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

我们在七聚体外壳蛋白I的γ1-COP亚基中发现了一个双等位基因K652E突变。 5名阿曼兄弟姐妹患有反复肺部感染的包裹性细菌CMV 和EBV病毒血症。四个年长的兄弟姐妹表现出严重的CD4+T淋巴细胞减少和T细胞增加 细胞凋亡：最小的出生后不久T细胞数量正常，但出现CMV病毒血症和CD4+T细胞 6个月时细胞淋巴细胞减少。 外壳蛋白使细胞膜变形，产生运输小泡。它们还与货物蛋白结合，以 正确地将它们分类到这些小泡中，并确保它们被输送到特定的细胞内隔间。在 早期的分泌系统，COPII复合体将货物从内质网(ER)运送到高尔基体， 而COPI复合体从高尔基到ER，从另一个方向运输选定的货物，以及 此外，还通过高尔基体介导镉依赖的转运。货运蛋白上的特定序列 被COPI识别的基序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)，它将COPI(驻留在高尔基膜的胞液一侧)连接到 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，与蛋白质有关 折叠(监护人)。一小部分KDEL蛋白从内质网泄漏，并从高尔基体被回收到内质网 通过KDELR和COPI。这种检索的缺陷导致内质网伴侣的丢失，这已经是 发现会诱发内质网应激。 初步数据显示，来自患者和小鼠的成纤维细胞为K652Eγ1-COP纯合 突变，表达突变蛋白，但证明有缺陷的COPI介导的转运，并且 突变破坏了突变的COPI复合体与KDELR的结合。突变的小鼠有严重的 低丙种球蛋白血症和抗体反应差。他们的B细胞内质网应激增加，受损 内质网应激抑制剂TUDCA挽救的免疫球蛋白(Ig)分泌。突变体中的T细胞 数量正常，但增加了内质网应激，并显示出细胞凋亡增加和IL-4减少 在体外持续激活后产生。 我们建议检验γ1-COP突变破坏COPI1-COP介导的交易的假设 导致内质网应激增加，损害B和T细胞功能，增加对细菌的易感性 和病毒感染，并导致持续病毒感染继发的CD4+T细胞淋巴细胞减少。 提出的研究将阐明一种新的单基因缺陷损害 COPI介导的转运导致CID合并CD4+T淋巴细胞减少，并将在临床前测试治疗 内质网应激缓解药物对本病的疗效。

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.