Genetics and Genomic Medicine Research and Teaching Department, University College London, UCL Great Ormond Street Institute of Child Health, 30 Guilford Street, London WC1N 1EH, United Kingdom.
Department of Human Genetics, Medical Research Institute, Alexandria University, 22 El-Guish Road, El-Shatby, Alexandria 21526, Egypt.
Hum Mol Genet. 2023 Oct 17;32(21):3090-3104. doi: 10.1093/hmg/ddad132.
Ciliopathies are inherited disorders caused by defective cilia. Mutations affecting motile cilia usually cause the chronic muco-obstructive sinopulmonary disease primary ciliary dyskinesia (PCD) and are associated with laterality defects, while a broad spectrum of early developmental as well as degenerative syndromes arise from mutations affecting signalling of primary (non-motile) cilia. Cilia assembly and functioning requires intraflagellar transport (IFT) of cargos assisted by IFT-B and IFT-A adaptor complexes. Within IFT-B, the N-termini of partner proteins IFT74 and IFT81 govern tubulin transport to build the ciliary microtubular cytoskeleton. We detected a homozygous 3-kb intragenic IFT74 deletion removing the exon 2 initiation codon and 40 N-terminal amino acids in two affected siblings. Both had clinical features of PCD with bronchiectasis, but no laterality defects. They also had retinal dysplasia and abnormal bone growth, with a narrowed thorax and short ribs, shortened long bones and digits, and abnormal skull shape. This resembles short-rib thoracic dysplasia, a skeletal ciliopathy previously linked to IFT defects in primary cilia, not motile cilia. Ciliated nasal epithelial cells collected from affected individuals had reduced numbers of shortened motile cilia with disarranged microtubules, some misorientation of the basal feet, and disrupted cilia structural and IFT protein distributions. No full-length IFT74 was expressed, only truncated forms that were consistent with N-terminal deletion and inframe translation from downstream initiation codons. In affinity purification mass spectrometry, exon 2-deleted IFT74 initiated from the nearest inframe downstream methionine 41 still interacts as part of the IFT-B complex, but only with reduced interaction levels and not with all its usual IFT-B partners. We propose that this is a hypomorphic mutation with some residual protein function retained, which gives rise to a primary skeletal ciliopathy combined with defective motile cilia and PCD.
纤毛病是由纤毛缺陷引起的遗传性疾病。影响游动纤毛的突变通常导致慢性黏液阻塞性肺鼻疾病原发性纤毛运动障碍（PCD），并与侧位缺陷有关，而广泛的早期发育和退行性综合征则是由影响初级（非运动）纤毛信号的突变引起的。纤毛的组装和功能需要货物在内鞭毛运输（IFT）的帮助下进行，IFT-B 和 IFT-A 衔接复合物辅助运输。在 IFT-B 中，伴侣蛋白 IFT74 和 IFT81 的 N 端控制微管运输到构建纤毛微管细胞骨架。我们在两个受影响的兄弟姐妹中检测到一个纯合的 3kb 内含子 IFT74 缺失，该缺失去除了外显子 2 的起始密码子和 40 个 N 端氨基酸。他们都有 PCD 的临床特征，包括支气管扩张，但没有侧位缺陷。他们还患有视网膜发育不良和骨骼生长异常，胸廓狭窄，肋骨短，长骨和手指缩短，颅骨形状异常。这类似于短肋胸发育不良，这是一种骨骼纤毛病，以前与初级纤毛中的 IFT 缺陷有关，而不是游动纤毛。从受影响的个体中收集的有纤毛的鼻上皮细胞中，有许多缩短的运动纤毛数量减少，微管排列紊乱，一些基底足的方向错误，以及纤毛结构和 IFT 蛋白分布的破坏。没有全长的 IFT74 表达，只有与 N 端缺失和下游起始密码子的框内翻译一致的截断形式。在亲和纯化质谱分析中，从最近的框内下游甲硫氨酸 41 起始的缺失外显子 2 的 IFT74 仍作为 IFT-B 复合物的一部分相互作用，但只有相互作用水平降低，而不是与所有通常的 IFT-B 伴侣相互作用。我们提出这是一种低等位基因突变，保留了一些残余的蛋白功能，导致原发性骨骼纤毛病合并运动纤毛和 PCD 缺陷。
