Gene preference in maple syrup urine disease

Abstract

Untreated maple syrup urine disease (MSUD) results in mental and physical disabilities and often leads to neonatal death. Newborn-screening programs, coupled with the use of protein-modified diets, have minimized the severity of this phenotype and allowed affected individuals to develop into productive adults. Although inheritance of MSUD adheres to rules for single-gene traits, mutations in the genes for E1alpha, E1beta, or E2 of the mitochondrial branched-chain alpha-ketoacid dehydrogenase complex can cause the disease. Randomly selected cell lines from 63 individuals with clinically diagnosed MSUD were tested by retroviral complementation of branched-chain alpha-ketoacid dehydrogenase activity to identify the gene locus for mutant alleles. The frequencies of the mutations were 33% for the E1alpha gene, 38% for the E1beta gene, and 19% for the E2 gene. Ten percent of the tested cell lines gave ambiguous results by showing no complementation or restoration of activity with two gene products. These results provide a means to establish a genotype/phenotype relationship in MSUD, with the ultimate goal of unraveling the complexity of this single-gene trait. This represents the largest study to date providing information on the genotype for MSUD.

Figure 1

Western blot of BCKD proteins in mitochondria. Mitochondria were prepared from EM6229 cells without viral transduction or after transduction with either the E2 or E1β vector, and 20 μg were resolved for immunoblots. Mitochondria from DG75 cells serve as a wild-type control line.

Figure 2

Percentage of cell lines with mutations in each of three genes that cause MSUD. A, Results of 63 cell lines using restoration of BCKD activity assay. Twenty-nine females and 34 males, excluding those with known Mennonite heritage, were tested. B, Summary of percentages based on mutations described elsewhere.