A Case of Neonatal Diabetes Mellitus Due to INS Gene Mutation with Maternal Mosaicism and Atypical Presentation.

Neonatal diabetes mellitus is a single gene defect that results in diabetes mellitus in the first 6 months of life. We report a child who was diagnosed to be hyperglycemic at 13 months of life and assumed to have type 1 diabetes mellitus and started on insulin. The child came to us at 2 and 1/2 years of age. He had exceptionally good blood glucose control. His history revealed that he was symptomatic with a voracious appetite and poor weight gain since the second half of infancy. Genetic testing revealed a heterozygous mutation of the INS gene (the gene that codes for insulin). The condition has autosomal dominant inheritance. Testing the parents revealed that the mother had 7.8% mosaicism for this variant in her lymphocyte DNA. Though this did not alter the management of the patient, it did help in counseling the parents regarding risk of recurrence in future pregnancies.