Genetic analysis of osteopetrosis in Pakistani families identifies novel and known sequence variants

Osteopetrosis is a genetically heterogenous, fatal bone disorder characterized by increased bone density. Globally, various genetic causes are reported for osteopetrosis with all forms of inheritance patterns. A precise molecular diagnosis is necessary for prognosis and for prescribing treatment paradigms in osteopetrosis. Here we report on thirteen individuals diagnosed with infantile malignant osteopetrosis coming from ten unrelated Pakistani families; nine of whom are consanguineous. We performed whole exome sequencing and Sanger sequencing in all families and identified homozygous variants in genes previously reported for autosomal recessive inheritance of osteopetrosis. All the identified variants are expected to affect the stability or length of gene products except one nonsynonymous missense variant. TCIRG1 was found as a candidate causal gene in majority of the families. We report six novel variants; four in TCIRG1 and one each in CLCN7 and OSTM1. Our combined findings will be helpful in molecular diagnosis and genetic counselling of patients with osteopetrosis particularly in populations with high consanguinity.

Two novel mutations in TCIRG1 induced infantile malignant osteopetrosis: a case report

BACKGROUND

Infantile malignant osteopetrosis (IMO) is a rare autosomal recessive disease characterized by a higher bone density in bone marrow caused by the dysfunction of bone resorption. Clinically, IMO can be diagnosed with medical examination, bone mineral density test and whole genome sequencing.

CASE PRESENTATION

We present the case of a 4-month-old male infant with abnormal skull development, hypocalcemia and premature closure of the cranial sutures. Due to the hyper bone density showed by his radiographic examination, which are characteristic patterns of IMO, we speculated that he might be an IMO patient. In order to confirm this diagnosis, a high-precision whole exome sequencing of the infant and his parents was performed. The analysis of high-precision whole exome sequencing results lead to the identification of two novel heterozygous mutations c.504-1G > C (a splicing site mutation) and c.1371delC (p.G458Afs*70, a frameshift mutation) in gene TCIRG1 derived from his parents. Therefore, we propose that there is a close association between these two mutations and the onset of IMO.

CONCLUSIONS

To date, these two novel mutations in gene TCIRG1 have not been reported in the reference gene database of Chinese population. These variants have likewise not been reported outside of China in the Genome Aggregation Database (gnomAD). Our case suggests that the use of whole exome sequencing to detect these two mutations will improve the identification and early diagnosis of IMO, and more specifically, the identification of homozygous individuals with TCIRG1 gene mutation. We propose that these mutations in gene TCIRG1 could be a novel therapeutic target for the IMO in the future.

Massive osteopetrosis caused by non-functional osteoclasts in R51Q SNX10 mutant mice

The R51Q mutation in sorting nexin 10 (SNX10) was shown to cause a lethal genetic disease in humans, namely autosomal recessive osteopetrosis (ARO). We describe here the first R51Q SNX10 knock-in mouse model and show that mice homozygous for this mutation exhibit massive, early-onset, and widespread osteopetrosis. The mutant mice exhibit multiple additional characteristics of the corresponding human disease, including stunted growth, failure to thrive, missing or impacted teeth, occasional osteomyelitis, and a significantly-reduced lifespan. Osteopetrosis in this model is the result of osteoclast inactivity that, in turn, is caused by absence of ruffled borders in the mutant osteoclasts and by their inability to secrete protons. These results confirm that the R51Q mutation in SNX10 is a causative factor in ARO and provide a model system for studying this rare disease.

Hematopoietic stem cell transplantation-induced bone remodeling in autosomal recessive osteopetrosis: Interaction between skeleton and hematopoietic and sensory nervous systems

OBJECTIVE

Autosomal recessive osteopetrosis (ARO) is a rare congenital disorder of defective bone resorption. The inability of osteoclasts to resorb bone compromises the development of bone marrow cavity, and ultimately, leads to defective hematopoiesis and death within the first decade. The only curative treatment currently available for certain forms of ARO is hematopoietic stem cell transplantation (HSCT). Infants over ten months of age suffering from ARO are defined as patients with advanced disease; HSCT to these patients is associated with high risk of transplant-related mortality (TRM). Because of the extreme variability of ARO clinical phenotypes, the most reliable predictive factor of TRM and graft failure risk is the residual bone marrow space volume.

CASE REPORT

We report clinical and radiological outcomes of one patient affected by ARO and treated with HSCT at advance stage of the disease. We describe the anomalies in various tissues, including bone marrow and bones at the moment of the diagnosis and document their gradual disappearance after HSCT until their complete resolution based on magnetic resonance imaging (MRI) observations. We provided radiological images of the cranial vault bone structure modifications, correlating the radiological appearance of the optical canals and nerves and of the cerebellum with the neurological manifestations of the disease.

CONCLUSIONS

Our results demonstrate that MRI is a highly sensitive technique that provides excellent images of bone marrow space before and after HSCT without exposing children to ionizing radiation. MRI also permits us to evaluate post-transplant skeletal remodeling and the deriving changes in the hematopoietic and sensory system.

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.

Autosomal recessive osteopetrosis with a unique imaging finding: multiple encephaloceles

Osteopetrosis is a hereditary form of sclerosing bone dysplasia with various radiological and clinical presentations. The autosomal recessive type, also known as malignant osteopetrosis, is the most severe type, with the early onset of manifestations. A 5-month-old infant was admitted to our hospital with recurrent respiratory tract infections. Chest X-ray and skeletal survey revealed the classic findings of osteopetrosis, including diffuse osteosclerosis and bone within a bone appearance. At follow-up, the patient presented with, thickened calvarium, multiple prominent encephaloceles, and dural calcifications leading to the intracranial clinical manifestations with bilateral hearing and sight loss. Autosomal recessive osteopetrosis is one of the causes of encephaloceles and this finding may become dramatic if untreated.