Vollrath Douglas, Yasumura Douglas, Benchorin Gillie, Matthes Michael T, Feng Wei, Nguyen Natalie M, Sedano Cecilia D, Calton Melissa A, LaVail Matthew M
Department of Genetics, Stanford University School of Medicine, Stanford, California, United States of America.
Beckman Vision Center, University of California San Francisco, San Francisco, California, United States of America.
PLoS Genet. 2015 Dec 11;11(12):e1005723. doi: 10.1371/journal.pgen.1005723. eCollection 2015 Dec.
Inherited photoreceptor degenerations (IPDs) are the most genetically heterogeneous of Mendelian diseases. Many IPDs exhibit substantial phenotypic variability, but the basis is usually unknown. Mutations in MERTK cause recessive IPD phenotypes associated with the RP38 locus. We have identified a murine genetic modifier of Mertk-associated photoreceptor degeneration, the C57BL/6 (B6) allele of which acts as a suppressor. Photoreceptors degenerate rapidly in Mertk-deficient animals homozygous for the 129P2/Ola (129) modifier allele, whereas animals heterozygous for B6 and 129 modifier alleles exhibit an unusual intermixing of degenerating and preserved retinal regions, with females more severely affected than males. Mertk-deficient mice homozygous for the B6 modifier allele display degeneration only in the far periphery, even at 8 months of age, and have improved retinal function compared to animals homozygous for the 129 allele. We genetically mapped the modifier to an approximately 2-megabase critical interval that includes Tyro3, a paralog of Mertk. Tyro3 expression in the outer retina varies with modifier genotype in a manner characteristic of a cis-acting expression quantitative trait locus (eQTL), with the B6 allele conferring an approximately three-fold higher expression level. Loss of Tyro3 function accelerates the pace of photoreceptor degeneration in Mertk knockout mice, and TYRO3 protein is more abundant in the retinal pigment epithelium (RPE) adjacent to preserved central retinal regions of Mertk knockout mice homozygous for the B6 modifier allele. Endogenous human TYRO3 protein co-localizes with nascent photoreceptor outer segment (POS) phagosomes in a primary RPE cell culture assay, and expression of murine Tyro3 in cultured cells stimulates phagocytic ingestion of POS. Our findings demonstrate that Tyro3 gene dosage modulates Mertk-associated retinal degeneration, provide strong evidence for a direct role for TYRO3 in RPE phagocytosis, and suggest that an eQTL can modify a recessive IPD.
遗传性光感受器退化（IPD）是孟德尔疾病中基因异质性最高的疾病。许多IPD表现出显著的表型变异性，但其原因通常未知。MERTK基因的突变会导致与RP38位点相关的隐性IPD表型。我们鉴定出了一种与Mertk相关的光感受器退化的小鼠基因修饰因子，其中C57BL/6（B6）等位基因起抑制作用。在纯合129P2/Ola（129）修饰等位基因的Mertk缺陷动物中，光感受器迅速退化，而杂合B6和129修饰等位基因的动物则表现出退化和保留的视网膜区域异常混合，雌性比雄性受影响更严重。纯合B6修饰等位基因的Mertk缺陷小鼠即使在8个月大时也仅在最外周出现退化，并且与纯合129等位基因的动物相比，其视网膜功能有所改善。我们通过基因定位将该修饰因子定位到一个约2兆碱基的关键区间，该区间包含Mertk的旁系同源基因Tyro3。外视网膜中Tyro3的表达随修饰基因型而变化，其方式具有顺式作用表达数量性状位点（eQTL）的特征，B6等位基因赋予的表达水平约高3倍。Tyro3功能丧失会加速Mertk基因敲除小鼠光感受器退化的速度，并且在纯合B6修饰等位基因的Mertk基因敲除小鼠保留的中央视网膜区域相邻的视网膜色素上皮（RPE）中，TYRO3蛋白更为丰富。在原代RPE细胞培养实验中，内源性人类TYRO3蛋白与新生光感受器外段（POS）吞噬体共定位，并且在培养细胞中表达小鼠Tyro3会刺激对POS的吞噬摄取。我们的研究结果表明，Tyro3基因剂量调节与Mertk相关的视网膜退化，为TYRO3在RPE吞噬作用中的直接作用提供了有力证据，并表明一个eQTL可以修饰隐性IPD。
