Mechanisms of a Novel Combined Immunodeficiency Caused by a Homozygous Mutation in COPG1

我们在七聚体外壳蛋白I（COPI）的γ1-COP亚基中鉴定了一个双等位基因K652 E突变。 复杂的5阿曼兄弟姐妹，患有复发性肺部感染与封装细菌，巨细胞病毒 EBV病毒血症4名年长的兄弟姐妹表现为严重的CD 4 + T细胞淋巴细胞减少症和T细胞增多症。 最年轻的在出生后不久有正常的T细胞数量，但发展为CMV病毒血症和CD 4 + T细胞减少。 6个月时细胞淋巴细胞减少 外壳蛋白使膜变形以产生运输囊泡。它们还与货物蛋白结合， 正确地将它们分类到这些囊泡中，并确保它们运输到特定的细胞内区室。在 在早期分泌系统中，COPII复合体将货物从内质网（ER）运输到高尔基体， 而COPI复合体在另一个方向上运输选择的货物，从高尔基体到ER， 另外介导通过高尔基体的CDC 42依赖性转运。货物蛋白上的特定序列 由COPI识别的包括二赖氨酸和二精氨酸基序。外壳蛋白也可以间接结合到 COPI通过作为货物受体的跨膜蛋白。其中包括KDEL受体 （KDELR），其将COPI（位于高尔基体膜的胞质侧）连接到细胞内的可溶性蛋白质。 高尔基体（腔侧）具有KDEL序列。KDEL蛋白是一种丰富的内质网蛋白， 折叠（伴侣）。一部分KDEL蛋白质从内质网中泄漏，并从高尔基体被回收到内质网中 通过KDELR和COPI。这种检索中的缺陷导致ER分子伴侣的丢失，这已经被证明是一种缺陷。 发现诱导ER应激。 初步数据显示，来自患者和K652 E γ1-COP纯合子小鼠的成纤维细胞 突变，表达突变蛋白，但表现出有缺陷的COPI介导的运输， 突变破坏了突变COPI复合物与KDELR的结合。突变小鼠具有严重的 低丙种球蛋白血症和抗体应答差。他们的B细胞ER应激增加， 免疫球蛋白（IG）分泌，这是由ER应激抑制剂TUDCA拯救。突变体的T细胞 数量正常，但ER应激增加，并显示凋亡增加和IL-4减少 在体外持续活化后产生。 我们建议验证γ1-COP突变破坏COPI介导的运输的假设 导致ER应激增加，损害B和T细胞功能，增加对细菌的易感性， 和病毒感染，并导致继发于持续病毒感染的CD 4 + T细胞淋巴细胞减少症。 这些研究将阐明一种新的单基因缺陷损害细胞增殖的机制。 COPI介导的运输导致CID伴CD 4 + T细胞淋巴细胞减少症，并将在临床前测试治疗药物 ER应激缓解药物在这种疾病中的疗效。

We identified a biallelic K652E mutation in the γ1-COP subunit of the heptameric coat protein I (COPI) complex in 5 Omani siblings, suffering from recurrent pulmonary infections with encapsulated bacteria, CMV and EBV viremia. The four older siblings presented with severe CD4+ T cell lymphopenia and increased T cell apoptosis; the youngest had normal T cell numbers shortly after birth, but developed CMV viremia and CD4+ T cell lymphopenia at 6 mo. Coat proteins deform membranes to generate transport vesicles. They also bind to cargo proteins to properly sort them into these vesicles, and ensure their transport to specific intracellular compartments. In the early secretory system, the COPII complex transports cargo from the endoplasmic reticulum (ER) to the Golgi, while the COPI complex transports select cargo in the other direction, from the Golgi to the ER, and additionally mediates CDC42-dependent transport through the Golgi. Specific sequences on cargo proteins recognized by COPI include the di-lysine and the di-arginine motifs. Coat proteins can also bind indirectly to COPI through transmembrane proteins that act as cargo receptors. These include the KDEL receptor (KDELR), which links COPI (residing on the cytosolic side of Golgi membrane) to soluble proteins within the Golgi (lumenal side) that have a KDEL sequence. KDEL proteins are abundant ER proteins involved in protein folding (chaperones). A fraction of KDEL proteins leak from the ER, and are retrieved from the Golgi to the ER through the KDELR and COPI. Defect in this retrieval leads to the loss of ER chaperones, which has been found to induce ER stress. Preliminary data show that fibroblasts from patients, and mice homozygous for the K652E γ1-COP mutation, express the mutant protein, but demonstrate defective COPI-mediated trafficking, and that the mutation disrupts the binding of the mutant COPI complex to KDELR. Mutant mice have severe hypogammaglobulinemia and poor antibody responses. Their B cells had increased ER stress and impaired immunoglobulin (Ig) secretion that was rescued by the ER stress inhibitor TUDCA. T cells from the mutant were normal in numbers, but had increased ER stress, and displayed increased apoptosis and diminished IL-4 production following sustained activation in vitro. We propose to test the hypothesis that the γ1-COP mutation disrupts COPI-mediated trafficking causing increased ER stress that impairs B and T cell function, increases susceptibility to bacterial and viral infection, and results in CD4+ T cell lymphopenia secondary to persistent viral infection. The studies proposed will elucidate the mechanisms by which a novel monogenic defect that impairs COPI-mediated trafficking leads to CID with CD4+ T cell lymphopenia, and will test pre-clinically the therapeutic efficacy of ER stress relieving drugs in this disease.