Enolase deficiency

Symptoms and signs

Symptoms of enolase deficiency include exercise-induced myalgia and generalized muscle weakness and fatigability, both with onset in adulthood. Symptoms also include muscle pain without cramps, and decreased ability to sustain long term exercise.

Causes

Genetics is found to be the cause of enolase deficiency. The individual in the first known case of this deficiency was heterozygous for the gene for β-enolase, and carried two missense mutations, one inherited from each parent. His muscle cells synthesized two forms of β-enolase, each carrying a different mutation. These mutations changed glycine at position 374 to glutamate (G374E) and glycine at position 156 to aspartate (G156D).

Pathophysiology

The enolase enzyme catalyzes the conversion of 2-phosphoglycerate to phosphoenolpyruvate; this is the ninth step in glycolysis. Enolase is a dimeric protein formed from three subunits, α, β, and γ, encoded by different genes. The αα homodimer assumes all enolase activity in the early stages of embryo development and in some adult tissues. In tissues that need large amounts of energy, the αγ and γγ in the brain, and αβ and ββ in striated muscles these forms of enolase are present. At all stages of development, β-enolase expression is only found in striated muscles. In adult humans, the ββ homodimer accounts for more than 90% of total enolase activity in muscle.

Two mutations in the ENO3 gene, the gene encoding β-enolase, is responsible for the deficiency, both mutations changed highly conserved amino acid residues. One of the changes was of a glycine residue at position 374 to aspartate, this amino acid change was located in close proximity to the His residue of human enolase, which is an important part of the β-enolase catalytic site, while the glycine at position 156 changed to glutamate, which may have brought about change the secondary structure of the enzyme. These mutations may impair activity by significantly reducing the steady- state level of the protein, rather than produce a non- functional mutant protein. Mutations of the β-enolase dimer complexes might result in incorrect folding and increased susceptibility to protein degradation thus causing the deficiency.

Similar mutations on yeast showed destabilization of the protein and decreased substrate affinity. Destabilization of the protein results in partial dissociation, some researchers propose that in muscle cells this dissociation may be perceived as an abnormality leading to degradation of the mutated enolase.

References

References

1. (1986). "Partial erythrocyte enolase deficiency: a hereditary disorder with variable clinical expression". Blood.
2. (1977). "Assignment of the human gene for enolase 1 to region pter in equilibrium p36 of chromosome 1". Cytogenetics and Cell Genetics.
3. (October 1972). "Chronic hemolytic anemia associated with erythrocyte enolase deficiency exacerbated by ingestion of nitrofurantoin". American Journal of Clinical Pathology.
4. (August 2001). "Beta-enolase deficiency, a new metabolic myopathy of distal glycolysis". Annals of Neurology.
5. (January 2008). "Effects of the G376E and G157D mutations on the stability of yeast enolase--a model for human muscle enolase deficiency". The FEBS Journal.