Monogenic diabetes and neonatal diabetes — branching viva

Branch 1 — The six-week-old with persistent hyperglycaemia

Stage 1. A six-week-old, small for gestational age, has persistent insulin-requiring hyperglycaemia found during a febrile illness. What is the diagnostic principle that governs any diabetes at this age? [1]

Diabetes under six months is almost never autoimmune type 1, so the working diagnosis is monogenic neonatal diabetes until a genetic test says otherwise; the work-up is molecular, not antibody-based. [1]

Stage 2 (release). The baby has mild developmental delay and hypotonia. Which gene group rises up your differential, and what is the clinical syndrome called? [2] [5]

The combination of neonatal diabetes with developmental delay and hypotonia points to an activating KCNJ11 or ABCC8 potassium-channel mutation with a DEND syndrome (developmental delay, epilepsy and neonatal diabetes). [2] [5]

Stage 3 (release). The panel confirms an activating KCNJ11 mutation. How does this change the treatment and why does it work? [3]

The child is switched from insulin to oral glibenclamide. The activating mutation holds the KATP channel open so the beta cell cannot depolarise; a sulfonylurea closes the channel directly, bypassing the faulty ATP-sensing step and restoring insulin release, which usually improves both glycaemia and the neurological features. [3]

Branch 2 — The antibody-negative twelve-year-old labelled type 1

Stage 1. A twelve-year-old, diagnosed type 1 at seven, has always been well controlled on small insulin doses. Her antibodies are negative and her C-peptide is normal; her slim father was diagnosed type 1 at fifteen. What is the unifying diagnosis? [4]

The most likely diagnosis is HNF1A-MODY: negative antibodies, a C-peptide persisting beyond any honeymoon, a lean phenotype, small insulin requirements, and a dominant family history of an affected parent. [4]

Stage 2 (release). The genetic panel confirms an HNF1A mutation. How does the treatment change? [1]

She is switched from insulin to a low-dose sulfonylurea, started low and titrated slowly because HNF1A-MODY beta cells are exquisitely sulfonylurea-sensitive and prone to hypoglycaemia on over-treatment; insulin is reserved for treatment failure. [1]

Stage 3. Why is this child's microvascular risk different from that of the glucokinase-MODY child in the next branch? [1]

HNF1A-MODY is a progressive secretory defect with a real retinopathy, nephropathy and neuropathy risk over time, so it warrants active treatment and annual screening; glucokinase-MODY is a stable raised set-point with a low complication rate and usually no drug. [1]

Branch 3 — The nine-year-old with a mildly raised fasting glucose

Stage 1. A nine-year-old has a fasting glucose of 6.8 millimoles per litre found on a routine school medical, rechecked and stable. She is asymptomatic, slim, and her mother has the same mildly raised glucose and never needed treatment. What is the likely diagnosis and the expected management? [1]

The likely diagnosis is glucokinase-MODY: a stable, mild fasting hyperglycaemia from birth with a dominant family history of a similarly mild, untreated glucose. Management is usually no pharmacotherapy, because microvascular complications are rare and unnecessary treatment adds hypoglycaemia and a wrong lifelong label. [1]

Stage 2 (release). Her paediatrician started her on metformin three months ago and she has had two hypoglycaemic episodes. What do you advise? [1]

Confirm the GCK mutation and step back from pharmacotherapy; the stable mild hyperglycaemia of glucokinase-MODY does not warrant a drug outside pregnancy, and the hypoglycaemia reflects over-treatment of a condition that rarely causes complications. [1]

Branch 4 — The fourteen-year-old with diabetes and bilateral renal cysts

Stage 1. A fourteen-year-old presents with polyuria and polydipsia; she is found to be diabetic, and a renal ultrasound performed for hypertension shows bilateral renal cysts with reduced renal size. What monogenic diagnosis unifies these findings? [5]

HNF1B-MODY (MODY5), the renal cysts and diabetes syndrome, unifies the diabetes with the renal cystic dysplasia and pancreatic hypoplasia; the renal and endocrine features share one gene. [5]

Stage 2. What investigations and surveillance does this diagnosis demand beyond the glucose? [1]

Image the pancreas for hypoplasia or atrophy, establish a baseline renal function and arrange longitudinal renal surveillance for progressive disease, screen for genitourinary anomalies, and offer genetic counselling and cascade testing of relatives, because the multi-system burden of HNF1B-MODY is managed as much through the kidneys and pancreas as through the glucose. [1]

Synthesis — The genetic testing pathway and genotype-matched drug choice

Tie the four branches together: suspect monogenic diabetes from the phenotype (young onset, lean, negative antibodies, preserved C-peptide, dominant pedigree), confirm with a targeted gene panel through a specialist service with counselling, then match the drug to the gene — no drug for glucokinase-MODY, a low-dose sulfonylurea for HNF1A and HNF4A, a switch to oral glibenclamide for potassium-channel neonatal diabetes, and insulin for INS and other permanent neonatal diabetes. [1] [3]