The ubiquitin-proteasome system and nonsense-mediated mRNA decay in hypertrophic cardiomyopathy

Abstract

Cardiomyopathies represent an important cause of cardiovascular morbidity and mortality due to heart failure, arrhythmias, and sudden death. Most forms of hypertrophic cardiomyopathy (HCM) are familial with an autosomal-dominant mode of inheritance. Over the last 20 years, the genetic basis of the disease has been largely unravelled. HCM is considered as a sarcomeropathy involving mutations in sarcomeric proteins, most often beta-myosin heavy chain and cardiac myosin-binding protein C. 'Missense' mutations, more common in the former, are associated with dysfunctional proteins stably integrated into the sarcomere. 'Nonsense' and frameshift mutations, more common in the latter, are associated with low mRNA and protein levels derived from the diseased allele, leading to haploinsufficiency of the remaining healthy allele. The two quality control systems responsible for the removal of the affected mRNAs and proteins are the nonsense-mediated mRNA decay (NMD) and the ubiquitin-proteasome system (UPS), respectively. This review discusses clinical and genetic aspects of HCM and the role of NMD and UPS in the regulation of mutant proteins, evidence for impairment of UPS as a pathogenic factor, as well as potential therapies for HCM.

Figure 1

The NMD. (A) A PTC located more than 50–55 nucleotides (nt) upstream of the last exon–exon junction within the mRNA (green region) elicits NMD, whereas mRNAs with PTCs downstream of this boundary (red region) escape NMD. (B) Mechanism of the action of NMD. During pre-mRNA splicing, a protein complex called exon junction complex (EJC) is deposited 20–24 nucleotides upstream of every exon–exon junction and functions hereby as a marker to dictate whether a stop codon is premature or not. The EJC containing the up-frameshift (UPF) 3 protein remains bound to the mRNA when exported to the cytoplasm. Here, a second NMD core protein, UPF2, binds to UPF3. In normal mRNAs, the EJCs are then displaced by the ribosome during the pioneer round of translation, and translation stops when the ribosome reaches the normal stop codon (B, top). In contrast, in PTC-bearing mRNAs, the ribosome is blocked at the PTC, and the EJC downstream of the PTC remains associated with the mRNA, because it cannot be displaced by the ribosome (B, bottom). This results in attachment of the so-called SURF complex, which comprises SMG-1 (suppressor with morphogenetic effect on genitalia), UPF1, and the eukaryotic release factors (eRF) 1 and eRF3, to the ribosome. Binding of UPF2 to UPF1 leads to its phosphorylation by SMG-1, which in turn drives dissociation of eRF1 and eRF3 and binding of SMG-7. Ultimately, the mRNA is degraded by different pathways including decapping or deadenylation. Figure adapted from Garneau et al.

Figure 2

The UPS. The UPS functions as an ATP-dependent proteolytic system that requires polyubiquitination via lysine 48 residues of the target protein prior to its degradation by the 26S proteasome. Polyubiquitination involves the concerted action of three enzymes: E1 (ubiquitin-activating), E2 (ubiquitin-conjugating), and E3 (ubiquitin ligase), the latter affording substrate specificity. The eukaryotic 26S proteasome is a large, multicatalytic protein complex composed of the 19S regulatory complex and the 20S proteasome. The damaged, misfolded, or mutant proteins are degraded by three major peptidase activities (chymotrypsin-like, trypsin-like, and caspase-like).