Molecular Basis of the Disease

Galactosemia is an autosomal recessive disorder caused by deficient or absent activity of one of three enzymes involved in the metabolic pathway to convert galactose to glucose: galactokinase (GALK), galactose-1-phosphate uridyl transferase (GALT), and UDP-galactose 4'-epimerase (GALE). The predominant form is classic galac-tosemia, which is due to a severe reduction or absence of the GALT enzyme, and has an incidence of 1 in 40,000 to 60,000 in European newborns (for review, see Reference 15).

The symptoms of classic galactosemia in neonates include poor feeding, vomiting, failure to thrive, lethargy, jaundice, occasionally diarrhea, and E. coli sepsis. The symptoms in an affected newborn can be obviated if a lactose-free diet is initiated within the first two weeks of life. Newborn screening for galactosemia is included in most states in the United States. Newborns with a positive screen are followed up with immediate dietary treatment and confirmatory biochemical analysis.

The GALT gene is located at 9q13, is about 4kb in length, and consists of 11 exons. More than 150 mutations in the GALT gene have been reported, most of which are private mutations.16 Q188R is the most frequent mutation associated with classic galactosemia in many populations, and accounts for 64% of disease alleles in Europeans, 60% to 70% in Americans, and 50% to 58% in Mexican Hispanics.16,17 Ethnic-specific mutations include K285N, S135L, IVS2-2A^G, and a 5kb deletion in Caucasian, African American, Hispanic, and Jewish patients, respectively. Due to differing ethnic backgrounds in different regions of the United States, disease allele prevalence may vary in different regions of the United States. For example, Q188R, S135L, K285N, L198P, Y209C, and F171S were reported to be the most prevalent mutations in a study based on individuals from the state of Georgia, while Q188R, K285N, IVS2-2A^G, S135L, and T138M are the most common mutant alleles observed in Texas newborns.18,19

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