Evidence of a de novo variant in FBN1:p.(Ala882Val) as the cause of congenital bilateral ectopia lentis in a crossbred horse

Background: Although several inherited ocular disorders have been extensively studied in horses, few reports of equine ectopia lentis exist and no genetic investigations have been performed. Ectopia lentis is reported to be caused by trauma and genetic variants responsible for ocular diseases, and/or systemic diseases in humans and other species. The most commonly reported genetic cause are dominant variants in FBN1. Here we examined a 3-day old Oldenburg x Thoroughbred colt due to concerns over bilateral ocular anomalies and hypothesized that either a recessive or a de novo mutation was the genetic cause. Results: Examination revealed bilateral microphakia and spherophakia with medioventral lens subluxation. Histopathology of the globes was consistent with ectopia lentis. Whole genome sequencing of the affected foal was conducted, and forty-six candidate genes were evaluated for SNPs and small INDELS. Testing both hypotheses, 82 variants were identified, of which 69 were present in publicly available data from 504 horses and not investigated further. Of the 13 remaining variants, two variants were found in 3' UTRs (ADAMTS17 and OAF), ten were intronic, and one was a coding variant located in fibrillin-1 (FBN1) (FBN1:p.(Ala882Val)). This variant was also computationally predicted to be deleterious to protein function. The affected foal was confirmed by Sanger sequencing to be heterozygous for this variant and his dam, sire, and five paternal half-siblings, all presumed clinically unaffected, were homozygous for the reference allele. Additionally, this same substitution is reported to be pathogenic causing Marfan syndrome in humans with a dominant mode of inheritance, of which ectopia lentis is a common feature. Conclusion: These findings support the de novo hypothesis with a variant in FBN1:p.(Ala882Val) as the likely cause of ectopia lentis in this foal. This is the first report of ectopia lentis caused by a genetic variant in the horse. Given, the role of FBN1 in ectopic lentis in humans and other species, FBN1 should be evaluated as a potential candidate when other horses with this condition are identified.

背景：尽管多种遗传性眼部疾病已在马中得到广泛研究，但关于马晶状体异位（equine ectopia lentis）的报道极少，且尚未开展相关遗传学研究。据报道，人类及其他物种的晶状体异位可由外伤、眼部疾病相关遗传变异和/或全身性疾病引发，其中最常见的遗传病因是FBN1基因的显性变异。本研究针对一匹因双侧眼部异常被送检的3日龄奥尔登堡马×纯血马杂交驹展开，推测其遗传病因可能为隐性突变或新生突变（de novo mutation）。结果：检查显示该驹存在双侧小晶状体（microphakia）与球形晶状体（spherophakia），伴腹内侧晶状体半脱位（medioventral lens subluxation）。眼球组织病理学（histopathology）检测结果符合晶状体异位的特征。对患病驹开展全基因组测序（whole genome sequencing），并针对46个候选基因的单核苷酸多态性（Single Nucleotide Polymorphisms, SNPs）及小型插入缺失变异（small INDELS）进行分析。在验证前述两种假说的过程中，共鉴定出82个变异，其中69个可在包含504匹马的公开数据集中被检测到，因此未做进一步研究。剩余13个变异中，2个位于3'非翻译区（3' untranslated regions, 3' UTRs）（分别对应ADAMTS17与OAF基因），10个位于内含子区，1个为原纤蛋白-1（fibrillin-1, FBN1）基因上的编码区变异：FBN1:p.(Ala882Val)。经计算预测，该变异会对蛋白质功能产生有害影响。桑格测序（Sanger sequencing）确认患病驹为该变异的杂合子，而其母本、父本及5匹假定临床健康的父系半同胞均为参考等位基因的纯合子。此外，已有报道称该相同氨基酸替换会引发人类马凡综合征（Marfan syndrome），该疾病以显性方式遗传，而晶状体异位正是其常见临床表现之一。结论：本研究结果支持新生突变假说，即FBN1基因p.(Ala882Val)变异可能是该驹晶状体异位的致病原因。这是首次报道马中由遗传变异引发的晶状体异位病例。鉴于FBN1在人类及其他物种晶状体异位发病机制中的作用，当后续发现其他罹患该病的马匹时，可将FBN1作为潜在候选基因进行检测。