Diabetes Mellitus Other Than Type 1 and Type 2
Genetic Defects of Beta-Cell Function: MODY
Key Characteristics
- Represents 1-5% of all diabetes.
- Autosomal dominant inheritance; penetrance spans 2-3 consecutive generations.
- Onset typically <25-30 years.
- Impaired residual insulin secretion (C-peptide present) without beta-cell autoantibodies.
Common MODY Syndromes
| Syndrome | Gene / Locus | Pathophysiology & Clinical Features | Management |
|---|---|---|---|
| MODY 1 | HNF4A (20q12) | Highly penetrant loss-of-function. Large birth weight, neonatal hyperinsulinemic hypoglycemia followed by progressive insulin secretory defect. | Low-dose sulfonylureas. |
| MODY 2 | GCK (7p15) | Defective glucokinase (glucose sensor). Shifted set-point causes mild, stable, non-progressive fasting hyperglycemia. HbA1c rarely >7.5%. Microvascular complications rare. | Diet/exercise. No medical therapy needed (except in pregnancy). |
| MODY 3 | HNF1A (12q24) | Most common. Progressive beta-cell dysfunction. Significant glucosuria. High risk for microvascular complications. | Low-dose sulfonylureas (highly sensitive). |
| MODY 5 | HNF1B (17q12) | Associated with renal cysts (RCAD), genitourinary malformations, hypomagnesemia, pancreatic exocrine insufficiency. | Insulin therapy (variable response to oral agents). |
Neonatal Diabetes Mellitus (NDM)
Epidemiology & Diagnosis
- Diagnosis <6 months of life mandatory for genetic evaluation (rarely autoimmune T1DM in this window).
- Presents with intrauterine growth retardation (IUGR), severe hyperglycemia, dehydration, and failure to thrive.
Classification & Pathophysiology
| Classification | Gene/Mechanism | Clinical Features | Management |
|---|---|---|---|
| TNDM1 (Transient) | 6q24 Overexpression (PLAGL1/HYMAI) | Presents day 1. Macroglossia, umbilical hernia. Remits ~3 months, relapses in adolescence. | Insulin initially. |
| PNDM (Permanent) | KCNJ11 / ABCC8 | Activating mutations in KATP channel (Kir6.2/SUR1). Channel remains open, preventing insulin release. Severe mutations cause DEND syndrome (developmental delay, epilepsy, neonatal diabetes). | High-dose sulfonylureas (overcomes channel defect). |
| PNDM (Permanent) | INS (Insulin gene) | Misfolded proinsulin causes severe ER stress and beta-cell apoptosis. | Insulin. |
| Syndromic | EIF2AK3 (Wolcott-Rallison) | PERK defect. Epiphyseal dysplasia, recurrent acute liver failure. | Insulin pump. |
| Syndromic | FOXP3 (IPEX) | Defective Treg cells. Polyendocrinopathy, severe enteropathy, eczema. | Bone marrow transplant. |
Mitochondrial and Other Genetic Syndromes
Mitochondrial Diabetes
- Maternally inherited mutations (e.g., np 3243 tRNA-leu).
- Defective oxidative phosphorylation impairs insulin secretion.
- Associated with MIDD (Maternally Inherited Diabetes and Deafness) and MELAS (mitochondrial encephalopathy, lactic acidosis, stroke-like episodes).
- Metformin contraindicated (mitochondrial toxin risk).
Wolfram Syndrome
- DIDMOAD: Diabetes Insipidus, Diabetes Mellitus, Optic Atrophy, Deafness.
- Recessive WFS1 mutation (Wolframin) located on chromosome 4p.
- Triggers chronic ER stress-mediated apoptosis of pancreatic beta cells and neurons.
Rogers Syndrome
- SLC19A2 gene mutation (Thiamine transporter defect).
- Triad: Thiamine-responsive megaloblastic anemia, sensorineural deafness, diabetes.
Genetic Defects in Insulin Action
Severe Insulin Resistance Syndromes
| Syndrome | Genetic Defect | Pathophysiology & Phenotype |
|---|---|---|
| Donohue Syndrome (Leprechaunism) | INSR (biallelic) | Extreme hyperinsulinemia, Elfin facies, profound lipodystrophy, fasting hypoglycemia (insulin cross-reacts with IGF-1 receptor), death in infancy. |
| Rabson-Mendenhall Syndrome | INSR | Extreme insulin resistance, acanthosis nigricans, dental/nail anomalies, pineal hyperplasia. Progresses to DKA. |
| Type A Insulin Resistance | INSR | Acanthosis nigricans, severe hyperandrogenism (PCOS phenotype), absence of obesity. |
| Type B Insulin Resistance | Acquired | Autoantibodies against the insulin receptor. Associated with systemic autoimmune diseases (e.g., Rheumatoid Arthritis). |
| Lipoatrophic Diabetes | LMNA, AGPAT2 | Dunnigan syndrome (Familial partial, LMNA 1q21). Berardinelli-Seip (Congenital generalized, 11q13). Severe insulin resistance, dyslipidemia. |
Exocrine Pancreas & Drug-Induced Diabetes
Cystic Fibrosis-Related Diabetes (CFRD)
- Progressive islet amyloid deposition and fibrosis.
- Impaired first-phase insulin secretion; concurrent insulin resistance during pulmonary exacerbations.
- Requires insulin therapy.
Drug-Induced Diabetes
- Immunosuppressants: Calcineurin inhibitors (Cyclosporine, Tacrolimus, Sirolimus) exhibit direct beta-cell toxicity.
- Chemotherapy: L-Asparaginase.
- Glucocorticoids: Induce severe peripheral insulin resistance.