Monogenic diabetes and neonatal diabetes — formative SAQs

SAQ 1 — The six-week-old with persistent hyperglycaemia (20 marks, ~15 minutes)

A six-week-old infant, born at term and small for gestational age, is admitted to the neonatal unit with poor weight gain and a febrile illness. Septic screening is initiated, and a point-of-care glucose reads 18 millimoles per litre. The hyperglycaemia persists once the illness resolves, and an insulin infusion at roughly 0.05 units per kilogram per hour is required to keep the glucose under 12. The baby has mild global developmental delay and hypotonia. [5]

Questions

1. State the single most important diagnostic principle in any baby with diabetes at this age, and justify it. (4 marks) [1]
2. Outline the genetic testing pathway and the gene group most likely here, giving your reasoning from the phenotype. (5 marks) [2] [5]
3. Describe how confirmation of the suspected gene changes the treatment, and the mechanism that makes the change work. (6 marks) [3]
4. Outline the family communication, counselling and disposition once the gene is confirmed. (5 marks) [1]

Model answer (must-hit)

1. Diabetes diagnosed before six months is almost never autoimmune type 1 and is presumed monogenic until genetic testing proves otherwise. Autoimmunity is vanishingly rare in this age window, so the work-up is molecular rather than antibody-based; relying on autoantibodies alone risks missing a treatable channel mutation. [1] [5]

2. Send a targeted next-generation sequencing panel covering the neonatal diabetes genes (KCNJ11, ABCC8, INS, 6q24 imprinting, and the rarer EIF2AK3, PDX1, FOXP3, GATA6), with whole-exome sequencing if unrevealing, through a specialist monogenic service with genetic counselling. The combination of neonatal-onset insulin-requiring diabetes with developmental delay and hypotonia points to an activating KCNJ11 or ABCC8 potassium-channel mutation with a DEND phenotype. [2] [5]

3. A confirmed activating KCNJ11 or ABCC8 mutation allows a switch from insulin to oral glibenclamide. The mechanism is that the activating mutation holds the ATP-sensitive potassium channel open so the beta cell cannot depolarise; a sulfonylurea binds the sulfonylurea receptor and closes the channel directly, bypassing the faulty ATP-sensing step and restoring insulin release. The switch usually improves glycaemic control and, because the same channel operates in the brain, often improves the neurological features of DEND. The conversion is best managed through a specialist centre. [3]

4. Offer genetic counselling to the parents, explain the autosomal-dominant or de-novo basis of the mutation and the recurrence risk, and arrange cascade testing of first-degree relatives. Disposition is lifelong shared care with a specialist monogenic diabetes service, a structured transition plan to adult care, and a correct patient identifier to prevent the wrong-diagnosis errors that recur across a lifetime of healthcare contacts. [1]

SAQ 2 — The twelve-year-old labelled type 1 for five years (20 marks, ~15 minutes)

A twelve-year-old diagnosed with type 1 diabetes at age seven is reviewed because her glucose control has always been excellent on small insulin doses and she has never had a severe hypoglycaemic event. On re-testing her islet autoantibody panel is negative and her C-peptide is well within the normal range. Her father, aged forty-two, was diagnosed with type 1 diabetes at fifteen and has always been slim. [4]

Questions

1. Give the most likely unifying diagnosis and the bedside features that support it. (5 marks) [4] [6]
2. Outline the confirmatory investigations and how a screening pathway helps target them. (4 marks) [1] [6]
3. Explain how the treatment changes once the diagnosis is molecularly confirmed, and why. (6 marks) [1]
4. Compare and contrast this condition with glucokinase-MODY, and explain why the drug choice differs. (5 marks) [1]

Model answer (must-hit)

1. The most likely diagnosis is HNF1A-MODY (formerly MODY3). The supporting features are the negative islet autoantibodies, the C-peptide that persists well beyond any honeymoon, the excellent control on small insulin doses, the lean phenotype, and the dominant family history — a father labelled type 1 who is slim and was diagnosed young. Most such children are initially mislabelled as type 1. [4] [6]

2. Confirm with a targeted monogenic gene panel including HNF1A, HNF4A, GCK and HNF1B through a specialist service with genetic counselling. A biomarker-based screening pathway combining young age at onset, negative antibodies and a measurable C-peptide (or a urine C-peptide creatinine ratio) identifies the highest-yield candidates for testing and is the recommended first step before the genetic panel. [1] [6]

3. Once HNF1A-MODY is confirmed, the child is switched from insulin to a low-dose sulfonylurea, started low and titrated slowly because these patients respond to doses far below those used in type 2 and are prone to hypoglycaemia on over-treatment. The rationale is that the defect is in the transcription factor that maintains the beta cell's glucose-sensing and secretory apparatus, and the residual beta cells are exquisitely sulfonylurea-sensitive, so the drug drives insulin release without injections. Insulin is reserved for those who fail or cannot tolerate a sulfonylurea. [1]

4. Glucokinase-MODY is a raised glucose set-point from a faulty glucose sensor, producing mild, stable fasting hyperglycaemia from birth that rarely progresses and rarely causes microvascular complications, so it usually needs no drug at all. HNF1A-MODY is a progressive insulin secretory defect with a real microvascular risk over time, which is why it warrants active treatment with a sulfonylurea and annual complication screening. Both are autosomal dominant and antibody-negative, but the natural history and the treatment differ because the gene and the mechanism differ. [1]