Triose Phosphate Isomerase Deficiency Is Caused by Altered Dimerization–Not Catalytic Inactivity–of the Mutant Enzymes

Abstract

Triosephosphate isomerase (TPI) deficiency is an autosomal recessive disorder caused by various mutations in the gene encoding the key glycolytic enzyme TPI. A drastic decrease in TPI activity and an increased level of its substrate, dihydroxyacetone phosphate, have been measured in unpurified cell extracts of affected individuals. These observations allowed concluding that the different mutations in the TPI alleles result in catalytically inactive enzymes. However, despite a high occurrence of TPI null alleles within several human populations, the frequency of this disorder is exceptionally rare. In order to address this apparent discrepancy, we generated a yeast model allowing us to perform comparative in vivo analyses of the enzymatic and functional properties of the different enzyme variants. We discovered that the majority of these variants exhibit no reduced catalytic activity per se. Instead, we observed, the dimerization behavior of TPI is influenced by the particular mutations investigated, and by the use of a potential alternative translation initiation site in the TPI gene. Additionally, we demonstrated that the overexpression of the most frequent TPI variant, Glu104Asp, which displays altered dimerization features, results in diminished endogenous TPI levels in mammalian cells. Thus, our results reveal that enzyme deregulation attributable to aberrant dimerization of TPI, rather than direct catalytic inactivation of the enzyme, underlies the pathogenesis of TPI deficiency. Finally, we discovered that yeast cells expressing a TPI variant exhibiting reduced catalytic activity are more resistant against oxidative stress caused by the thiol-oxidizing reagent diamide. This observed advantage might serve to explain the high allelic frequency of TPI null alleles detected among human populations.

Degradation of Functional Triose Phosphate Isomerase Protein UnderliessugarkillPathology

“…Alternatively, it is possible that mutations abrogating isomerase activity are lethal (do not cause disease) and that the specific mutations that result in disease disrupt a hitherto novel function of TPI. This interpretation is consistent with recent findings in a yeast TPI model, where transgenic expression of multiple human mutant TPI proteins in vivo demonstrated catalytic isomerase activity associated with the pathogenic mutant proteins (Ralser et al 2006).…”

“…Studies have also documented markedly elevated DHAP levels, suggesting that the anemia may be caused by toxicity of DHAP or one of its catabolites (Zanella et al 1985;Eber et al 1991;Hollan et al 1997;Karg et al 2000;Olah et al 2002Olah et al , 2005. Interestingly, DHAP levels in nonerythrocytes are only modestly increased, presumably owing to it being a substrate for phospholipid biosynthesis in other cell types, and a yeast model of TPI deficiency reports that proteins bearing the human disease mutations maintain catalytic activity (Orosz et al 2001(Orosz et al , 2006Ralser et al 2006).…”

The Response to Heat Shock and Oxidative Stress inSaccharomyces cerevisiae

“…Hence, any growth condition that influences glycolytic activity can potentially alter the flux of glucose equivalents through the pentose phosphate pathway leading to the generation of NADPH. This has been shown experimentally, since reducing the activity of glycolytic enzymes such as triosephosphate isomerase (TPI) or glyceraldehyde 3-phosphate dehydrogenase (GAPDH) confers resistance against oxidative stress conditions (Ralser et al 2006(Ralser et al , 2007. Quantitative metabolomic analysis has directly confirmed that reduced TPI or GAPDH activity redirects glucose equivalents to the pentose phosphate pathway, increasing the concentration of pentose phosphate pathway metabolites and shifting the NADPH/ NADP + ratio to a more reduced state (Ralser et al 2007).…”

In silico prediction of the effects of mutations in the human triose phosphate isomerase gene: Towards a predictive framework for TPI deficiency

“…Indeed methylglyoxal has already been shown to be neurotoxic and therefore may contribute to the neurological impairment seen in many TPI deficiency patients (Ahmed et al, 2003;de Arriba et al, 2006;de Arriba et al, 2007). Abnormal dimerisation behaviour of diseaseassociated TPI variants has been suggested as a potential contributing factor in the pathogenesis of TPI deficiency (Ralser et al, 2006;Rodriguez-Almazan et al, 2008;Roland et al, 2016). In recent years the suggestion has arisen that this may be as a consequence of the formation of toxic protein aggregates in the brain due to the misfolding of the variant TPI (Olah et al, 2002;Olah et al, 2005)).…”