Maple Syrup Urine Disease, Type 1b (BCKDHB)
Maple syrup urine disease, type 1b is an autosomal recessive disorder that is caused by pathogenic variants in the gene BCKDHB. While it is found in different ethnicities around the world, it is more prevalent in individuals of Ashkenazi Jewish descent due to the presence of a founder mutation. There are several different presentations of maple syrup urine disease, but the most common, and the most severe, is the classic infantile presentation. Affected infants cannot break down certain amino acids, and buildup of toxic metabolites occurs and gives the urine a sweet scent. Without intervention, affected infants will experience vomiting and lethargy and will develop seizures and ataxic movements. Progression to coma and death within the first weeks of life will occur if untreated. Although dietary management of the disease is possible, the outcome is unpredictable and a significant portion of affected individuals are mentally impaired or experience neurological complications. Some patients may have a later onset of symptoms, but the disease may still cause brain damage or death. It is not currently possible to predict the severity of the disease based on the genotype.
For information about carrier frequency and residual risk, please see the residual risk table.