rs71180793 — OBSCN OBSCN c.23838del

OBSCN — When the Sarcomere's Scaffold Carries a Frameshift

Obscurin is one of the largest proteins in the human body — a giant cytoskeletal scaffold encoded by the OBSCN gene on chromosome 1q42. In cardiac and skeletal muscle, obscurin anchors the sarcoplasmic reticulum to the sarcomere¹¹ obscurin anchors the sarcoplasmic reticulum to the sarcomere
The protein integrates structural and signalling functions at the M-band of the sarcomere, coordinates myofibrillogenesis, and regulates calcium cycling during contraction and relaxation. The rs71180793 variant is a single-nucleotide deletion (c.23838del) that shifts the reading frame in the OBSCN coding sequence, creating a premature stop and truncating the obscurin protein at around serine-7947 (isoform C numbering). Whether this frameshift is sufficient alone to cause cardiomyopathy remains contested — ClinVar records conflicting interpretations ranging from benign to uncertain significance — but a growing body of evidence links OBSCN protein-truncating variants as a class to cardiomyopathy risk, particularly in adults.

The Mechanism

The rs71180793 deletion removes one cytosine from a homopolymeric CCCCC run in exon coding sequence, causing a frameshift at p.Ser7947²² frameshift at p.Ser7947
The exact amino acid position varies by isoform; p.Ser6990fs in isoform b. The truncated protein loses obscurin's C-terminal kinase domains and the titin-binding SK3 domain, which are critical for sarcoplasmic reticulum organization and RhoGEF signalling. Mechanistically, the likely disease model is haploinsufficiency³³ haploinsufficiency
One functional copy produces insufficient obscurin protein to maintain normal sarcomere architecture: truncating mutations reduce total obscurin immunoreactive material to 45–72% of normal in explanted DCM hearts. This partial depletion may disrupt SR anchorage and calcium handling, increasing arrhythmia susceptibility. Murine obscurin-knockout models confirm the downstream consequences: hearts lacking obscurin show larger ventricular volumes, reduced fractional shortening, impaired beta-adrenergic response, and increased ventricular tachycardia inducibility⁴⁴ hearts lacking obscurin show larger ventricular volumes, reduced fractional shortening, impaired beta-adrenergic response, and increased ventricular tachycardia inducibility
Knockout model; end-diastolic volume increased 20%.

The Evidence

Evidence for OBSCN truncating variants as a cardiac risk class is accumulating, though the evidence for rs71180793 specifically remains emerging. In the largest study to date, Wu et al. 2021 performed whole-exome sequencing on 986 HCM patients and 761 controls⁵⁵ Wu et al. 2021 performed whole-exome sequencing on 986 HCM patients and 761 controls
Replication cohort: 529 HCM patients and 307 controls; combined OR 3.58 and identified 28 qualifying OBSCN truncating variants in 2.6% of HCM patients versus 0.8% of controls (combined OR 3.58, p<0.001). Carriers had significantly worse outcomes over a mean 3.3-year follow-up: adjusted HR 3.1 (95% CI 1.40–6.70) for cardiovascular death and 2.63 (95% CI 1.21–5.71) for all-cause mortality. A separate proteomic study of explanted DCM hearts⁶⁶ explanted DCM hearts
30 patients with familial DCM; whole-exon sequencing of 58 disease genes identified five OBSCN mutations in four samples, with haploinsufficient protein expression in all three assayed samples. The Genomics England cohort analysis⁷⁷ Genomics England cohort analysis
926 adult cardiomyopathy patients independently confirmed that OBSCN was among the top mutated genes in adult DCM, enriched for protein-truncating variants.

The picture is more nuanced for rs71180793 itself. ClinVar lists three submissions with conflicting results: one benign call (Labcorp Genetics), one uncertain significance (Greenwood Genetic Center), and one likely benign (CeGaT). A separate submission from PreventionGenetics classifies it as likely benign for OBSCN-related disorder. The global deletion allele frequency of approximately 0.36% (gnomAD exomes) places it above the typical threshold for high-penetrance dominant cardiomyopathy genes. This variant likely has incomplete penetrance, or operates as a low-effect risk allele requiring additional genetic or environmental co-factors. Bi-allelic (homozygous) carriers are essentially absent in population databases, consistent with severe disease or embryonic lethality; heterozygous carrier status is the clinically relevant state.

Practical Implications

Given the conflicting ClinVar evidence and the emerging-to-moderate population data on OBSCN truncating variants as a class, rs71180793 heterozygous carriers warrant a measured response: not alarm, but not dismissal. Cardiac echocardiography can rule out subclinical cardiomyopathy; an ECG or Holter monitor can assess for arrhythmia burden. Decisions about intensity of follow-up should factor in personal and family history of cardiomyopathy, arrhythmia, or unexplained syncope. Carriers who participate in high-intensity competitive sport may wish to discuss pre-participation cardiac screening with a sports cardiologist.

Interactions

OBSCN truncating variants may compound with pathogenic variants in other sarcomere genes — particularly titin (TTN) and desmoplakin (DSP)⁸⁸ titin (TTN) and desmoplakin (DSP)
Both identified alongside OBSCN mutations in DCM explant cohorts — to produce more severe phenotypes. OBSCN's titin-binding domain positions it in the same structural circuit as TTN, and co-occurrence of truncating variants in both genes may produce additive haploinsufficiency beyond what either alone causes. No formal compound action data exist for rs71180793 specifically, but clinicians evaluating OBSCN carriers for cardiomyopathy risk should consider comprehensive sarcomere gene panel testing.