In muscle cytochrome oxidase (COX) negative fibers (mitochondrial mosaics) have often been visualized.

Background/Objectives: Triosephosphate Isomerase (TPI) is a glycolytic enzyme known to be associated with TPI deficiency, a severe form of childhood-onset glycolytic enzymopathy associated with hemolytic anemia, neuromuscular impairment and early death. Most often the disease results from the common TPI1^E105D mutation, which can be either homozygous or compound heterozygous with another allele. Methods: We purified TPI^R5G protein, studied mutant protein biochemistry, established and characterized TPI^R5G/f.s.patient cells, and investigated newly identified compounds for their efficacy in vitro using Western blot and TPI activity assays. Results: We identified novel TPI1 alleles that result in TPI Deficiency with an atypical presentation lacking anemia and with more slowly developing neurologic and locomotor impairment. The patient was found to be compound heterozygous with a missense mutation resulting in an R5G amino acid substitution and a frameshift mutation that is a predicted null allele. To better understand disease pathogenesis in this patient, we expressed and purified the TPI^R5G human protein and studied it biochemically in addition to studying TPI^R5G/f.s.patient cells. We discovered that purified TPI^R5G protein has wildtype activity with modestly increased dimer stability. We also discovered that steady-state TPI protein levels were markedly reduced, suggesting that the instability of the mutant protein underlies disease pathogenesis. We tested compounds recently identified in a screen for novel TPI Df therapies for their efficacy in TPI^R5G/f.s.patient cells. All three compounds significantly increased TPI protein levels in patient cells. As expected, since the mutant protein retains essentially wild type activity, the increase in TPI protein levels also resulted in a significant increase in TPI activity. Conclusions: These results establish TPI^R5G as a TPI Df allele, demonstrate that reduced stability of the mutant protein underlies pathogenesis akin to other disease-causing alleles, and suggest that recently discovered developing therapies will likely function broadly and should be developed as potential TPI Df therapies.