Distichiasis, an ocular disorder in which aberrant cilia (eyelashes) grow from the opening of the Meibomian glands of the eyelid, has been reported in Friesian horses. These misplaced cilia can cause discomfort, chronic keratitis, and corneal ulceration, potentially impacting vision due to corneal fibrosis, or, if secondary infection occurs, may lead to loss of the eye. Friesian horses represent the vast majority of reported cases of equine distichiasis, and as the breed is known to be affected with inherited monogenic disorders, this condition was hypothesized to be a simply inherited Mendelian trait. A genome wide association study (GWAS) was performed using the Axiom 670 k Equine Genotyping array (MNEc670k) utilizing 14 cases and 38 controls phenotyped for distichiasis. An additive single locus mixed linear model (EMMAX) approach identified a 1.83 Mb locus on ECA5 and a 1.34 Mb locus on ECA13 that reached genome-wide significance (pcorrected = 0.016 and 0.032, respectively). Only the locus on ECA13 withstood replication testing (p = 1.6 × 10− 5, cases: n = 5 and controls: n = 37). A 371 kb run of homozygosity (ROH) on ECA13 was found in 13 of the 14 cases, providing evidence for a recessive mode of inheritance. Haplotype analysis (hapQTL) narrowed the region of association on ECA13 to 163 kb. Whole-genome sequencing data from 3 cases and 2 controls identified a 16 kb deletion within the ECA13 associated haplotype (ECA13:g.178714_195130del). Functional annotation data supports a tissue-specific regulatory role of this locus. This deletion was associated with distichiasis, as 18 of the 19 cases were homozygous (p = 4.8 × 10− 13). Genotyping the deletion in 955 horses from 54 different breeds identified the deletion in only 11 non-Friesians, all of which were carriers, suggesting that this could be causal for this Friesian disorder. This study identified a 16 kb deletion on ECA13 in an intergenic region that was associated with distichiasis in Friesian horses. Further functional analysis in relevant tissues from cases and controls will help to clarify the precise role of this deletion in normal and abnormal eyelash development and investigate the hypothesis of incomplete penetrance.

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全基因组测序发现 ECA13 上有一个 16 KB 的缺失，与弗里斯兰马的双毛病相关

据报道，弗里斯兰马患有双睫症，这是一种眼部疾病，异常的纤毛（睫毛）从眼睑睑板腺的开口处生长出来。这些错位的纤毛会引起不适、慢性角膜炎和角膜溃疡，可能因角膜纤维化而影响视力，或者，如果发生继发感染，可能会导致失明。弗里斯兰马代表了绝大多数报告的马双鞭毛病病例，并且由于已知该品种受到遗传性单基因疾病的影响，因此这种情况被假设为简单遗传的孟德尔性状。使用 Axiom 670 k 马基因分型芯片 (MNEc670k) 进行了全基因组关联研究 (GWAS)，利用 14 个病例和 38 个对照进行了双毛虫表型分析。加性单基因座混合线性模型 (EMMAX) 方法识别出 ECA5 上的 1.83 Mb 基因座和 ECA13 上的 1.34 Mb 基因座，达到全基因组显着性（p分别为 0.016 和 0.032）。只有 ECA13 上的基因座经受住了复制测试（p = 1.6 × 10−5，病例：n = 5，对照：n = 37）。在 14 个病例中的 13 个病例中发现了 ECA13 上 371 kb 的纯合性 (ROH)，为隐性遗传模式提供了证据。单倍型分析 (hapQTL) 将 ECA13 上的关联区域缩小至 163 kb。来自 3 个病例和 2 个对照的全基因组测序数据发现 ECA13 相关单倍型 (ECA13:g.178714_195130del) 内存在 16 kb 缺失。功能注释数据支持该基因座的组织特异性调节作用。这种缺失与双鞭毛病相关，因为 19 例中有 18 例是纯合子 (p = 4.8 × 10− 13)。对来自 54 个不同品种的 955 匹马的缺失进行基因分型，仅在 11 匹非弗里斯马中发现了该缺失，所有这些马都是携带者，这表明这可能是这种弗里斯马疾病的原因。这项研究在 ECA13 基因间区域发现了一个 16 kb 的缺失，该区域与弗里斯兰马的双鞭毛病相关。对病例和对照的相关组织进行进一步的功能分析将有助于阐明这种缺失在正常和异常睫毛发育中的确切作用，并研究不完全外显率的假设。