The nanophthalmos protein TMEM98 inhibits MYRF self-cleavage and is required for eye size specification

The precise control of eye size is essential for normal vision. TMEM98 is a highly conserved and widely expressed gene which appears to be involved in eye size regulation. Mutations in human TMEM98 are found in patients with nanophthalmos (very small eyes) and variants near the gene are associated in population studies with myopia and increased eye size. As complete loss of function mutations in mouse Tmem98 result in perinatal lethality, we produced mice deficient for Tmem98 in the retinal pigment epithelium (RPE), where Tmem98 is highly expressed. These mice have greatly enlarged eyes that are very fragile with very thin retinas, compressed choroid and thin sclera. To gain insight into the mechanism of action we used a proximity labelling approach to discover interacting proteins and identified MYRF as an interacting partner. Mutations of MYRF are also associated with nanophthalmos. The protein is an endoplasmic reticulum-tethered transcription factor which undergoes autoproteolytic cleavage to liberate the N-terminal part which then translocates to the nucleus where it acts as a transcription factor. We find that TMEM98 inhibits the self-cleavage of MYRF, in a novel regulatory mechanism. In RPE lacking TMEM98, MYRF is ectopically activated and abnormally localised to the nuclei. Our findings highlight the importance of the interplay between TMEM98 and MYRF in determining the size of the eye.

眼球大小的精确调控对于正常视觉功能至关重要。跨膜蛋白98（TMEM98）是一种高度保守且广泛表达的基因，其似乎参与眼球大小的调控过程。在罹患小眼球症（nanophthalmos，即眼球体积显著缩小）的患者体内，可检测到人类TMEM98基因发生突变；而在人群关联研究中，该基因区域附近的遗传变异与近视及眼球体积增大存在显著关联。由于小鼠Tmem98基因的完全功能缺失突变会导致围产期致死，我们构建了在视网膜色素上皮（Retinal Pigment Epithelium, RPE）——该组织中TMEM98呈高表达状态——中特异性敲除Tmem98的小鼠模型。该模型小鼠的眼球显著增大，且结构极易受损，同时伴随视网膜过薄、脉络膜受压以及巩膜变薄的表型特征。为深入解析其作用机制，我们采用邻近标记（proximity labelling）技术筛选互作蛋白，并鉴定出MYRF为TMEM98的互作伴侣蛋白。MYRF基因突变同样与小眼球症的发生存在关联。该蛋白是一种锚定在内质网上的转录因子，可通过自蛋白水解切割释放其N端结构域；该结构域随后易位至细胞核内，发挥转录因子的调控功能。我们发现，TMEM98通过一种全新的调控机制抑制MYRF的自切割过程。在缺失TMEM98的RPE细胞中，MYRF发生异位激活并异常定位于细胞核内。本研究结果揭示了TMEM98与MYRF之间的相互调控在决定眼球大小过程中的关键作用。