Primary Effects of HCM Mutations in Humans And Cats

Hypertrophic cardiomyopathy (HCM) is the most commonly inherited cardiomyopathy. In cats it is even more common: 15% of a large population of unselected outbred cats has HCM. HCM-causing mutations in sarcomeric proteins have been proposed to increase myofilament Ca2+-sensitivity but may have additional primary effects.We have previously found that in dilated cardiomyopathy (DCM)-causing mutations in thin filament proteins the relationship between phosphorylation at Ser 22 and 23 of cardiac troponin I (TnI) and Ca2+-sensitivity was abolished. This phenomenon has been termed ‘uncoupling’ and has been found for all thin-filament protein DCM-causing mutations investigated in vitro and in skinned myofibrils. It is associated with a blunted response to β1-adrenergic stimulation and reduced cardiac reserve that is potentially disease-causing.Interestingly, uncoupling might also be a characteristic of HCM. Uncoupling has been demonstrated in a number of HCM mutants in TnI and troponin T. To determine whether either of these phenomena is common to HCM mutations we have studied them using the in vitro motility assay. We have looked at mutations in TPM1 (E180G) and TNNT2 (Δ14, Δ28+7, R92L, R92Q, ΔE160, S179F, K273E and K280N) using recombinant and mutant cardiac troponin. All of these mutations showed uncoupling. The ACTC E99K mutation showed uncoupling in both isolated thin filaments and single myofibrils.Preliminary experiments were carried out with troponin from a Ragdoll cat heart with HCM due to the R820W mutation in MYBPC3 and a non-affected cat as control. The mutant thin filaments showed an increase in Ca2+-sensitivity and were also uncoupled. Thus, troponin from HCM cats was abnormal even though the mutation was in MyBP-C that was not present in the assay.Overall these results suggest the Ca2+-sensitising and uncoupling properties of HCM mutations may be more widely distributed than previously thought.