Dominant-negative GCMB mutations cause an autosomal dominant form of hypoparathyroidism.
BACKGROUND
Hypoparathyroidism (HP) is characterized by low PTH levels, hypocalcemia, and hyperphosphatemia. Heterozygous mutations in pre-pro-PTH or the calcium-sensing receptor (CaSR) cause some forms of autosomal dominant HP (AD-HP). Furthermore, homozygous mutations in glial cells missing B (GCMB) have been implicated in autosomal recessive HP (AR-HP). In most other HP patients, however, the molecular defect remains undefined.


OBJECTIVE
Our objectives were to determine the genetic defect in the affected members of two unrelated families with AD-HP and define the underlying disease mechanism.


METHODS
Several family members affected by AD-HP were investigated. The proband in family A had low calcium detected on routine blood testing, whereas the proband in family B had symptomatic hypocalcemia.


METHODS
Mutational analysis of the genes encoding pre-pro-PTH, CaSR, and GCMB was performed using PCR-amplified genomic DNA of the probands and other available members of each family. The identified GCMB mutants were characterized by Western blot analysis and luciferase reporter assay using DF-1 fibroblasts.


RESULTS
Two novel heterozygous mutations located in the last GCMB exon (c.1389delT and c.1399delC in families A and B, respectively) were identified that both lead to frame-shifts and replacement of the putative second transactivation domain within carboxyl-terminal region by unrelated amino acid sequence. The mutant GCMB proteins were well expressed, and both showed dose-dependent inhibition of the transactivation capacity of wild-type protein in luciferase reporter assays.


CONCLUSIONS
The dominant-negative effect observed in vitro for both GCMB mutations provides a plausible explanation for the impaired PTH secretion observed in the two unrelated families with AD-HP.
Mannstadt
Michael
M
Endocrine Unit, Massachusetts General Hospital, Thier 1051, 55 Fruit Street, Boston, Massachusetts 02114, USA.
Bertrand
Guylène
G
Muresan
Mihaela
M
Weryha
Georges
G
Leheup
Bruno
B
Pulusani
Sirish R
SR
Grandchamp
Bernard
B
Jüppner
Harald
H
Silve
Caroline
C
D000328:Adult; D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D002478:Cells, Cultured; D002645:Chickens; D004252:DNA Mutational Analysis; D005190:Family; D005260:Female; D017353:Gene Deletion; D005799:Genes, Dominant; D006801:Humans; D007011:Hypoparathyroidism / Q000235:genetics* / Q000378:metabolism; D008297:Male; D008875:Middle Aged; D008969:Molecular Sequence Data; D009154:Mutation / Q000502:physiology; D009687:Nuclear Proteins / Q000235:genetics*; D010281:Parathyroid Hormone / Q000378:metabolism; D010375:Pedigree; D014157:Transcription Factors / Q000235:genetics*
C119532:GCM2 protein, human; D009687:Nuclear Proteins; D010281:Parathyroid Hormone; D014157:Transcription Factors