This missense variant replaces arginine with histidine at codon 810 in the fibronectin type 3 domain C6 of the MYBPC3 protein. Computational prediction is inconclusive regarding the impact of this variant on protein structure and function (internally defined REVEL score threshold 0.5 < inconclusive < 0.7, PMID: 27666373). A functional study has shown that this variant had a 44% prolonged MyBPC protein half-life compared to wild-type (PMID: 32841044). However, clinical relevance of this observation is not clear. This variant has been reported in over 30 individuals affected with hypertrophic cardiomyopathy (PMID: 12951062, 15519027, 15936968, 20031618, 20624503, 21185001, 21302287, 25524337, 26090888, 27483260, 27532257, 27600940, 35626289). One of these individuals also carried a different pathogenic missense variant in the MYBPC3 gene (PMID: 12951062). This variant has been reported in homozygosity in two individuals affected with hypertrophic cardiomyopathy (PMID: 12951062, 30847666). It has been shown that this variant segregates with disease in multiple affected individuals in several families (PMID: 21185001; communication with an external laboratory, ClinVar Variation ID: 42620). This variant has been identified in 12/249184 chromosomes in the general population by the Genome Aggregation Database (gnomAD). Based on the available evidence, this variant is classified as Likely Pathogenic.

The p.R810H variant (also known as c.2429G>A), located in coding exon 25 of the MYBPC3 gene, results from a G to A substitution at nucleotide position 2429. The arginine at codon 810 is replaced by histidine, an amino acid with highly similar properties. This alteration has been reported in multiple unrelated individuals with hypertrophic cardiomyopathy (HCM), though in some cases, a second alteration was also detected or clinical details were limited (Van Driest et al. J Am Coll Cardiol. 2004;44(9):1903-10; Van Driest et al. Mol & Genet Metab. 2005;85(4):280-5; Kaski et al. Circ Cardiovasc Genet. 2009;2(5):436-41; Roncarati et al. Cell Physiol. 2011;226(11):2894-900; Coppini et al. J Am Coll Cardiol. 2014;64(24):2589-600; Liu et al. Sci Rep. 2015;5:11411; Bagnall RD et al. Circ Genom Precis Med. 2022 Dec;15(6):e003686; Field E et al. J Med Genet. 2022 Aug;59(8):768-775; Sepp R et al. Diagnostics (Basel). 2022 May;12(5)). One study reported this alteration in the homozygous state in an individual with a more severe presentation, and an additional publication reported this alteration to co-segregate with disease in two siblings (Nanni et al. Biochem Biophys Res Commun. 2003;309(2):391-8; Maron et al. Am J Cardiol. 2011;107(4):604-8). In one or more case control studies, this variant was found to be a low penetrance variant associated with MYBPC3-related cardiomyopathy (Meisner JK et al. Circulation, 2025 Mar;151:783-798). This amino acid position is highly conserved in available vertebrate species. In addition, this alteration is predicted to be deleterious by in silico analysis. Based on the majority of available evidence to date, this variant is likely to be pathogenic.

This sequence change in MYBPC3 is predicted to replace arginine with histidine at codon 810, p.(Arg810His). The arginine residue is highly conserved (100 vertebrates, UCSC), and is located in the fibronectin (C6) type III domain (amino acids 774-870), a region that is defined as a mutational hotspot (PMID: 32841044). There is a small physicochemical difference between arginine and histidine. The highest population minor allele frequency in the population database gnomAD v2.1 is 0.01% (11/112,984 alleles) in the European (non-Finnish) population. This variant has been reported in multiple individuals with hypertrophic cardiomyopathy (HCM; PMID: 2861529, 15519027, 27483260, 27532257, 27600940, 25637381, 25351510, 20031618, 15936968, 21302287). Of those individuals, one individual was homozygous for the variant with a severe phenotype and one was compound heterozygous for the variant with a milder phenotype (PMID:12951062; 18761664). The prevalence of the variant in affected individuals is significantly increased compared with the prevalence in controls (OR = 39.3) (PMID: 32841044). The variant has been reported to segregate in two affected family members from one family with MYBPC3-related cardiomyopathy (PMID: 21158001). Computational evidence is uninformative for the missense substitution (REVEL = 0.618). Another missense variant (c.2429G>T p.Arg810Leu), with a similar physicochemical difference in the same codon has been classified as likely pathogenic for MYBPC3-related HCM (ClinVar Variation ID:181127). Based on the classification scheme RMH Modified ACMG/AMP Guidelines v1.6.1, this variant is classified as LIKELY PATHOGENIC. Following criteria are met: PM1, PM3_Supporting, PS4

The p.Arg810His variant in MYBPC3 has been identified in the heterozygous state in >35 individuals with HCM and segregated with disease in 3 affected relatives from 3 families (Nanni 2003 PMID: 12951062, Van Driest 2004 PMID: 15519027, Van Driest 2005 PMID: 15936968, Kaski 2009 PMID: 20031618, Roncarati 2011 PMID: 21302287, Maron 2011 PMID: 21185001, Rubattu 2015 PMID: 27483260, Walsh 2017 PMID: 27532257, Invitae pers. comm., Ambry pers. comm., GeneDx pers. comm., LMM data). It was also identified in 9 individuals with additional disease-causing variants in cardiomyopathy related genes (Nanni 2003 PMID: 12951062, Van Driest 2005 PMID: 15936968, Liu 2015 PMID: 26090888, Ambry pers. comm., GeneDx pers. comm., LMM data). On average, these individuals with a second variant had an earlier age of onset than the heterozygous individuals. This variant has also been identified in 0.009% (11/112984) of European chromosomes by gnomAD (http://gnomad.broadinstitute.org) and has been reported by other clinical laboratories in ClinVar (Variation ID# 42620). Computational prediction tools and conservation analyses are consistent with pathogenicity. Based on criteria selected, this variant would be classified as uncertain significance; however, the available evidence is sufficiently borderline and the earlier age of onset in individuals with additional disease-causing variants provided additional evidence not accounted for by the current rules. Therefore, although additional studies are required to fully establish its clinical significance, this variant meets criteria to be classified as likely pathogenic for autosomal dominant HCM. ACMG/AMP Criteria applied: PS4_Moderate, PP1, PP3, with adjustment based on clinical judgment.

Based on the classification scheme VCGS_Germline_v1.3.4, this variant is classified as Likely pathogenic. Following criteria are met: 0102 - Loss of function is a known mechanism of disease in this gene and is associated with hypertrophic cardiomyopathy 4 (HCM; MIM#115197). (I) 0108 - This gene is associated with both recessive and dominant disease. Dominant inheritance is frequently reported in adult onset conditions, however recessive inheritance results in a more severe early onset phenotype (OMIM). (I) 0115 - Variants in this gene are known to have variable expressivity (PMID: 32841044). (I) 0200 - Variant is predicted to result in a missense amino acid change from arginine to histidine. (I) 0251 - Variant is heterozygous. (I) 0304 - Variant is present in gnomAD (v2&v3) <0.01 (20 heterozygotes, 0 homozygotes). (SP) 0309 - An alternative amino acid change at the same position has been observed in gnomAD (v3) (1 heterozygote, 0 homozygotes). (I) 0502 - Missense variant with conflicting in silico predictions and very high conservation. (I) 0602 - Variant is located in a hotspot region or cluster of pathogenic variants in the C6 domain (PMID: 32841044). (SP) 0708 - Other missense variants comparable to the one identified in this case have conflicting previous evidence for pathogenicity. Alternate changes to glycine and cysteine at the same residue have previously been reported as VUS. Additionally, an alternate change to leucine has been reported as both VUS and likely pathogenic (ClinVar, PMIDs: 20624503, 21835320, 28840316, 30847666). (I) 0802 - This variant has moderate previous evidence of pathogenicity in unrelated individuals. Historically, this variant had conflicting interpretations and was more commonly regarded as a VUS, however, more recent studies have upgraded the classification to likely pathogenic. The variant has been observed in more than thirty individuals with HCM. Additionally, individuals harbouring a second pathogenic variant, either compound heterozygous in MYBPC3 or in a different HCM-associated gene, generally present with a more severe phenotype than heterozygotes (ClinVar, VCGS, PMIDs: 12951062, 30847666, 32841044, 33558530). (SP) 1010 - Functional evidence for this variant is inconclusive. In a functional study using rat ventricular myocytes, the variant was not shown to impact myofilament assembly similar to pathogenic variants in the C10 domain, however it did show a significantly increased protein half-life compared to wild type. The authors concluded that further work is required to elucidate the pathogenic mechanism of variants located in the C3 and C6 domains (PMID: 32841044). (I) 1208 - Inheritance information for this variant is not currently available in this individual. (I) Legend: (SP) - Supporting pathogenic, (I) - Information, (SB) - Supporting benign

In silico analysis supports that this missense variant has a deleterious effect on protein structure/function; This variant is associated with the following publications: (PMID: 25335496, 15115610, 28790153, 25524337, 21302287, 20031618, 23299917, 25637381, 23549607, 18761664, 21185001, 12951062, 27483260, 27600940, 27532257, 15519027, 15936968, 25351510, 21415409, 30847666, 33558530, 28615295, 33782553, 34542152, 24033266, 32841044, 28408708, 35626289, 37396317, 20624503, 34400558, 22958901, 26090888, 36252119, 36243179, 36437915, 37652022)

PP1, PS4

This missense variant replaces arginine with histidine at codon 810 in the fibronectin type 3 domain C6 of the MYBPC3 protein. Computational prediction suggests that this variant may have a deleterious impact on protein structure and function. A functional study has shown that this variant had a 44% prolonged MyBPC protein half-life compared to wild-type (PMID: 32841044). However, clinical relevance of this observation is not clear. This variant has been reported in over 30 individuals affected with hypertrophic cardiomyopathy (PMID: 12951062, 15519027, 15936968, 20031618, 20624503, 21185001, 21302287, 25524337, 26090888, 27483260, 27532257, 27600940, 35626289, 37477868, 38002985, 38489124, 38757491). One of these individuals also carried a different pathogenic missense variant in the MYBPC3 gene (PMID: 12951062). This variant has been reported in homozygosity in two individuals affected with hypertrophic cardiomyopathy (PMID: 12951062, 30847666) as well as in an individual affected with dilated cardiomyopathy who also carried an additional pathogenic truncation variant in the same gene (PMID: 38795101). It has been shown that this variant segregates with disease in multiple affected individuals in several families (PMID: 21185001ClinVar SCV000059138.7). This variant has been identified in 12/249184 chromosomes in the general population by the Genome Aggregation Database (gnomAD). Based on the available evidence, this variant is classified as Likely Pathogenic.

This sequence change replaces arginine, which is basic and polar, with histidine, which is basic and polar, at codon 810 of the MYBPC3 protein (p.Arg810His). This variant is present in population databases (rs375675796, gnomAD 0.01%). This missense change has been observed in individuals with hypertrophic cardiomyopathy (PMID: 15519027, 25524337, 27483260, 28615295; internal data). ClinVar contains an entry for this variant (Variation ID: 42620). An algorithm developed to predict the effect of missense changes on protein structure and function (PolyPhen-2) suggests that this variant is likely to be disruptive. This variant disrupts the p.Arg810 amino acid residue in MYBPC3. Other variant(s) that disrupt this residue have been observed in individuals with MYBPC3-related conditions (PMID: 25524337; internal data), which suggests that this may be a clinically significant amino acid residue. For these reasons, this variant has been classified as Pathogenic.

MYBPC3: PM5, PS4:Moderate, PM2:Supporting, PP1