Context: Glial cells missing B (GCMB), the mammalian homolog of the Drosophila GCM gene, encodes a 506-amino-acid parathyroid-specific transcription factor. To date, only two different heterozygous GCMB mutations have been reported in three kindreds with autosomal dominant hypoparathyroidism.
Objective: Our objective was to investigate a family with autosomal dominant hypoparathyroidism for PTH, CaSR, and GCMB mutations.
Methods: Leukocyte DNA was used with exon-specific primers for PCR amplification and the DNA sequences of the PCR products determined. Functional analyses using fluorescence microscopy, EMSAs, and luciferase reporter assays were undertaken. Informed consent was obtained using protocols approved by a national ethical committee.
Results: DNA sequence analysis revealed an A to C transversion at codon 502 of GCMB, which altered the wild-type asparagine (Asn) to histidine (His). Functional studies, using transient transfections of COS7 cells with GCMB wild-type and mutant (Asn502His) tagged constructs, demonstrated that the wild-type and mutant proteins localized to the nucleus and retained the ability to bind the GCM-consensus DNA recognition motif. However, a luciferase reporter assay demonstrated that the Asn502His mutation resulted in a reduction in gene transactivation. Moreover, cotransfection of the wild-type with mutant did not lead to an increase in luciferase activity, thereby demonstrating a dominant-negative effect of the Asn502His mutant that would be consistent with an autosomal dominant inheritance.
Conclusion: Our results, which have identified the first dominant missense GCMB mutation, help to increase our understanding of the mechanism underlying gene transactivation that is a prerequisite for the function of this parathyroid gland-specific transcription factor.