Altered Energetics of Exercise Explain Risk of Rhabdomyolysis in Very Long-Chain Acyl-CoA Dehydrogenase Deficiency.

Rhabdomyolysis is common in very long-chain acyl-CoA dehydrogenase deficiency (VLCADD) and other metabolic myopathies, but its pathogenic basis is poorly understood. Here, we show that prolonged bicycling exercise against a standardized moderate workload in VLCADD patients is associated with threefold bigger changes in phosphocreatine (PCr) and inorganic phosphate (Pi) concentrations in quadriceps muscle and twofold lower changes in plasma acetyl-carnitine levels than in healthy subjects. This result is consistent with the hypothesis that muscle ATP homeostasis during exercise is compromised in VLCADD. However, the measured rates of PCr and Pi recovery post-exercise showed that the mitochondrial capacity for ATP synthesis in VLCADD muscle was normal. Mathematical modeling of oxidative ATP metabolism in muscle composed of three different fiber types indicated that the observed altered energy balance during submaximal exercise in VLCADD patients may be explained by a slow-to-fast shift in quadriceps fiber-type composition corresponding to 30% of the slow-twitch fiber-type pool in healthy quadriceps muscle. This study demonstrates for the first time that quadriceps energy balance during exercise in VLCADD patients is altered but not because of failing mitochondrial function. Our findings provide new clues to understanding the risk of rhabdomyolysis following exercise in human VLCADD.

Hyper IgD syndrome (HIDS) associated with in vitro evidence of defective monocyte TNFRSF1A shedding and partial response to TNF receptor blockade with etanercept.

Hereditary periodic fever syndromes comprise a group of distinct disease entities linked by the defining feature of recurrent febrile episodes. Hyper IgD with periodic fever syndrome (HIDS) is caused by mutations in the mevalonate kinase (MVK) gene. The mechanisms by which defects in the MVK gene cause febrile episodes are unclear and there is no uniformly effective treatment. Mutations of the TNFRSF1A gene may also cause periodic fever syndrome (TRAPS). Treatment with the TNFR-Fc fusion protein, etanercept, is effective in some patients with TRAPS, but its clinical usefulness in HIDS has not been reported. We describe a 3-year-old boy in whom genetic screening revealed a rare combination of two MVK mutations producing clinical HIDS as well as a TNFRSF1A P46L variant present in about 1% of the population. In vitro functional assays demonstrated reduced receptor shedding in proband's monocytes. The proband therefore appears to have a novel clinical entity combining Hyper IgD syndrome with defective TNFRSF1A homeostasis, which is partially responsive to etanercept.