Nephrogenic diabetes insipidus results from a novel in-frame deletion of gene in monozygotic-twin boys and their mother and grandmother.

Objectives: Mutations in the gene are the most common cause of nephrogenic diabetes insipidus (NDI). In-frame deletions of the gene are a rare variant that results in NDI. We report a novel variant of the p.H138del in an NDI family with twin male patients and three female carriers of different clinical phenotypes.

Methods: The proband's blood genome was sequenced with a panel, and the variants were classified according to ACMG/AMP (2015) guidelines. X chromosome inactivation (XCI) was analyzed in the peripheral blood of his mother, grandmother, and maternal aunt, respectively. The haplotypes of the X chromosome were determined using their STR loci.

Results: A novel in-frame deletion in the gene was detected in monozygotic-twin boys, and his mother, grandmother, and maternal aunt were heterozygous carriers. The two boys showed typical NDI, and their mother and grandmother presented polydipsia, polydipsia, and polyuria, but the maternal aunt did not have similar symptoms. The blood XCI results of the mother, grandmother, and maternal aunt showed random inactivation (36.18 , 48.37, and 49.30 %, respectively). The X haplotype indicated that the variant of the mother and grandmother was on their activated X chromosomes(Xa), while the maternal aunt's variant was on her inactivated X chromosome(Xi).

Conclusions: In-frame deletion of the gene within its functional domain can significantly affect protein function, which is one of the vital causes of NDI. The clinical variability of female carriers of is associated with underlying environmental and epigenetic factors or complex recombination of the X chromosomes.