DEFICIENT DNA-LIGASE ACTIVITY IN THE METABOLIC DISEASE TYROSINEMIA TYPE-I

Hereditary tyrosinemia type I (HT1) is an autosomal recessive inborn e rror of metabolism caused by the deficiency of fumarylacetoacetate hyd rolase, the last enzyme in the tyrosine catabolism pathway. This defec t results in accumulation of succinylacetone (SA) that reacts with ami no acids and proteins to form stable adducts via Schiff base formation , lysine being the most reactive amino acid. HT1 patients surviving be yond infancy are at considerable risk for the development of hepatocel lular carcinoma, and a high level of chromosomal breakage is observed in HT1 cells, suggesting a defect in the processing of DNA. In this pa per we show that the overall DNA-ligase activity is low in HT1 cells ( about 20% of the normal value) and that Okazaki fragments are rejoined at a reduced rate compared with normal fibroblasts. No mutation was f ound by sequencing the ligase I cDNA from HT1 cells, and the level of expression of the ligase I mRNA was similar in normal and HT1 fibrobla sts, suggesting the presence of a ligase inhibitor. SA was shown to in hibit in vitro the overall DNA-ligase activity present in normal cell extracts. The activity of purified T4 DNA-ligase, whose active site is also a lysine residue, was inhibited by SA in a dose-dependent manner . These results suggest that accumulation of SA reduces the overall li gase activity in HT1 cells and indicate that metabolism errors may pla y a role in regulating enzymatic activities involved in DNA replicatio n and repair.