Research Laboratory of Human Genome and Multifactorial Diseases, Faculty of Pharmacy, University of Monastir, Street Avicenne, 5000, Monastir, Tunisia.
Biochemistry Laboratory (LR 00SP03), Bechir Hamza Children's Hospital, Tunis, Tunisia.
Diagn Pathol. 2022 May 6;17(1):44. doi: 10.1186/s13000-022-01221-8.
Ocular cystinosis is a rare autosomal recessive disorder characterized by intralysosomal cystine accumulation in renal, ophthalmic (cornea, conjunctiva), and other organ abnormalities. Patients with ocular cystinosis are mostly asymptomatic and typically experience mild photophobia due to cystine crystals in the cornea observed accidently during a routine ocular examination. The ocular cystinosis is associated with different mutations in CTNS gene. Cysteamine therapy mostly corrects the organ abnormalities.
This study was performed in collaboration with the department of ophthalmology of Farhat Hached Hospital. The Optical Coherence Tomography (OCT) of the cornea and retinal photography were used to search cystine crystals within the corneas and conjunctiva in eight Tunisian patients. Screening for the common 57-kb deletion was performed by standard multiplex PCR, followed by direct sequencing of the entire CTNS gene.
The studied patients were found to have cystine crystal limited anterior corneal stroma and the conjunctiva associated with retinal crystals accumulation. CTNS gene sequencing disclosed 7 mutations: three missense mutations (G308R, p.Q88K, and p.S139Y); one duplication (C.829dup), one framshift mutation (p.G258f), one splice site mutation (c.681 + 7delC) and a large deletion (20,327-bp deletion). Crystallographic structure analysis suggests that the novel mutation p.S139Y is buried in a first transmembrane helix closed to the lipid bilayer polar region, introducing a difference in hydrophobicity which could affect the hydrophobic interactions with the membrane lipids. The second novel mutation p.Q88K which is located in the lysosomal lumen close to the lipid membrane polar head region, introduced a basic amino acid in a region which tolerate only uncharged residue. The third missense mutation introduces a positive change in nonpolar tail region of the phospholipid bilayer membrane affecting the folding and stability of the protein in the lipid bilayer.
Our data demonstrate that impaired transport of cystine out of lysosomes is the most common, which is obviously associated with the mutations of transmembrane domains of cystinosine resulting from a total loss of its activity.
眼型胱氨酸贮积症是一种罕见的常染色体隐性遗传病，其特征是溶酶体中胱氨酸蓄积，导致肾脏、眼部（角膜、结膜）和其他器官异常。眼型胱氨酸贮积症患者大多无症状，通常在常规眼部检查中偶然发现角膜中的胱氨酸晶体而出现轻度畏光。眼型胱氨酸贮积症与 CTNS 基因突变有关。半胱氨酸治疗可纠正大部分器官异常。
本研究与法哈特·哈切德医院眼科合作进行。应用光学相干断层扫描（OCT）检查角膜和视网膜摄影，以发现 8 名突尼斯患者角膜和结膜内的胱氨酸晶体。采用标准多重 PCR 筛查常见的 57-kb 缺失，然后对整个 CTNS 基因进行直接测序。
研究患者的角膜前基质和结膜中发现胱氨酸晶体，伴有视网膜晶体堆积。CTNS 基因测序发现 7 种突变：3 种错义突变（G308R、p.Q88K 和 p.S139Y）；1 种重复（C.829dup）、1 种移码突变（p.G258f）、1 种剪接位点突变（c.681 + 7delC）和 1 种大片段缺失（20327-bp 缺失）。晶体结构分析表明，新型突变 p.S139Y 位于靠近脂双层极性区的第一个跨膜螺旋内，导致疏水性不同，可能影响与膜脂的疏水相互作用。位于靠近溶酶体腔的脂膜极性头部区域的第二个新型突变 p.Q88K 引入了一个碱性氨基酸，该区域只能容忍不带电荷的残基。第三个错义突变引入了磷脂双层膜非极性尾部的正变化，影响了蛋白在脂质双层中的折叠和稳定性。
我们的数据表明，溶酶体中胱氨酸转运受损最为常见，这显然与胱氨酸跨膜结构域的突变有关，导致其完全丧失活性。
