from Osx1Cre.Efnb2(Δ/Δ) neonates showed impaired support of osteoclastogenesis but no significant change in Rankl (Tnfsf11) levels, whereas Adamts4 levels were significantly reduced. A population of ADAMTS4(+) early hypertrophic chondrocytes seen in controls was absent from Osx1Cre.Efnb2(Δ/Δ) neonates. This suggests that Osx1Cre-expressing cells, including hypertrophic chondrocytes, are dependent on ephrin B2 for their production of cartilage-degrading enzymes, including ADAMTS4, and this might be required for attachment of osteoclasts and osteoblasts to the cartilage surface during endochondral ossification.

Radiologic resolution of malignant infantile osteopetrosis skeletal changes following hematopoietic stem cell transplantation

INTRODUCTION

Hematopoietic stem cell transplantation (HSCT) is the only known curative treatment of malignant infantile osteopetrosis (MIOP). In this study, short-term serial bone surveys were used to assess radiologic evolution of skeletal changes after HSCT in MIOP.

MATERIALS AND METHODS

Baseline whole-body bone survey was performed in all patients. HSCT was successful in 14 patients (11 with full chimerism, three with mixed chimerism) in whom follow-up bone surveys were carried out at 6 and 12 months after HSCT.

RESULTS

Normal corticomedullary differentiation was evident in five (P = 0.06) and 12 (P < 0.005) patients at 6 and 12 months, respectively. Abnormal endobone appearance in long bones, present in 11 participants at baseline exam, disappeared in eight (P = 0.008) and all (P = 0.001) patients at 6 and 12 months, respectively. In 6-month follow-up, rachitic changes significantly disappeared (P < 0.01) in long bones; however, they were evident in ribs of 12 patients (P = 0.50). No patient had rickets in ribs or long bones after 12 months.

CONCLUSION

We observed considerable resolution of MIOP skeletal changes after HSCT in all patients with either full or mixed chimerism. Rachitic changes in long bones, attenuated corticomedullary differentiation, and endobone appearance were the first to resolve. We propose using single long bone plain x-ray to demonstrate short-term skeletal response to HSCT.

Genetics, pathogenesis and complications of osteopetrosis

Human osteopetrosis is a rare genetic disorder caused by osteoclast failure, which ranges widely in severity. In the most severe forms, deficient bone resorption prevents enlargement of bone cavities, impairing development of bone marrow, leading to hematological failure. Closure of bone foramina causes cranial nerve compression with visual and hearing deterioration. Patients also present with osteosclerosis, short stature, malformations and brittle bones. This form is fatal in infancy, has an autosomal recessive inheritance and is cured with hematopoietic stem cell transplantation, with a rate of success <50% and unsatisfactory rescue of growth and visual deterioration. It relies on loss-of-function mutations of various genes, including the TCIRG1 gene, encoding for the a3 subunit of the H+ATPase and accounting for >50% of cases, the ClCN7 and the OSTM1 genes, which have closely related function and account for approximately 10% of cases, also presenting with neurodegeneration. Further genes are implicated in rare forms with various severities and association with other syndromes and, recently, the RANKL gene has been found to be mutated in a subset of patients lacking osteoclasts. Autosomal recessive osteopetrosis may also have intermediate severity, with a small number of cases due to loss-of-function mutations of the CAII or the PLEKHM1 genes. Dominant negative mutations of the ClCN7 gene cause the so-called Albers-Schönberg disease, which represents the most frequent and heterogeneous form of osteopetrosis, ranging from asymptomatic to intermediate/severe, thus suggesting additional genetic/environmental determinants affecting penetrance. Importantly, recent work has demonstrated that osteoblasts may also contribute to the pathogenesis of the disease, either because they are affected by intrinsic defects, or because their activity may be enhanced by deregulated osteoclasts abundantly present in most forms. Therapy is presently unsatisfactory and effort is necessary to unravel the gene defects yet unrecognized and identify new treatments to improve symptoms and save life.

A clinical and molecular overview of the human osteopetroses

The osteopetroses are a heterogeneous group of bone remodeling disorders characterized by an increase in bone density due to a defect in osteoclastic bone resorption. In humans, several types can be distinguished and a classification has been made based on their mode of inheritance, age of onset, severity, and associated clinical symptoms. The best-known forms of osteopetrosis are the malignant and intermediate autosomal recessive forms and the milder autosomal dominant subtypes. In addition to these forms, a restricted number of cases have been reported in which additional clinical features unrelated to the increased bone mass occur. During the last years, molecular genetic studies have resulted in the identification of several disease-causing gene mutations. Thus far, all genes associated with a human osteopetrosis encode proteins that participate in the functioning of the differentiated osteoclast. This contributed substantially to the understanding of osteoclast functioning and the pathogenesis of the human osteopetroses and will provide deeper insights into the molecular pathways involved in other bone pathologies, including osteoporosis.

Localization of the gene causing the osteopetrotic phenotype in the incisors absent (ia) rat on chromosome 10q32.1

The incisors absent rat is an osteopetrotic animal model. Segregation analysis in 37 affected animals from an outcross enabled us to assign the disease causing gene to a 4.7-cM interval on rat chromosome 10q32.1. Further analysis of the genes mapped in this region will provide more insight into the underlying pathogenesis.

INTRODUCTION

Many of the insights into the factors that regulate the differentiation and activation of osteoclasts are gained from different spontaneous and genetically induced osteopetrotic animal models. The osteopetrotic incisors absent (ia) rat exhibits a generalized skeletal sclerosis and a delay of tooth eruption. Although the ia rat has well been studied phenotypically, the genetic defect still remains unknown.

MATERIAL AND METHODS

To map the ia locus, we outcrossed the inbred ia strain with the inbred strain Brown Norway. Intercrossing F1 animals produced the F2 generation. Thirty-one mutant F2 animals and six mutant F4 animals were available for segregation analysis.

RESULTS

Segregation analysis enabled us to assign the disease causing gene to rat chromosome 10q32.1. Homozygosity for the ia allele was obtained for two of the markers analyzed (D10Rat18 and D10Rat84). Key recombinations delineate a candidate region of 4.7 cM flanked by the markers D10Rat99 and D10Rat17.

CONCLUSION

We have delineated a 4.7-cM region on rat chromosome 10q32.1 in which the gene responsible for the osteopetrotic phenotype of the ia rat is located. Although the sequence of this chromosomal region is not complete, over 140 known or putative genes have already been assigned to this region. Among these, several candidate genes with a putative role in osteoclast functioning can be identified. However, at this point, it cannot be excluded that one of the genes with a currently unknown function is involved in the pathogenesis of the ia rat. Further analysis of the genes mapped in this region will provide us more insight into the pathogenesis of this osteopetrotic animal model.

Apparent cure of a newborn with malignant osteopetrosis using prednisone therapy

A newborn girl with hemorrhagic purpura, suspected neonatal sepsis, and pale and dry skin was lethargic with remarkable hepatosplenomegaly, convergent strabismus, severe anemia, and elevated alkaline phosphatase activity. Radiographs showed a generalized increase in bone density, small medullary cavities, sclerosis of the skull and vertebrae, transverse wavy stripes of sclerotic bone in the metaphyses, and bone-in-bone appearance in phalanges of hands and feet. On this basis, she was diagnosed with malignant infantile osteopetrosis. On the first day of life, the infant was given a blood transfusion and vitamin K (1 mg intravenously [iv]). Corticosteroid therapy was started with prednisone (2 mg/kg per day). She showed marked improvement of symptoms. On the 26th day and 42nd day of life, she received additional blood transfusions. On the 49th day, the patient was discharged and corticosteroid therapy was continued at a regimen of 5 mg/day. Subsequent blood sample analyses revealed normal values for age. At 1 year of life, a bone marrow sample showed normal white and red cell lineages. X-ray confirmed attenuation of the bone sclerosis; therefore, bone marrow transplantation (BMT) was not implemented. At the age of 1.5 years, prednisone therapy was discontinued gradually and withdrawn before the age of 2 years. Subsequent follow-up showed normalization of all radiological and hematologic parameters. At present, the patient is 3 years old and appears healthy with apparently complete regression of the disease.

Osteopetrosis--improvement of hematologic findings with age

A three months old girl was admitted to the hospital for evaluation of pallor and abdominal distension. There was anemia, thrombocytopenia and extramedullary hematopoiesis. In the long bone roentgenogram medullary areas could not be distinguished and bone densities were increased. The patient was diagnosed for osteopetrosis with these findings and prednisolone treatment was begun and then discontinued since it was not effective. In the follow up, the need for blood transfusions decreased and then disappeared. We present a case which was diagnosed as osteopetrosis and followed up by blood transfusions and of which hematologic findings improved with age.

Osteogenesis imperfecta and other heritable disorders of bone

This chapter summarizes the many recent advances in our understanding of the principal heritable disorders of bone. In the course of little more than a decade many diseases that were recognizable only by their clinical and radiological features have become explicable in molecular terms. Large numbers of mutations of the genes coding for collagen, for alkaline phosphatase, for the cell surface receptors for parathyroid hormone and for calcium, and for a number of other proteins, are recognized. The chapter covers the many variants of osteogenesis imperfecta, the most common heritable cause of fractures. It also covers osteopetrosis, hypophosphatasia, pseudohypoparathyroidism (with Albright's hereditary osteodystrophy), familial benign hypercalcaemia, autosomal dominant hypocalcaemia and the molecular causes of some chondrodysplasias.

Retroviruses and bone diseases

In 1980, retroviruses were shown to be pathogenic to humans, and experimentation on animals involving retroviruses as causal agents of tumors and degenerative diseases of bone, brain, and lung gained interest. Osteopetrosis, which can be either inherited in rodents or retrovirally induced in cats, is exemplary. Because of replication cycle, retroviruses can be propagated not only as infectious agents but also as cellular genes. If a retroviral infection occurs in germ line cells, the viral genes, which must integrate in the host's DNA, can be passed on to the progeny and inherited as Mendelian characteristics. Therefore, a retroviral etiology could account for diseases that present either as sporadic (infectious) or familial (inherited), although they may be similar in their clinical manifestations. This approach led to the finding of 2 new human retroviruses: 1 in a patient who had sporadic benign osteopetrosis, and the other in a patient who had sporadic paraarticular osteoma. In both patients, the retrovirus was isolated from mononuclear blood cells, not from bone cells, because of the links between bone and the immune system. A systematic search for retroviruses in patients who have sporadic bone disease, which also may appear as inherited disease, has yet to be performed. Patients with sporadic disease could be managed by antiretroviral agents such as Zidovudin.

Bone metabolism in the osteopetrotic rat mutation microphthalmia blanc

We have examined parameters of bone metabolism in a new mutation, microphthalmia blanc (mib), in the rat exhibiting a skeletal sclerosis at birth that improves with age. There were no significant differences in the rate of bone formation during the first postnatal month except a temporary reduction in mutants at 3 weeks that coincided with compromised nutrition at weaning. At birth the ruffled border in mutant osteoclasts was absent or poorly developed and mRNA analyses of mutant bone compared to normal bone showed significant reductions in the messages for the osteoclast-specific genes carbonic andydrase II and tartrate-resistant ATPase. These distinctive ultrastructural and molecular differences were not present 1 month later. These data show that the transient osteopetrosis in mib rats results from a perinatal reduction in ultrastructural and enzymatic features of active osteoclasts and is not complicated by elevations in bone formation. The molecular basis for both the production and resolution of these abnormalities deserves further study.

Bone marrow transplantation for autosomal recessive osteopetrosis. A report from the Working Party on Inborn Errors of the European Bone Marrow Transplantation Group

The outcomes of 69 patients who received allogeneic bone marrow grafts for autosomal recessive osteopetrosis in the period between 1976 and 1994 were analyzed retrospectively. Four patients received bone marrow transplants (BMT) without prior myeloablative conditioning; transient osteoclast function was demonstrated in one of them. Sixty-five patients received myeloablative pretreatment. Recipients of a genotypically human leukocyte antigen (HLA)-identical BMT had an actuarial probability for 5-year survival, with osteoclast function, of 79%; recipients of a phenotypically HLA-identical bone marrow graft from a related or unrelated donor, or one HLA-mismatched graft from a related donor, had an actuarial probability for 5-year survival, with osteoclast function, of 38%; patients who received a graft from an HLA-haplotype mismatched related donor had a probability for 5-year survival of only 13%. The main problems in haplotype-nonidentical BMT were graft failure and BMT-related complications such as sepsis, bleeding, and interstitial pneumonia. Osteoclast function developed in all patients with full engraftment. Recovery of osteoclast function was associated with severe hypercalcemia in 24% of the patients with engraftment, especially those older than 2 years of age. At the time of BMT, severe visual impairment was present in 35% of the patients; of the 15 patients who had visual impairment at the time that a successful BMT was performed, two had improvement after BMT (13%). Within the total group, one patient had neurodegeneration. Engraftment of healthy donor cells had no influence on the progression of that abnormality and BMT thus had no beneficial effect on this phenotype of osteopetrosis. In general, however, early BMT remains the only curative treatment for autosomal recessive osteopetrosis.

Neonatal reductions in osteoclast number and function account for the transient nature of osteopetrosis in the rat mutation microphthalmia blanc (mib)

We have examined the general and skeletal manifestations of osteopetrosis in a new, mild osteopetrotic mutation in the rat, microphalmia blanc (mib). Newborn mutant (mib) rats exhibit the typical skeletal deformities and sclerosis of osteopetrosis at birth, which are reduced significantly during the first postnatal month but don't disappear entirely up to 8 months later. Osteoclast numbers, staining for TRAP and TraATPase, and bone resorption are reduced in mutants during the first 2 postnatal weeks but improve by 1 month. In mutants, serum concentrations of calcium and phosphorus are normal, but 1,25(OH)2 D levels are higher at 1 week than those in normal littermates. Neonatally, mutants exhibit extramedullary hemopoiesis in the spleen. These results are interpreted to mean that the transient perinatal skeletal sclerosis in mib rats is caused by reduced production and function of osteoclasts in this period. The recent description of transient, perinatal osteopetrosis in a child suggests that analyses of the early differences between mild and severe animal mutations might distinguish those children with osteopetrosis who need treatment from those who do not.