Low catalytic activity is insufficient to induce disease pathology in triosephosphate isomerase deficiency.

Joanna Segal,Bernd Timmermann,Michael Mülleder,Ludger Hartmann,Heinrich Schrewe,Martin Klingenspor,Antje Krüger,Thure Adler,Beata Lukaszewska‐Mcgreal,Markus Ralser,Helmut Fuchs,Sabine M Hölter,Lillian Garrett,Thomas Klopstock,Mariia Yuneva,Ingo Voigt,Hans Lehrach,Julia Calzada‐Wack,Dirk H Busch,Birgit Rathkolb,Jan Rozman,Wolfgang Wurst,Valérie Gailus‐Durner,Manuela Scholze‐Wittler,Steve Michel,Nana‐Maria Grüning,Ralf Fischer,Eckhard Wolf,Lore Becker,Frauke Neff,Martin Hrabě De Angelis ,Ildikó Rácz

doi:10.1002/jimd.12105

Abstract

Triosephosphate isomerase (TPI) deficiency is a fatal genetic disorder characterized by hemolytic anemia and neurological dysfunction. Although the enzyme defect in TPI was discovered in the 1960s, the exact etiology of the disease is still debated. Some aspects indicate the disease could be caused by insufficient enzyme activity, whereas other observations indicate it could be a protein misfolding disease with tissue‐specific differences in TPI activity. We generated a mouse model in which exchange of a conserved catalytic amino acid residue (isoleucine to valine, Ile170Val) reduces TPI specific activity without affecting the stability of the protein dimer. TPIIle170Val/Ile170Val mice exhibit an approximately 85% reduction in TPI activity consistently across all examined tissues, which is a stronger average, but more consistent, activity decline than observed in patients or symptomatic mouse models that carry structural defect mutant alleles. While monitoring protein expression levels revealed no evidence for protein instability, metabolite quantification indicated that glycolysis is affected by the active site mutation. TPIIle170Val/Ile170Val mice develop normally and show none of the disease symptoms associated with TPI deficiency. Therefore, without the stability defect that affects TPI activity in a tissue‐specific manner, a strong decline in TPI catalytic activity is not sufficient to explain the pathological onset of TPI deficiency.

Highlights

Triosephosphate isomerase (TPI) deficiency was among the first human genetic disorders discovered by systematic enzymological screens more than 60 years ago.[1]
The syndrome is characterized by severe progressive neuromuscular degeneration, neurologic dysfunction that is associated with impaired synaptic vesicle dynamics, hemolytic anemia and associated susceptibility to infections, and episodic hypotonia.[2,3,4]
The answer to the disease etiology question is not provided by two mouse models either. One of these carries an aspartate to glycine substitution at codon 49, positioned in the dimer interface,[27] and a second has a phenylalanine to serine substitution at amino acid 57,28 which affects the stability of the protein and leads to its degradation. Both mouse models recapitulate the key symptom of TPI deficiency, hemolytic anemia, but, as they both combine a stability defect with a decline in TPI activity that penetrates in a tissue-specific manner, they do not enable the distinguishing of the consequences of activity decline from structural defect

Summary

| INTRODUCTION

Triosephosphate isomerase (TPI) deficiency was among the first human genetic disorders discovered by systematic enzymological screens more than 60 years ago.[1]. Roland et al obtained a surprising result upon expressing the sugar kill allele in combination with a catalytically inactive TPI mutant allele, TPIMet11Lys.[26] This compound heterozygous mutant was rescued from the behavioral and longevity phenotypes observed in the sugar kill homozygotes without restoring the overall TPI activity In this particular model, it is not TPI activity but the degradation of the protein that causes all symptoms. One of these carries an aspartate to glycine substitution at codon 49, positioned in the dimer interface,[27] and a second has a phenylalanine to serine substitution at amino acid 57,28 which affects the stability of the protein and leads to its degradation Both mouse models recapitulate the key symptom of TPI deficiency, hemolytic anemia, but, as they both combine a stability defect with a decline in TPI activity that penetrates in a tissue-specific manner, they do not enable the distinguishing of the consequences of activity decline from structural defect. In the absence of the protein stability defect that produces tissue-specific TPI activity differences, a decline in TPI activity is not sufficient to cause symptoms of TPI deficiency

| RESULTS

| DISCUSSION

| MATERIALS AND METHODS

Findings

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