Novel mutations of TCIRG1 cause a malignant and mild phenotype of autosomal recessive osteopetrosis (ARO) in four Chinese families.

Human autosomal recessive osteopetrosis (ARO), also known as infantile malignant osteopetrosis, is a rare genetic bone disorder that often causes death. Mutations in T-cell immune regulator 1 (TCIRG1) are a frequent cause of human ARO. Six additional genes (TNFSF11, TNFRSF11A, CLCN7, OSTM1, SNX10, PLEKHM1) were also found to be associated with human ARO. In order to expand the mutation spectrum and clinical diversity for a better understanding of the ARO phenotype and to further investigate the clinical characteristics of benign subjects with ARO, we here report five individuals with ARO from four unrelated Chinese families. X-ray examination was conducted and bone turnover markers were assayed. The gene of T-cell immune regulator 1 (TCIRG1) was screened and analyzed. Monocyte-induced osteoclasts were prepared and their resorption ability was studied in vitro. We identified five novel mutations (c.66delC, c.1020+1_1020+5dup, c.2181C>A, c.2236+6T>G, c.692delA) in these patients. Four patients displayed a malignant phenotype, three of them died, and one who received bone marrow transplantation survived. The remaining one, a 24-year-old male from a consanguineous family, was diagnosed based on radiological findings but presented no neurological or hematological defects. He was homozygous for c.2236+6T>G in intron 18; this mutation influenced the splicing process. An in vitro functional study of this novel splicing defect showed no resorption pits on dentine slices. TCIRG1-dependent osteopetrosis with a mild clinical course was observed for the first time in Chinese population. The present findings add to the wide range of phenotypes of Chinese patients with TCIRG1-dependent ARO and enrich the database of TCIRG1 mutations.

Rapid gene identification in a Chinese osteopetrosis family by whole exome sequencing.

Osteopetrosis is a rare genetically heterogeneous disorder of bone metabolism characterized by increased skeleton density. In the past, standard methods for genetic diagnosis of osteopetrosis have primarily been performed by candidate gene screening and positional cloning. However, these methods are time and labor consumptive; and the genetic basis of approximately 30% of the cases is yet to be elucidated. Here, we employed whole exome sequencing of two affected individuals from an osteopetrosis family to identify a candidate mutation in CLCN7 (Y99C). It was identified from a total of 1757 and 1728 genetic variations found in either patient, which were then distilled using filtering strategies and confirmed using Sanger sequencing. We identified this mutation in six family members, while not in population matched controls. This mutation was previously found in osteopetrosis patients by other researchers. Our evolutionary analysis also indicated that it is under extremely high selective pressure, and is likely to be critical for the correct function of ClC-7, and thus is likely to be the responsible cause of disease. Collectively, our data further indicated that mutation (Y99C) may be a cause of osteopetrosis, and highlights the use of whole exome sequencing as a valuable approach to identifying disease mutations in a cost and time efficient manner.

A unique mutation underlying carbonic anhydrase II deficiency syndrome in patients of Arab descent.

We have investigated, in the genomic DNA of ten Tunisian patients, the presence of a splice junction mutation at the 5' end of intron 2 in the carbonic anhydrase II gene (CAII) previously described in six CAII-deficient patients presumed to be of Arab origin. All our patients were homozygous for this mutation and were mentally retarded, a characteristic feature of the phenotype of patients with an Arabic background. This mutation is found exclusively in patients with an Arabic background and thus may be confined to this ethnic group.