In the thick of it: HCM-causing mutations in myosin binding proteins of the thick filament

Abstract

In the 20 years since the discovery of the first mutation linked to familial hypertrophic cardiomyopathy (HCM), an astonishing number of mutations affecting numerous sarcomeric proteins have been described. Among the most prevalent of these are mutations that affect thick filament binding proteins, including the myosin essential and regulatory light chains and cardiac myosin binding protein (cMyBP)-C. However, despite the frequency with which myosin binding proteins, especially cMyBP-C, have been linked to inherited cardiomyopathies, the functional consequences of mutations in these proteins and the mechanisms by which they cause disease are still only partly understood. The purpose of this review is to summarize the known disease-causing mutations that affect the major thick filament binding proteins and to relate these mutations to protein function. Conclusions emphasize the impact that discovery of HCM-causing mutations has had on fueling insights into the basic biology of thick filament proteins and reinforce the idea that myosin binding proteins are dynamic regulators of the activation state of the thick filament that contribute to the speed and force of myosin-driven muscle contraction. Additional work is still needed to determine the mechanisms by which individual mutations induce hypertrophic phenotypes.

Figure 1. Myosin Binding Proteins of the Thick Filament

Simplified diagram depicting major proteins of the thick and thin filaments. Myosin ELC and RLC are shown binding to myosin S1 near the junction with myosin S2. cMyBP-C is shown in a hypothetical arrangement extending from the thick filament to the thin filament, but the precise position of cMyBP-C relative to myosin is not known. Individual cMyBP-C domains are numbered C0-C10 starting at the N-terminus of cMyBP-C; PA denotes a proline/alanine rich linker sequence between C0 and C1; m indicates the position of the cMyBP-C regulatory motif between domains C1 and C2 that contains PKA phosphorylation sites. TnC, troponin C; TnI, troponin I; TnT, troponin T; Tm, tropomyosin; LMM, light meromyosin.

Figure 2. MYBPC3 Sequence Variants Linked to HCM

Mutations in the MYBPC3 gene shown relative to individual domains of the cMyBP-C protein. Blue, Missense mutations that cause single amino acid substitutions; Green, Insertions or deletions predicted to cause reading frameshifts; Red, Nonsense mutations predicted to introduce premature termination codons; Pink, Splice site donor/acceptor mutations; Gold, In-frame insertions or deletions. Domain boundaries were as described by Carrier et al..

Figure 3. cMyBP-C HCM Missense Mutations by Domain

Multiple variants at the same codon or variants in consecutive codons are shown in bold.

Figure 4. Incorporation of the A31P cMyBP-C Missense Mutation in Sarcomeres of A31P Affected Cats

Top Row: Immunofluorescent localization of cMyBP-C (green) was performed using a polyclonal antibody against cMyBP-C. The antibody recognized cMyBP-C in myofibrils from (A) unaffected wild-type, (B) heterozygous, and (C) homozygous A31P cats. Counter-staining with an antibody to myomesin (red) in (A) indicates M-line position. Bottom Row: Immunofluorescent localization of A31P mutant cMyBP-C (green) was performed using an affinity purified antibody specific for the A31P mutation. The antibody did not cross-react with wild-type cMyBP-C in myofibrils from (D) unaffected wild-type cats, but recognized A31P cMyBP-C in myofibrils from (E) heterozygous and (F) homozygous A31P cats. Counter-staining with an antibody to myomesin (red) indicates M-line position in (D). (G), Merged image of the same myofibril as shown in (F) showing counter-staining with an antibody to myomesin (red) to indicate M-line position.