Genetic and phenotypic heterogeneity in KIAA0753-related ciliopathies.

Primary ciliopathies are heterogenous disorders resulting from perturbations in primary cilia form and/or function. Primary cilia are cellular organelles which mediates key signaling pathways during development, including in the brain. The Sonic hedgehog (Shh) pathway is mediated by primary cilia and required for neuroepithelium and central nervous system development. The proteins which transduce Shh signaling are located within the primary cilia. Joubert syndrome is a primary ciliopathy characterized by cerebellar/brain stem malformation, hypotonia, and developmental delays. Variants in more than 35 genes are associated with Joubert syndrome, including the gene KIAA0753. KIAA0753 is localized at the basal body/centrosome and is part of a complex required for primary ciliogenesis. The phenotypic spectrum of human patients with biallelic pathogenic variants in KIAA0753 (OMIM 617127) is broad and not well-characterized. Here we describe four individuals with biallelic pathogenic KIAA0753 variants, including 5 novel variants. We report in vitro results assessing the function of each variant indicating that KIAA0753 protein is produced, but is not fully competent to promote primary ciliogenesis. Ablation of KIAA0753 in vitro blocks primary ciliogenesis and Shh pathway activity. Correspondingly, KIAA0753 patient fibroblasts have a deficit in primary ciliation and improper Shh and Wnt signaling, with a particularly blunted response to Shh pathway stimulation. Our work expands the phenotypic spectrum of KIAA0753 ciliopathies. We also demonstrate the utility of patient-focused functional assays for proving causality of genetic variants and understanding the relationship between variants and phenotypes as part of the ongoing effort to definitively categorize variants of unknown significance.

原发性纤毛疾病是由原发性纤毛形态和/或功能紊乱引起的异质性疾病。原发性纤毛是细胞器，在发育过程中（包括在大脑中）介导关键的信号通路。音猬因子（Shh）通路由原发性纤毛介导，是神经上皮和中枢神经系统发育所必需的。传导Shh信号的蛋白质位于原发性纤毛内。乔贝综合征是一种原发性纤毛疾病，其特征为小脑/脑干畸形、肌张力减退和发育迟缓。超过35个基因的变异与乔贝综合征有关，包括KIAA0753基因。KIAA0753定位于基体/中心体，是原发性纤毛发生所需复合物的一部分。具有KIAA0753双等位基因致病变异的人类患者（在线人类孟德尔遗传数据库编号617127）的表型谱很广泛且特征不明确。在此我们描述了4名具有双等位基因致病性KIAA0753变异的个体，包括5种新型变异。我们报告了评估每种变异功能的体外实验结果，结果表明KIAA0753蛋白能够产生，但不完全具备促进原发性纤毛发生的能力。体外敲除KIAA0753会阻断原发性纤毛发生和Shh通路活性。相应地，KIAA0753患者的成纤维细胞在原发性纤毛形成方面存在缺陷，Shh和Wnt信号传导异常，对Shh通路刺激的反应尤其迟钝。我们的工作扩展了KIAA0753纤毛疾病的表型谱。我们还证明了以患者为中心的功能检测在证明基因变异的因果关系以及理解变异与表型之间的关系方面的实用性，这是对意义未明变异进行明确分类的持续努力的一部分。