Molecular basis for Duarte and Los Angeles variant galactosemia.

Human orythrocytes that are homozygous for the Duarte enzyme variant of galactosemia (D/D) have a characteristic isoform on isoelectric focusing and 50% reduction in galactose-1-phosphate uridyltransferase (GALT) enzyme activity.
The Duarte biochemical phenotype has a molecular genotype of N314D/N314D. The characteristic Duarte isoform is also associated with a variant called the " Los Angeles (LA) phenotype, " which has increased GALT enzyme activity.
We evaluated GALT enzyme activity and screened the GALT genes of 145 patients with one or more N314D-containing alleles.
We found seven with the LA biochemical phenotype, and all had a 1721C-- > T transition in exon 7 in cis with the N314D missense mutation.
The 1721C-- > T transition is a neutral polymorphism for leucine at amino acid 218 (L218L). In pedigree analyses, this 1721C-- > T transition segregated with the LA phenotype of increased GALT activity in three different biochemical phenotypes (LA/N, LA/G, and LA/D). To determine the mechanism for increased activity of the LA variant, we compared GALT mRNA, protein abundance, and enzyme thermal stability in lymphoblast cell lines of D and LA phenotypes with comparable genotypes.
GALT protein abundance was increased in LA compared to D alleles, but mRNA was similar among all genotypes.
When LA/D and D/D GALT biochemical phenotypes were compared to N/N GALT phenotypes, both had 50%, as compared to 21%, reduction in GALT activity in the wild type (N/N) after exposure at identical initial enzyme activity to 50 degrees C for 15 min.
We conclude that the codon change N314D in cis with the base-pair transition 1721C-- > T produces the LA variant of galactosemia and that this nucleotide change increases GALT activity by increasing GALT protein abundance without increasing transcription or decreasing thermal lability.
A favorable codon bias for the mutated codon with consequently increased translation rates is postulated as the mechanism.. 
