Maple Syrup Urine Disease Type 1B
Summary
Maple syrup urine disease (MSUD) is a metabolic condition where the body cannot break down three branched-chain amino acids — leucine, isoleucine, and valine — found in all protein-containing foods. These amino acids and their toxic byproducts accumulate in blood and tissues, causing a characteristic sweet maple syrup odour in urine and earwax. The BCKDHB gene encodes one subunit of the branched-chain alpha-keto acid dehydrogenase complex that processes these amino acids.
Practical takeaway
This matters when planning children. Carrier testing through a GP or genetic counselling service can determine your partner's status. If you are both carriers, a genetic counsellor can discuss dietary management, newborn screening protocols, and the importance of metabolic emergency planning. The reassurance: with early detection and dietary management, children with MSUD can achieve normal intellectual development. Realised does not calculate reproductive risk or advise on reproductive decisions.
Evidence detail
What Carrier Status Means For You
You are not affected. One working copy of BCKDHB produces sufficient enzyme for normal branched-chain amino acid metabolism. Your carrier status becomes relevant if your partner also carries a BCKDHB variant (or a variant in BCKDHA or DBT, the other MSUD genes). In that case, each child has a 25% chance of having MSUD, 50% carrier, 25% unaffected.
Population Context
MSUD is most common in the Old Order Mennonite community (~1 in 380 births, carrier frequency ~1 in 10) due to a founder effect. In the Ashkenazi Jewish population, carrier frequency is approximately 1 in 100-120. General population incidence is approximately 1 in 185,000. The condition occurs in all ethnic groups.
Limitations
23andMe tests 2 of over 80 known pathogenic BCKDHB variants. MSUD can also be caused by variants in BCKDHA and DBT — a negative BCKDHB result does not rule out MSUD carrier status via other genes. Comprehensive carrier screening through a clinical laboratory covers all three MSUD genes and more variants.