Clinical presentation and analysis of genotype-phenotype correlations in patients with malignant infantile osteopetrosis

Generation of an induced pluripotent stem cell line (SYSUSCi004-A) from a patient with Infantile Malignant Osteopetrosis

Infantile Malignant Osteopetrosis (IMO) is a rare, severe autosomal recessive form of osteopetrosis. Here, the peripheral blood mononuclear cells (PBMCs) extracted from a patient with IMO carrying a compound heterozygous mutation in T cell immune regulator 1, ATPase H + transporting V0 subunit a3 (TCIRG1) gene (c.242delC; c.1114C > T) were successfully reprogrammed using Sendai virus encoding the four Yamanaka factors. The generated hiPSCs, IMO-hiPSCs, displayed typical embryonic stem cell-like morphology and were verified by expression of pluripotency markers such as OCT4, SOX2, NANOG, TRA-1-60 and SSEA4, as well as in vivo and in vitro differentiation into derivatives of three germ layers.

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.

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.

Osteoclast rich osteopetrosis due to defects in the TCIRG1 gene

Discovery that mutations in TCIRG1 (also known as Atp6i) gene are responsible for most instances of autosomal recessive osteopetrosis (ARO) heralded a new era for comprehension and treatment of this phenotypically heterogeneous rare bone disease. TCIRG1 encodes the a3 subunit, an essential isoform of the vacuolar ATPase proton pump involved in acidification of the osteoclast resorption lacuna and in secretory lysosome trafficking. TCIRG1 defects lead to inefficient bone resorption by nonfunctional osteoclasts seen in abundance on bone marrow biopsy, delineating this ARO as 'osteoclast-rich'. Presentation is usually in early childhood and features of extramedullary haematopoiesis (hepatosplenomegaly, anaemia, thrombocytopenia) due to bone marrow fibrosis, and cranial nerve impingement (blindness in particular). Impaired dietary calcium uptake due to high pH causes the co-occurrence of rickets, described as "osteopetrorickets". Osteoclast dysfunction leads to early death if untreated, and allogeneic haematopoietic stem cell transplantation is currently the treatment of choice. Studies of patients as well as of mouse models carrying spontaneous (the oc/oc mouse) or targeted disruption of Atp6i (TCIRG1) gene have been instrumental providing insight into disease pathogenesis and development of novel cellular therapies that exploit gene correction.

Osteopetrosis associated with PLEKHM1 and SNX10 genes, both involved in osteoclast vesicular trafficking

The clinical and radiological variability seen in different forms of osteopetrosis, all due to impaired osteoclastic bone resorption, reflect many causal genes. Both defective differentiation of osteoclasts from hematopoietic stem cells as well as disturbed functioning of osteoclasts can be the underlying pathogenic mechanism. Pathogenic variants in PLEKHM1 and SNX10 can be classified among the latter as they impair vesicular transport within the osteoclast and therefore result in the absence of a ruffled border. Some of the typical radiological hallmarks of osteopetrosis can be seen, and most cases present as a relatively mild form segregating in an autosomal recessive mode of inheritance.

Osteoclast-poor osteopetrosis

Osteopetrosis (OPT) is a rare inherited bone disease characterized by a bone resorption defect, due to osteoclast malfunction (in osteoclast-rich, oc-rich, OPT forms) or absence (in oc-poor OPT forms). This causes severe clinical abnormalities, including increased bone density, lack of bone marrow cavity, stunted growth, macrocephaly, progressive deafness, blindness, hepatosplenomegaly, and severe anemia. The oc-poor subtype of OPT is ultra-rare in humans. It is caused by mutations in either the tumor necrosis factor ligand superfamily member 11 (TNFSF11) gene, encoding RANKL (Receptor Activator of Nuclear factor-kappa B [NF-κB] Ligand) which is expressed on cells of mesenchymal origin and lymphocytes, or the TNFRSF member 11A (TNFRSF11A) gene, encoding the RANKL functional receptor RANK which is expressed on cells of myeloid lineage including osteoclasts. Clinical presentation is usually severe with onset in early infancy or in fetal life, although as more patients are reported, expressivity is variable. Phenotypic variability of RANK-deficient OPT sometimes includes hypogammaglobulinemia or radiological features of dysosteosclerosis. Disease progression is somewhat slower in RANKL-deficient OPT than in other 'malignant' subtypes of OPT. While both RANKL and RANK are essential for normal bone turnover, hematopoietic stem cell transplantation (HSCT) is the treatment of choice only for patients with the RANK-deficient form of oc-poor OPT. So far, there is no cure for RANKL-deficient OPT.

Early treatment of osteopetrosis: Paradigm shift to marrow cell transplantation

The osteopetroses reflect alterations of a special cell type, the osteoclast, belonging to the myeloid lineage. We have known this since the 1970s, confirmed by a myriad of reports featuring details that guided subsequent molecular diagnosis and treatment. This review is a tribute to two pioneers in the field: Donald G. Walker PhD (1925-1979) and Sandy C. Marks Jr. PhD (1937-2002), who explored osteopetrosis pathophysiology and treatment. Using spontaneous mutant models of osteopetrosis in mice, rats, and rabbits, they demonstrated the cellular basis of osteopetrosis while also advancing understanding of the hematological origin of osteoclasts. This became the foundation for life-saving treatment by hematopoietic stem cell transplantation. Their prose was uncomplicated, experiments were straightforward, and conclusions were based on facts explaining why their teaching became influential worldwide. I never met Dr. Walker but spoke with Dr. Marks on several occasions. Both inspired my work and, now appreciating how they shaped the osteoclast/osteopetrosis scientist community, we must thank these eminent scientists for being mentors of all of us.