A novel GCM2 mutation identified in an infant with familial isolated hypoparathyroidism

Background: Isolated hypoparathyroidism comprises a set of heterogeneous inherited diseases associated with abnormal calcium metabolism exclusively due to parathyroid hormone (PTH) deficiency. Isolated hypoparathyroidism can be either sporadic or inherited. Genetic causes that impair the synthesis or secretion of PTH, such as calcium-sensing receptor and PTH defects, or defects in the development of the parathyroid gland [glial cell missing 2, (GCM2)], have been established as causes of familial isolated hypoparathyroidism. Transcription factor GCM2 is a crucial regulator of parathyroid gland homeostasis. Transmission of pathogenic variants encoding GCM2 occurs in an autosomal recessive or dominant manner. Case Presentation: Herein, we describe the case of a 12-year-old boy, born to consanguineous parents, who presented with abnormal movement during the first week of birth. Laboratory results revealed hypocalcemia, hyperphosphatemia, and low PTH levels. Genetic testing detected a novel homozygous variant in the GCM2 gene, c.391C>T (p.Arg131*). Although this variant has not been previously described, it is likely the pathogenic cause of this condition. Conclusion: To the best of authors' knowledge, this variant has not been listed in any database. Proper replacement therapy is likely to have good long-term outcomes for our patient. [JBCGenetics 2022; 5(1.000): 25-28

Mutations in ARMC9, which Encodes a Basal Body Protein, Cause Joubert Syndrome in Humans and Ciliopathy Phenotypes in Zebrafish

Joubert syndrome (JS) is a recessive neurodevelopmental disorder characterized by hypotonia, ataxia, abnormal eye movements, and variable cognitive impairment. It is defined by a distinctive brain malformation known as the "molar tooth sign" on axial MRI. Subsets of affected individuals have malformations such as coloboma, polydactyly, and encephalocele, as well as progressive retinal dystrophy, fibrocystic kidney disease, and liver fibrosis. More than 35 genes have been associated with JS, but in a subset of families the genetic cause remains unknown. All of the gene products localize in and around the primary cilium, making JS a canonical ciliopathy. Ciliopathies are unified by their overlapping clinical features and underlying mechanisms involving ciliary dysfunction. In this work, we identify biallelic rare, predicted-deleterious ARMC9 variants (stop-gain, missense, splice-site, and single-exon deletion) in 11 individuals with JS from 8 families, accounting for approximately 1% of the disorder. The associated phenotypes range from isolated neurological involvement to JS with retinal dystrophy, additional brain abnormalities (e.g., heterotopia, Dandy-Walker malformation), pituitary insufficiency, and/or synpolydactyly. We show that ARMC9 localizes to the basal body of the cilium and is upregulated during ciliogenesis. Typical ciliopathy phenotypes (curved body shape, retinal dystrophy, coloboma, and decreased cilia) in a CRISPR/Cas9-engineered zebrafish mutant model provide additional support for ARMC9 as a ciliopathy-associated gene. Identifying ARMC9 mutations as a cause of JS takes us one step closer to a full genetic understanding of this important disorder and enables future functional work to define the central biological mechanisms underlying JS and other ciliopathies