Inheritance of Mildly Activating ABCC8 Mutation From a Mother With MODY Causes Permanent Neonatal Diabetes Mellitus (NDM) in Two Siblings Who Also Carry a Second Inactivating Mutation: Genetic Testing Allows for Improved Treatment With Sulfonylureas (SU)
Authors: Salguero Bermonth Maria Victoria , Letourneau-Freiberg Lisa , Devine Nancy , Greeley Siri Atma W 
Published: May 2021

Abstract
Background: Heterozygous activating mutations in KCNJ11 or ABCC8 are the most common cause of neonatal diabetes (NDM). ABCC8 (SUR1) mutations more often cause transient NDM. Inactivating ABCC8 mutations can cause congenital hyperinsulinism (HI), but very rarely will such mutations be inherited together. Mildly activating KATP mutations can also be a cause of MODY, but even if genetic testing is considered, many commercial testing panels do not include these genes, despite the significant difference in treatment that can result due to sulfonylurea (SU) responsiveness.
Clinical Case: The proband was diagnosed with DM at 11 months old and fortuitously treated with SU for 3 years. He was switched to insulin and had poor DM control thereafter. Sister was diagnosed at 3.5 months old and had poor DM control on insulin. Mother was diagnosed with DM at 27 years old and treated with various medications including insulin. Genetic testing revealed that mother carried ABCC8 mutation R1380C previously described to cause transient NDM and/or later-onset DM consistent with her phenotype. Both children inherited this mutation from her and inherited a variant (L1148R) from their father without diabetes that has been reported in association with HI. The L1148R allele may reduce cell surface expression thereby increasing the relative expression and pathogenic effect of the R1380C allele that has not previously been described to cause permanent NDM.
We assessed SU responsiveness by measuring maximal beta-cell function through combined mixed meal and arginine testing. Mother exhibited easily detectable C-peptide levels at baseline that improved by SU treatment. In contrast, the children displayed almost undetectable baseline beta-cell function with variable response to SU: the sister who had been chronically poorly controlled on insulin therapy displayed barely improved C-peptide production, while her brother who had previously been treated with SU as an infant had markedly improved beta-cell function on SU. Within two months of continued treatment with high doses of SU only, he was able to start lowering his SU dose with improved glycemia. His sister was started on high-dose SU in addition to insulin, but continued to have difficulty adhering to her treatment regimen. Her blood sugar improved after the addition of long-acting GLP-1 agonist (liraglutide) but she later became pregnant and returned to insulin only. Her glycemic control improved when re-started on SU after pregnancy. The mother exhibited excellent DM on a lower dose of exclusive SU therapy.
Clinical Lesson: Genetic testing can dramatically alter management and must be pursued in both NDM and family members with diabetes later in life. Careful assessment of clinical characteristics along with genetic testing for segregation patterns in family members can greatly improve understanding of the causality of previous uncharacterized variants.