TCIRG1 Transgenic Rescue of Osteoclast Function Using Induced Pluripotent Stem Cells Derived from Patients with Infantile Malignant Autosomal Recessive Osteopetrosis

Background:Osteoclasts are hematopoietic stem cell-derived multinucleated cells necessary for bone remodeling and resorption. TCIRG1 encodes a protein that is an adenosine triphosphate (ATP)-dependent vacuolar proton pump required for this process. Recessive loss-of-function mutations in both copies of this gene lead to impairment of osteoclast function, with increased bone density, increased skeletal mass, and early mortality.Methods:We isolated fibroblasts from a patient with the compound heterozygous TCIRG1 mutations c.1549G>A (p.517D>N) and c.2236C>T (p.746Q>X), and reprogrammed them into iPS (induced pluripotent stem) cells. The function of osteoclasts derived from these cells was then rescued by transgenic expression of TCIRG1 cDNA.Results:In addition to the known effects of TCIRG1 loss of function, iPS cell-derived osteoclasts from this patient had reduced expression of the bone remodeling enzymes cathepsin K (CTSK) and tartrate-resistant acid phosphatase (TRAP), leading to reduced in vitro bone remodeling. Expression of both genes and pit formation were restored in iPS cell-derived osteoclasts following transgenic restoration of TCIRG1 expression.Conclusions:Transgenic overexpression of TCIRG1 was sufficient to restore osteoclast function in iPS cell-derived osteoclasts from a patient with infantile malignant autosomal-recessive osteopetrosis.Clinical Relevance:This work provides a proof of concept for an autologous approach to treating osteopetrosis, potentially avoiding the risks associated with hematopoietic stem cell transplantation in a young patient population.