Case Report: A Novel Homozygous Variant of the CTSK Gene in Rare Pycnodysostosis.

Pycnodysostosis (PD) is a rare autosomal recessive skeletal dysplasia from impaired bone resorption due to osteoclastic dysfunction. The features of PD are deformity of the skull, maxilla, and phalanges; osteosclerosis; and bone fragility. We describe the case of a patient with complaints of multiple fractures of the lower extremities in the anamnesis and pain in the lower extremities, cervical spine, and shoulder girdle during physical exertion. Genetic testing revealed a novel homozygous variant c.704T>C (p.Leu235Pro) in the CTSK gene. Biallelic pathogenic variants in this gene lead to PD. Thus, the diagnosis in the patient was established by finding a novel likely pathogenic variant in the CTSK gene.