Absence of myofibrillar myopathy in Quarter Horses with a histopathological diagnosis of type 2 polysaccharide storage myopathy and lack of association with commercial genetic tests

Background Genetic tests for variants in MYOT (P2; rs1138656462) , FLNC (P3a; rs1139799323 or P3b; rs1142918816) and MYOZ3 (P4; rs1142544043) genes are offered commercially to diagnose myofibrillar myopathy (MFM) and type 2 polysaccharide storage myopathy (PSSM2) in Quarter Horses (QH). Objectives To determine if PSSM2‐QH has histopathological features of MFM. To compare genotype and allele frequencies of variants P2, P3, P4 between control‐QH and PSSM2‐QH diagnosed by histopathology. Study design Retrospective cross‐sectional. Methods The study includes a total of 229 healthy control‐QH, 163 PSSM2‐QH GYS1 mutation negative. Desmin stains of gluteal/semimembranosus muscle were evaluated. Purported disease alleles P2, P3a, P3b, P4 were genotyped by pyrosequencing. Genotype, allele frequency and total number of variant alleles or loci were compared between phenotypes using additive/genotypic and dominant models and quantitative effects evaluated by multivariable logistic regression. Results Histopathological features of MFM were absent in all QH. A P variant allele at any locus was not associated ( P > .05) with a histopathological diagnosis of PSSM2 and one or more P variants were common in control‐QH (57%) and PSSM2‐QH (61%). Allele frequencies (control/PSSM2) were: 0.24/0.21 (P2), 0.07/0.12 (P3a), 0.07/0.11 (P3b) and 0.06/0.08 (P4). P3a and P3b loci were not independent ( r 2 = 0.894); and not associated with PSSM2 histopathology comparing the haplotype of both P3a and P3b variants to other haplotypes. A receiver operator curve did not accurately predict the PSSM2 phenotype (AUC = 0.67, 95% CI 0.62‐0.72), and there was no difference in the total number of variant loci or total variant allele count between control‐QH and PSSM2‐QH. Main limitations P3a and P3b were not in complete linkage disequilibrium. Conclusions The P2, P3 and P4 variants in genes associated with human MFM were not associated with PSSM2 in 392 QH. Their use would improperly diagnose PSSM2/MFM in 57% of healthy QH and fail to diagnose PSSM2 in 40% of QH with histopathological evidence of PSSM2.