Enhanced but hypofunctional osteoclastogenesis in an autosomal dominant osteopetrosis type II case carrying a c.1856C>T mutation in CLCN7

Abstract

Type II autosomal dominant osteopetrosis (ADO2), which is the most common form of osteopetrosis, is caused by heterozygous mutations in the chloride channel 7 (CLCN7) gene. The osteopetrosis of ADO2 has been attributed to hypofunctional osteoclasts. The mechanism underlying the abnormality in osteoclast function remains largely unknown. This study was designed to investigate gene mutations and osteoclast function in a case that was clinically diagnosed as ADO2. Genomic DNA was extracted from blood samples of this patient, and the 25 exons of CLCN7 were amplified. Peripheral blood from the ADO2 subject and a healthy age- and sex-matched control was used to evaluate osteoclastogenesis, osteoclast morphology, and bone resorption. Analysis of DNA from the patient showed a germline heterozygous missense mutation, c.1856C>T (p.P619L), in exon 20 of CLCN7. A similar homozygous mutation at this site was previously reported in a patient with autosomal recessive osteopetrosis. When cultured, the peripheral blood mononuclear cells (PBMCs) from the ADO2 patient spontaneously differentiated into mature osteoclasts in vitro. The ADO2 patient’s PBMCs formed enhanced, but heterogeneous, osteoclasts in both the presence and absence of macrophage-colony stimulating factor, and nuclear factor-ĸB ligand. Bone resorption was reduced in the ADO2 patient’s osteoclasts, which exhibited aberrant morphology and abnormal distribution of integrin a_vβ₃. Gene analysis found increased c-fos expression and reduced RhoA and integrin beta 3 expression in ADO2 cells. In conclusion, our data suggest that enhanced, heterogeneous osteoclast induction may be an intrinsic characteristic of ADO2.

Osteopetrosis: What makes bones too dense?

A mutation that reduces activity of bone-resorbing cells is responsible for a rare inherited disorder characterised by dense, brittle bones. Healthy bones undergo constant remodeling; osteoclasts resorb mature bone and osteoblasts generate new tissue. Patients with autosomal dominant osteopetrosis Type II (ADO2), the most common form of osteopetrosis, show impaired bone remodelling, resulting in overly dense, fragile bone tissue. No treatment is available, and although mutations in the chloride channel 7 gene (CLCN7) have been implicated, the mechanism underlying the disease remains unclear. Xijie Yu at Sichuan University and colleagues investigated gene mutations and osteoclast function in a recently diagnosed patient. The researchers found a single mutation of CLCN7 and higher than normal osteoclast generation; however, the osteoclasts were irregularly shaped and showed poor bone resorption. These results may help to identify treatments for osteopetrosis.

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