The contribution of molecular genetic methods to the diagnosis of classical galactosemia and investigation of genotype-phenotype correlation

Objective: Classical galactosemia is a disorder caused by pathogenic GALT gene variants leading to deficiency of galactose 1-phosphate uridyl transferase enzyme. This condition is inherited in an autosomal recessive pattern. Although most of the pathogenic variants identified are small sequence alterations, gross mutations have also been reported (1%) in GALT gene, which has 11 exons. We aimed to reveal genetic ethiology of patients with classical galactosemia and investigate the frequency and distribution of gross and small GALT mutations for molecular diagnostic approach and search for genotype-phenotype correlation. Material-Methods: Clinically diagnosed 90 patients with galactosemia were included in this study. DNA samples were isolated from peripheral blood and algorithmic testing strategy for GALT gene (NM_000155.3) was performed with Sanger sequencing in all study group, and MLPA in selected patients with normal sequencing result. Results: We identified 17 known mutations and four novel variants (p.R67Pfs*19, p.S236Rfs*30, p.S156*, p.V243I) in 90 patients, but no gross alteration by MLPA analysis. In this study, the most common mutations are p.Q188R(42%) in exon 6, p.E340* (15%) in exon 10 and p.R67Pfs*19 in exon 2. The mutation detection rate was 90% through Sanger sequencing. Conclusions: According to our results, the frequency of p.Q188R in Turkish popupation was the most common mutation as in Europe. Second most frequent mutation was p.E340* which is known to be specific for population in Turkey. The third most frequent mutation (8%) was a novel single nucleotide deletion leading to protein truncation (p.R67Pfs*19), found to aggregate in patients from eastern Turkey. It may be concluded that an algorithmic analysis of exons 6 and 10 would delineate molecular genetic diagnosis for 65% of the patients. The negative MLPA result may be explained by drift away from the prediagnosis after clinical re-evaluation of the patients. In the case that there is no pathogenic variant is found in sequencing, clinical re-evaluation should be considered before MLPA.