This poster (#343) will be presented at the ACMG Annual Meeting by Dr. Alkorta-Aranburu, molecular fellow, Thursday March 26: 10:30am-noon. Stop by and learn more.

Neonatal diabetes mellitus (NDM) is usually defined as the occurrence of insulin-requiring diabetes in the first 6 months of life, with an estimated prevalence of 1 in 90,000-160,000 births. NDM may be caused by several different genetic abnormalities and might either be transient (TNDM) or permanent (PNDM). While the majority of NDM cases have a monogenic etiology, approximately 70% of TNDM cases are caused by overexpression of at least two imprinted genes, PLAGL1 and HYMAI, both located on chromosome 6q24. The expression of these loci is restricted to only the paternal copies, while the maternal copies are silenced as a result of differential methylation of the promoter (differentially methylated region, DMR). Multiple techniques have been used to detect 6q24 abnormalities that can lead to disruption of expression, including paternal uniparental disomy (UPD6), paternal duplications and methylation abnormalities, which are characteristic of the disease.

In this study, we evaluated a methylation-specific multiplex ligation dependent probe amplification (MS-MLPA) assay for the molecular diagnosis of TNDM. Fifty samples including 7 with known 6q24 abnormalities and 43 UPD6-negative NDM patients lacking a genetic diagnosis were investigated. All 6q24 aberrations identified previously were detected, validating the assay. 6q24 abnormalities were identified in nine UPD6-negative patients (21%). Interestingly, a de novo deletion overlapping the TNDM DMR region was identified in one patient. Loss of the methylation signature in the PLAGL1/HYMAI DMR was consistent with a maternal deletion in this patient resulting in a TNDM phenotype.

The identification and correct classification of monogenic neonatal diabetes is clinically important to facilitate family counseling and provide adequate medical treatment. Moreover, outside of some syndromic forms of NDM, there are no clear distinguishable clinical features at presentation with hyperglycemia to help predict if a child with NDM will have TNDM or PNDM. Until recently, patients with NDM were treated solely with insulin. However, recent data demonstrated that the majority of NDM mutations are amenable to oral sulfonylurea therapy, allowing insulin discontinuation and glycemic control improvement. While the efficacy of sulfonylurea therapy in TNDM is less well established, some TNDM patients with 6q24 abnormalities respond to sulfonylurea and other non-insulin based therapies when diabetes reoccurs. Identifying the correct genetic cause of NDM has clear treatment benefits and highlights the crucial role of genetic testing in all NDM patients.

 This data provides evidence for a new causal mechanism of 6q24 loss of methylation expanding the spectrum of 6q24 abnormalities associated with TNDM. Moreover, our results demonstrate that this MS-MLPA assay represents a reliable diagnostic method to identify all currently known 6q24 abnormalities associated with TNDM.