Identification of a novel mutation disrupting the DNA binding activity of GCM2 in autosomal recessive familial isolated hypoparathyroidism.

H ypoparathyroidism is a heterogeneous group of disorders with both acquired and inherited causes, each presenting clinically with hypocalcaemia. Familial cases of hypoparathyroidism may be due to an isolated defect of the parathyroid glands or be a component of a syndrome disorder, examples of which include DiGeorge, hypoparathyroidism-retardation-dysmorphism, and KennyCaffey syndrome. Familial isolated hypoparathyroidism (FIH) is characterised by hypocalcaemia and hyperphosphataemia and may be due to an inherited deficiency or the abnormal activity of parathyroid hormone (PTH). FIH is heterogeneous with X linked, autosomal dominant, and autosomal recessive modes of inheritance reported. Mutations in the PTH gene on chromosome 11p have been described in both autosomal dominant and autosomal recessive forms of the disorder. 4 Mature PTH is generated by cleavage of the signal peptide from the prepro-PTH, followed by successive proteolysis of pro-PTH within the Golgi apparatus. A requirement for normal PTH processing is emphasised by the detection of FIH disease causing mutations, which include two missense amino acid substitutions within the signal region of the prepro-PTH gene, leading to inefficient cleavage by signal peptidases. 5 Heterozygous defects in the calcium sensing receptor (CASR) have been detected in autosomal dominant FIH and sporadic cases of hypoparathyroidism. All known CASR mutations in FIH lead to receptor activation. 10 The CASR normally functions to provide a negative feedback loop, whereby binding of extracellular Ca activates the receptor, resulting in decreased PTH secretion by parathyroid cells, but also through inhibition of calcium reabsorption in the renal distal tubule. Constitutive activation of this receptor therefore results in hypocalcaemia and hypercalciuria. Notwithstanding these reports, the molecular genetic basis of most cases of autosomal recessive FIH remains unknown. Recently, a homozygous deletion within the human GCM2 (glial cells missing, Drosophila homologue B, previously referred to as GCMB) gene has been shown to underlie hypoparathyroidism in one patient from a kindred with no consanguinity. GCM2 belongs to a family of novel transcription factors characterised by an N-terminal DNA binding domain, known as the gcm motif. 13 This motif, unique to the GCM family, spans approximately 150 amino acids, exhibiting approximately 92% identity between human and mouse homologues. Human GCM1 and GCM2, the two identified isoforms of GCM, display additional homology to their mouse counterparts outside this region with GCM2 showing 68% and 47% similarity overall to Gcm2 and GCM1, respectively. Despite limited characterisation of human GCM, studies on the expression pattern of mouse Gcm2 protein have shown it to be confined to the developing parathyroid gland and homozygous mutant mice lacking the Gcm2 gene demonstrate absence of parathyroid tissue and consequent hypoparathyroidism. Taken together, these studies provide compelling evidence for a critical role of GCM2 in normal parathyroid gland function; however the molecular basis and interactions required in this process remain unclear. Here we report a consanguineous Pakistani family with multiple individuals presenting in early life with autosomal recessive FIH. Mutation studies revealed a novel homozygous missense mutation of the DNA binding domain of GCM2, functional analysis of which now provides an important tool for the further delineation of GCM2 bioactivity and molecular mechanisms necessary for parathyroid gland development.