Congenital, X-linked, Nephrogenic Diabetes Insipidus (NDI) is a rare disorder in which the kidney is insensitive to the antidiuretic hormone, vasopressin. The gene responsible for this type of NDI, the V2 vasopressin receptor, has been cloned and mapped to Xq28. We report the case of a boy, 2.5 month old, who presented with nephrogenic diabetes insipidus (NDI). The mother and the 7 year old sister of the boy also had the NDI phenotype but did not seek medical attention until the presentation of the boy to our department. The mutational analysis of the patient showed the R337stop mutation, also founded to the mother's genotype analysis. The allele separation in mother revealed the second X chromosomal allele with a 12- bp in-frame deletion. The same in-frame deletion was also found in his sister's genotype. This deletion of four amino acids (Arg-247 to Gly-250) has been previously described but was suggested not to be linked with the NDI phenotype. However, in our case, the only possible cause of NDI phenotype in the boy's sister was the 12-bp in-frame deletion.
Keywords: 12bp-ICL3 deletion mutant; nephrogenic diabetes insipidus.