VUGREK Lab Profile

Research Focus: Molecular mechanisms of S-Adenosylhomocysteine hydrolase deficiency

S-adenosylhomocysteine hydrolase (SAHH) catalyzes the reversible hydrolysis of S-adenosylhomocysteine (SAH) to adenosine (Ado) and L-homocysteine (Hcy). In eukaryotes, this is the major route for disposal of the SAH formed as a product of many S-adenosylmethionine (SAM)-dependent methyltransferases. Thus, SAH hydrolysis is believed to play a critical role in the regulation of biological methylation, which on the other hand is important for post-translational modifications. Also, methylation is an important part of the histone code that regulates cell type–specific gene expression programs. Interestingly, aberrant methylation is a widespread phenomenon in cancer and may be among the earliest changes during oncogenesis. Aberrant methylation processes might be the reason for the severe pathologic effect of the new genetic disorder of SAHH deficiency in human. Namely, SAHH deficiency results in an intracellular accumulation of SAH (150-fold) and SAM (30-fold). SAH is a strong competitive inhibitor of many SAM-dependent methyltransferases. Therefore, the significant imbalance in the SAM/SAH ratio in SAHH deficient patients might be the reason for other abnormalities such as myopathy, retarded psychomotor development and signs of mental retardation (Baric et al, 2004). Further, bioinformatics results from the human genome project show that there exist a large number of methyltransferases, but their substrates have not been characterised yet. Methylation may thus be far more important than previously thought, which is consistent with the severe pathologic effect of SAHH deficiency.
The molecular basis for SAHH deficiency has not yet been resolved. However, several individuals with extremely high levels of SAH share not yet characterized point mutations in the SAHH gene. The severeness of the disease is variable and causes serious health problems with a possible lethal outcome if some of the point mutations occur in combination. To elucidate the impact of each of the point mutations on the enzymatic activity of SAHH, we made recombinant protein for functional analysis. Using plasmid expression vectors and heterologous expression in bacteria we have identified a temperature sensitive mutation. The functional analysis will allow conclusions about the involvement of the identified mutations in protein activity and herein serve as basis for our hypothesis that irregular methylation processes caused by inactivation of S-Adenosylhomocysteine hydrolase are the reason for the severe pathologic effect of the new genetic disorder of SAHH deficiency in human.