Maturity-Onset Diabetes of the Young (MODY) is a form of monogenic diabetes, typically characterized by an autosomal dominant inheritance pattern, onset usually before 25 years of age, and impairment in insulin secretion with minimal or no defects in insulin action. Unlike more common forms of diabetes, such as Type 1 and Type 2, MODY is not primarily driven by lifestyle factors.

It is thought that around 1-2% of patients with diabetes mellitus have MODY, and around 90% are misclassified as having either type 1 or type 2 diabetes mellitus.

Classification and Genetics

MODY is a heterogenous group, with over 14 types identified as of the latest revision, each corresponding to mutations in different genes. The most common types are MODY2 (GCK mutation) and MODY3 (HNF1A mutation), but others include MODY1 (HNF4A), MODY4 (PDX1), MODY5 (HNF1B), and so on.

MODY 3 accounts for around 60% of cases with MODY 2 causing 20% of cases.

Each subtype varies in its clinical presentation, management, and prognosis, emphasizing the importance of precise genetic diagnosis in order to guide treatment strategy and genetic counselling.

Presentation

• Patients with MODY often present with mild non-ketotic hyperglycemia that is often detected incidentally or during routine screening.
• It may also be discovered during pregnancy.
• Unlike Type 1 diabetes, patients with MODY usually do not present with diabetic ketoacidosis except under severe stress conditions, and unlike Type 2 diabetes, they are often of normal weight and do not exhibit signs of insulin resistance.

The specific clinical manifestations and complications can vary depending on the subtype of MODY. For example:

• individuals with MODY2 generally have mild, stable fasting hyperglycemia and rarely develop severe complications
• with MODY3 or MODY1 may have progressive hyperglycemia and are at higher risk for complications typically associated with diabetes, such as retinopathy, nephropathy, and cardiovascular disease.

Diagnosis

MODY should be suspected in individuals with persistent, asymptomatic hyperglycemia detected before the age of 25, without the typical features of Type 1 or Type 2 diabetes.

The diagnosis is confirmed by genetic testing, which is crucial for identifying the specific type of MODY, as this has direct implications for management. It's important to remember, however, that genetic testing can be expensive and may not be available in all settings.