Methylmalonic Aciduria and Homocystinuria, Cobalamin D Type (MMADHC)
Methylmalonic aciduria and homocystinuria, cobalamin D type is an autosomal recessive, pan-ethnic disease caused by pathogenic variants in the MMADHC gene. The clinical features are variable, and patients may have either isolated methylmalonic aciduria, isolated homocystinuria, or a combination of both. Onset may be in infancy, childhood or adolescence. The most common presentation is during the newborn period, where a previously normal infant begins vomiting, and develops lethargy and hypotonia due to an excess of ammonia in the blood. Without immediate treatment, the resulting brain disease can be fatal. Later-onset forms of the disease may also be fatal if not identified and treated promptly. Even with treatment, some patients experience long-term complications. Life expectancy is variable depending on the number and length of metabolic crises and the severity of the resulting damage. No clear genotype-phenotype correlation is known.
For information about carrier frequency and residual risk, please see the residual risk table.