Molecular characterization of galactokinase deficiency in Japanese patients

Galactokinase (GALK) deficiency is an autosomal recessive disorder, which c auses cataract formation in children not maintained on a lactose-free diet. We characterized the human GALK gene by screening a Japanese genomic DNA p hage library, and found that several nucleotides in the 5'-untranslated reg ion and introns 1, 2, and 5 in our GALK genomic analysis differed from publ ished data. A 20-bp tandem repeat was found in three places in intron 5, wh ich were considered insertion sequences. We identified five novel mutations in seven unrelated Japanese patients with GALK deficiency. There were thre e missense mutations and two deletions. All three missense mutations (R256W , T344M, and G349S) occurred at CpG dinucleotides, and the T344M and G349S mutations occurred in the conserved region. The three missense mutations le d to a drastic reduction in GALK activity when individual mutant cDNAs were expressed in a mammalian cell system. These findings indicated that these missense mutations caused GALK deficiency. The two deletions, of 410delG an d 509-510delGT, occurred at the nucleotide repeats GGGGGG and GTGTGT, respe ctively, and resulted in in-frame nonsense codons at amino acids 163 and 20 1. These mutations arose by slipped strand mispairing. All five mutations o ccurred at hot spots in the CpG dinucleotide for missense mutations and in short direct repeats for deletions. These five mutations in Japanese have n ot yet been identified in Caucasians. We speculate that the origin of GALK mutations in Japanese is different from that in Caucasians.