Long-term survival in infantile malignant autosomal recessive osteopetrosis secondary to homozygous p.Arg526Gln mutation in CLCN7

A novel compound heterozygous mutation of the CLCN7 gene is associated with autosomal recessive osteopetrosis

Osteopetrosis is a genetic condition of the skeleton characterized by increased bone density caused by osteoclast formation and function defects. Osteopetrosis is inherited in the form of autosomal dominant and autosomal recessive manner. We report autosomal recessive osteopetrosis (ARO; OMIM 611490) in a Chinese case with a history of scarce leukocytosis, vision and hearing loss, frequent seizures, and severe intellectual and motor disability. Whole-exome sequencing (WES) followed by Sanger sequencing revealed novel compound heterozygous mutations in the chloride channel 7 (CLCN7) gene [c.982-1G > C and c.1208G > A (p. Arg403Gln)] in the affected individual, and subsequent familial segregation showed that each parent had transmitted a mutation. Our results confirmed that mutations in the CLCN7 gene caused ARO in a Chinese family. Additionally, our study expanded the clinical and allelic spectrum of the CLCN7 gene and enhanced the applications of WES technology in determining the etiology of prenatal diagnoses in fetuses with ultrasound anomalies.

Adult Osteosclerotic Metaphyseal Dysplasia With Progressive Osteonecrosis of the Jaws and Abnormal Bone Resorption Pattern Due to a LRRK1 Splice Site Mutation

Osteosclerotic metaphyseal dysplasia (OSMD) is a rare autosomal recessive sclerosing skeletal dysplasia. We report on a 34-year-old patient with sandwich vertebrae, platyspondyly, osteosclerosis of the tubular bones, pathologic fractures, and anemia. In the third decade, he developed osteonecrosis of the jaws, which was progressive in spite of repeated surgical treatment over a period of 11 years. An iliac crest bone biopsy revealed the presence of hypermineralized cartilage remnants, large multinucleated osteoclasts with abnormal morphology, and inadequate bone resorption typical for osteoclast-rich osteopetrosis. After exclusion of mutations in TCIRG1 and CLCN7 we performed trio-based exome sequencing. The novel homozygous splice-site mutation c.261G>A in the gene LRRK1 was found and co-segregated with the phenotype in the family. cDNA sequencing showed nearly complete skipping of exon 3 leading to a frameshift (p.Ala34Profs*33). Osteoclasts differentiated from the patient's peripheral blood monocytes were extremely large. Instead of resorption pits these cells were only capable of superficial erosion. Phosphorylation of L-plastin at position Ser5 was strongly reduced in patient-derived osteoclasts showing a loss of function of the mutated LRRK1 kinase protein. Our analysis indicates a strong overlap of LRRK1-related OSMD with other forms of intermediate osteopetrosis, but an exceptional abnormality of osteoclast resorption. Like in other osteoclast pathologies an increased risk for progressive osteonecrosis of the jaws should be considered in OSMD, an intermediate form of osteopetrosis. © 2020 The Authors. Journal of Bone and Mineral Research published by American Society for Bone and Mineral Research.

Sclerosing bone dysplasias with hallmarks of dysosteosclerosis in four patients carrying mutations in SLC29A3 and TCIRG1

The osteopetroses and related sclerosing bone dysplasias can have a broad range of manifestations. Especially in the milder forms, sandwich vertebrae are an easily recognizable and reliable radiological hallmark. We report on four patients from three families presenting with sandwich vertebrae and platyspondyly. The long bone phenotypes were discordant with one patient showing modeling defects and patchy osteosclerosis, while the second displayed only metaphyseal sclerotic bands, and the third and fourth had extreme metaphyseal flaring with uniform osteosclerosis. Two of the four patients had experienced pathological fractures, two had developmental delay, but none showed cranial nerve damage, hepatosplenomegaly, or bone marrow failure. According to these clinical features the diagnoses ranged between intermediate autosomal recessive osteopetrosis and dysosteosclerosis. After exclusion of mutations in CLCN7 we performed gene panel and exome sequencing. Two novel mutations in SLC29A3 were found in the first two patients. In the third family a TCIRG1 C-terminal frameshift mutation in combination with a mutation at position +4 in intron 2 were detected. Our study adds two cases to the small group of individuals with SLC29A3 mutations diagnosed with dysosteosclerosis, and expands the phenotypic variability. The finding that intermediate autosomal recessive osteopetrosis due to TCIRG1 splice site mutations can also present with platyspondyly further increases the molecular heterogeneity of dysosteosclerosis-like sclerosing bone dysplasias.

Osteogenesis imperfecta with ectopic mineralizations in dentin and cementum and a COL1A2 mutation

We report a Thai father (patient 1) and his daughter (patient 2) affected with osteogenesis imperfecta type IV and dentinogenesis imperfecta. Both were heterozygous for the c.1451G>A (p.Gly484Glu) mutation in COL1A2. The father, a Thai boxer, had very mild osteogenesis imperfecta with no history of low-trauma bone fractures. Scanning electron micrography of the primary teeth with DI of the patient 2, and the primary teeth with DI of another OI patient with OI showed newly recognized dental manifestations of teeth with DI. Normal dentin and cementum might have small areas of ectopic mineralizations. Teeth affected with DI have well-organized ectopic mineralizations in dentin and cementum. The "French-fries-appearance" of the crystals at the cemento-dentinal junction and abnormal cementum have never been reported to be associated with dentinogenesis imperfecta, either isolated or osteogenesis imperfecta-associated. Our study shows for the first time that abnormal collagen fibers can lead to ectopic mineralization in dentin and cementum and abnormal cementum can be a part of osteogenesis imperfecta.

ClC-7 expression levels critically regulate bone turnover, but not gastric acid secretion

Mutations in the 2Cl(-)/1H(+)-exchanger ClC-7 impair osteoclast function and cause different types of osteoclast-rich osteopetrosis. However, it is unknown to what extent ClC-7 function has to be reduced to become rate-limiting for bone resorption. In osteoclasts from osteopetrosis patients expression of the mutated ClC-7 protein did not correlate with disease severity and resorption impairment. Therefore, a series of transgenic mice expressing ClC-7 in osteoclasts at different levels was generated. Crossing of these mice with Clcn7(-/-) mutants rescued the osteopetrotic phenotype to variable degrees. One resulting double transgenic line mimicked human autosomal dominant osteopetrosis. The trabecular bone of these mice showed a reduction of osteoblast numbers, osteoid, and osteoblast marker gene expression indicative of reduced osteoblast function. In osteoclasts from these mutants ClC-7 expression levels were 20 to 30% of wildtype levels. These reduced levels not only impaired resorptive activity, but also increased numbers, size and nucleus numbers of osteoclasts differentiated in vitro. Although ClC-7 was expressed in the stomach and PTH levels were high in Clcn7(-/-) mutants loss of ClC-7 did not entail a relevant elevation of gastric pH. In conclusion, we show that in our model a reduction of ClC-7 function by approximately 70% is sufficient to increase bone mass, but does not necessarily enhance bone formation. ClC-7 does not appear to be crucially involved in gastric acid secretion, which explains the absence of an osteopetrorickets phenotype in CLCN7-related osteopetrosis.

Identification of potential microRNA-target pairs associated with osteopetrosis by deep sequencing, iTRAQ proteomics and bioinformatics

MicroRNAs aberrantly express in many human diseases including some metabolic bone disorders. They have been found to be associated with osteoclast differentiation and function, which makes them attractive candidates for the therapy of bone. However, the potential clinical application of microRNAs in therapeutics rests heavily upon our in-depth understanding of microRNAs and their targets. To identify potential microRNA-target pairs associated with osteopetrosis, we performed a system approach including deep sequencing, iTRAQ quantitative proteomics, and bioinformatics in the peripheral blood mononuclear cells (PBMCs) taken from patients with osteopetrosis and health donors. Notably, 123 differently expressed microRNAs, 173 differently expressed proteins, and 117 computationally predicted microRNA-target pairs with reciprocally expressed level in PBMCs were found in the two sample groups. Functional annotation identified that the microRNA-target pairs were involved in cell growth, differentiation, cellular signaling network, and the network highlighted the microRNA-target pair of has-miR-320a and ADP ribosylation factor 1 (Arf1) potentially associated with CLCN7 mutations in osteopetrosis. The pair of has-miR-320a and Arf1 was further verified by real-time PCR, western blot, and the interaction between has-miR-320a and its targeted sequence on the Arf1 mRNAs was confirmed by luciferase assay. Collectively, the present study established a new system approach for the investigation of microRNAs, and the microRNA-target pairs, particular has-miR-320a and Arf1, may have important roles in osteopetrosis.

Severe neuronopathic autosomal recessive osteopetrosis due to homozygous deletions affecting OSTM1

Autosomal recessive osteopetrosis (ARO, MIM 259700) is a genetically heterogeneous rare skeletal disorder characterized by failure of osteoclast resorption leading to pathologically increased bone density, bone marrow failure, and fractures. In the neuronopathic form neurological complications are especially severe and progressive. An early identification of the underlying genetic defect is imperative for assessment of prognosis and treatment by hematopoietic stem cell transplantation. Here we describe for the first time homozygous microdeletions of different sizes affecting the OSTM1 gene in two unrelated consanguineous families with children suffering from neuronopathic infantile malignant osteopetrosis. Patients showed an exceptionally severe phenotype with variable CNS malformations, seizures, blindness, and deafness. Multi-organ failure due to sepsis led to early death between six weeks and five months of age in spite of intensive care treatment. Analysis of the breakpoints revealed different mechanisms underlying both rearrangements. Microdeletions seem to represent a considerable portion of OSTM1 mutations and should therefore be included in a sufficient diagnostic screening.

Osteopetrosis, osteopetrorickets and hypophosphatemic rickets differentially affect dentin and enamel mineralization

Osteopetrosis (OP) is an inherited disorder of defective bone resorption, which can be accompanied by impaired skeletal mineralization, a phenotype termed osteopetrorickets (OPR). Since individuals with dysfunctional osteoclasts often develop osteomyelitis of the jaw, we have analyzed, if dentin and enamel mineralization are differentially affected in OP and OPR. Therefore, we have applied non-decalcified histology and quantitative backscattered electron imaging (qBEI) to compare the dental phenotypes of Src(-/-), oc/oc and Hyp(-/0) mice, which serve as models for OP, OPR and hypophosphatemic rickets, respectively. While both, Src(-/-) and oc/oc mice, were characterized by defects of molar root formation, only oc/oc mice displayed a severe defect of dentin mineralization, similar to Hyp(-/0) mice. Most importantly, while enamel thickness was not affected in either mouse model, the calcium content within the enamel phase was significantly reduced in oc/oc, but not in Src(-/-) or Hyp(-/0) mice. Taken together, these data demonstrate that dentin and enamel mineralization are differentially affected in Src(-/-) and oc/oc mice. Moreover, since defects of dental mineralization may trigger premature tooth decay and thereby osteomyelitis of the jaw, they further underscore the importance of discriminating between OP and OPR in the respective individuals.

Omi, a recessive mutation on chromosome 10, is a novel allele of Ostm1

Large-scale N-ethyl-N-nitrosourea (ENU) mutagenesis has provided many rodent models for human disease. Here we describe the initial characterization and mapping of a recessive mutation that leads to degeneration of the incisors, failure of molars to erupt, a grey coat colour, and mild osteopetrosis. We mapped the omi mutation to chromosome 10 between D10Mit214 and D10Mit194. The Ostm1 gene is a likely candidate gene in this region and the grey-lethal allele, Ostm1 ( gl ), and omi mutations fail to complement each other. We show that om/om mice have reduced levels of Ostm1 protein. To date we have not been able to identify the causative mutation. We propose that omi is a novel hypomorphic mutation affecting Ostm1 expression, potentially in a regulatory element.