Date of Award

2000

Degree Type

Thesis

Degree Name

Bachelor of Science Honours

Faculty

Faculty of Communication, Health and Science

First Advisor

Luba Kalaydjieva

Second Advisor

Dr Dora Ancgelicheva

Abstract

Galactokinase deficiency is an autosomal-recessive inborn error of galactose metabolism whose major clinical manifestation is the development of cataracts during the first months of life. This metabolic disorder is caused by defects in the first enzyme of the Leloir pathway, galactokinase, encoded by the gene GALK1 on chromosome 17q24. Despite the identification of a number of conserved domains in GALK1, understanding of the functional significance of these regions and the molecular basis of the disorder is limited. This is largely due to the rarity of the disease and the fact that the small number of GALK1 mutations identified to-date are confined to individual families, thus precluding extensive genotype/phenotype correlations. The current investigation involved an analysis of the GALK1 gene in nine patients with biochemical phenotypes indicative of classical galactokinase deficiency. Sequencing of the entire coding region, flanking intronic sequence and both the 5'UIR and 3’UTR of GALK1 using Dye Terminator chemistry and the ABI377 DNA Analyzer, revealed four novel mutations in two unrelated patients with galactokinase deficiency. Three of these were amino acid substitutions: 1569C→ Tin exon 2 (R68C), 7093C→ Tin exon 6 (f288M) and 7538G→ C in exon 8 (A384P). In addition, a single base deletion was found in exon 5 (2833delC), predicted to result in a shift of the reading frame and a premature termination codon at position 263. Sequence analysis in a third patient detected a 563C→ A transversion (P28T), previously identified as a common mutation in six Bulgarian Gypsy families (Kalaydjieva et al., 1999). The remaining six patients, belonging to socially and geographically dispersed Gypsy groups from Spain, Hungary and Bulgaria were screened for P28T using a PCR-based Ava I restriction assay. All were homozygous for P28T. Haplotype analysis established a common origin of the mutation, providing further evidence that P28T is a founder mutation among patients with Gypsy ethnicity and signaling the mutation is more widespread than initial indications suggest. To determine the carrier frequency and distribution of the P28T mutation, 227 unrelated individuals originating from various Gypsy groups in Europe, were screened using the Ava I assay. Carrier frequency in the Vlax Roma was calculated at 2.7% and 1.8% in Spanish Gypsies. These results indicate that the prevalence of the P28T mutation warrants further investigation preceeding the development of newborn screening strategies and dietary intervention in these groups in a bid to prevent infantile blindness. The findings also allow the identification of a large sample of carriers suitable for future investigations into genotype/ phenotype correlations. Sequence analysis of the GALK1 gene in two Afro/Hispanic patients presenting with a rare atypical phenotype with metabolic symptoms of galactokinase deficiency and normal RBC galactokinase activity, revealed no plausible disease-causing mutations. In fact, lack of similarity in the GALK1 gene in both patients, despite sharing a common ethnic background, suggested exclusion of GALK1 in this phenotype.

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