South Asian-Specific MYBPC3 Δ25bp Deletion Carriers Display Hypercontraction and Impaired Diastolic Function Under Exercise Stress

Abstract

Background: A 25-base pair (25bp) intronic deletion in the MYBPC3 gene enriched in South Asians (SAs) is a risk allele for late-onset left ventricular (LV) dysfunction, hypertrophy, and heart failure (HF) with several forms of cardiomyopathy. However, the effect of this variant on exercise parameters has not been evaluated. Methods: As a pilot study, 10 asymptomatic SA carriers of the MYBPC3 ^Δ25bp variant (52.9 ± 2.14 years) and 10 age- and gender-matched non-carriers (NCs) (50.1 ± 2.7 years) were evaluated at baseline and under exercise stress conditions using bicycle exercise echocardiography and continuous cardiac monitoring. Results: Baseline echocardiography parameters were not different between the two groups. However, in response to exercise stress, the carriers of Δ25bp had significantly higher LV ejection fraction (%) (CI: 4.57 ± 1.93; p < 0.0001), LV outflow tract peak velocity (m/s) (CI: 0.19 ± 0.07; p < 0.0001), and higher aortic valve (AV) peak velocity (m/s) (CI: 0.103 ± 0.08; p = 0.01) in comparison to NCs, and E/A ratio, a marker of diastolic compliance, was significantly lower in Δ25bp carriers (CI: 0.107 ± 0.102; p = 0.038). Interestingly, LV end-diastolic diameter (LVID_dia) was augmented in NCs in response to stress, while it did not increase in Δ25bp carriers (CI: 0.239 ± 0.125; p = 0.0002). Further, stress-induced right ventricular systolic excursion velocity s' (m/s), as a marker of right ventricle function, increased similarly in both groups, but tricuspid annular plane systolic excursion increased more in carriers (slope: 0.008; p = 0.0001), suggesting right ventricle functional differences between the two groups. Conclusions: These data support that MYBPC3 ^Δ25bp is associated with LV hypercontraction under stress conditions with evidence of diastolic impairment.

Keywords: 25bp deletion; DOSA study; MYBPC3; South Asians; hypertrophic cardiomyopathy; myosin binding protein-C; stress echocardiography; ventricular diastolic dysfunction.

Figure 1

MYBPC3^Δ25bp genotyping and prevalence. (A) Schematic diagram of the MYBPC3 gene that includes the location of MYBPC3^Δ25bp variant in Intron 32. Exon 33 is highlighted in blue as a potential skipped exon and altered splicing exon when the MYBPC3^Δ25bp variant is pathogenic. (B) cMyBP-C domains are drawn with interacting partners, proline-alanine (Pro-Ala)-rich region, phosphorylation motif, and potential C10 domain that could be modified if exon 33 is altered (DC10). (C) Agarose gel electrophoresis of PCR-based genotyping of the MYBPC3^Δ25bp variant. (D) MYBPC3^Δ25bp distribution of 3,432 South Asian participants. Among carriers of the MYBPC3^Δ25bp variant, 6.61% of carriers (227, red color sector) were heterozygous (Het), and 0.14% of carriers (5, blue color sector) were homozygous (Homo). NCs, non-carriers.

Figure 2

Flow chart depicting the recruitment process, employed questionnaires, and echocardiography and clinical data analyses for the genotype-phenotype study. (A) Carriers of MYBPC3^Δ25bp variant and age- and gender-matched non-carriers 18 years of age and older of US SA ancestry were recruited. Then, past medical and medication history was collected using a prerecruitment questionnaire, and, if no contraindication, subjects were scheduled to perform an exercise stress test using a bicycle ergometer. (B) On the test day, informed consent was obtained; then, a comorbidity questionnaire was used to collect comprehensive medical, medication, social, habit, and family history information from all participants. To look for other genetic modifiers via NGS testing, a 10-ml blood sample was collected. Compliance with exercise stress test instructions was evaluated via a pre-procedure questionnaire. Cardiac function was evaluated by ECG and TTE at rest and by bicycle ergometer during exercise stress testing for any detectable underlying risk factor. (C) While blinded, two observers independently measured echocardiography variables. Statistical significance (p < 0.05) was calculated using two-way ANOVA and simple linear regression, and the results were reported as mean ± SEM. SA = South Asian, NGS = next-generation sequencing. TTE, transthoracic echocardiography.

Figure 3

Spectral Doppler data depict LVOT peak velocity. Representative apical five-chamber view at baseline, 45 (W), and 105 (W) in a NC (upper panel) subject and a MYBPC3^Δ25bp variant carrier (lower panel). NC, non-carriers.

Figure 4

Prevalent South Asian-specific MYBPC3^Δ25bp variant is associated with hypercontraction and impaired relaxation under exercise stress (10 NCs vs. 10 MYBPC3^Δ25bp carriers). (A) A significant difference in LVID_dia is observed between carriers of the MYBPC3^Δ25bp variant and NCs (Two-way ANOVA, p = 0.0002; CI: 0.239 ± 0.125). In response to stress, LVID_dia increases significantly from baseline in NCs, whereas it does not significantly change in MYBPC3^Δ25bp carriers. (B) The graph depicts a non-significant difference between MYBPC3^Δ25bp variant carriers and NCs in LVID_s in response to stress (Two-way ANOVA, p = 0.061; CI: 0.09 ± 0.09). In addition, carriers of the MYBPC3^Δ25bp variant show significantly higher (C) ejection fraction (CI: 4.57 ± 1.93) and (D) LVOT peak velocity (CI: 0.197 ± 0.069) compared to NCs (Two-way ANOVA, p < 0.0001, both). (E) With exercise, the average E/A ratio shows a significant difference between MYBPC3^Δ25bp carriers and NCs (Two-way ANOVA, p < 0.038; CI: 0.107 ± 0.102). (F) The graph depicts significantly higher aortic valve peak velocity in MYBPC3^Δ25bp variant carriers compared to NCs (Two-way ANOVA, p < 0.012; CI: 0.103 ± 0.081). LVID_dia, left ventricular internal diameter in diastole; LVID_s, left ventricular internal diameter in systole; LVOT, left ventricular outflow tract; NC, non-carriers.