Departments of Dermatology and Cutaneous Biology, Philadelphia, Pennsylvania 19107.
Division of Neurology, The Children's Hospital of Philadelphia, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104.
J Biol Chem. 2010 Mar 26;285(13):10005-10015. doi: 10.1074/jbc.M109.093666. Epub 2010 Jan 27.
Ullrich congenital muscular dystrophy (UCMD) is a disabling and life-threatening disorder resulting from either recessive or dominant mutations in genes encoding collagen VI. Although the majority of the recessive UCMD cases have frameshift or nonsense mutations in COL6A1, COL6A2, or COL6A3, recessive structural mutations in the COL6A2 C-globular region are emerging also. However, the underlying molecular mechanisms have remained elusive. Here we identified a homozygous COL6A2 E624K mutation (C1 subdomain) and a homozygous COL6A2 R876S mutation (C2 subdomain) in two UCMD patients. The consequences of the mutations were investigated using fibroblasts from patients and cells stably transfected with the mutant constructs. In contrast to expectations based on the clinical severity of these two patients, secretion and assembly of collagen VI were moderately affected by the E624K mutation but severely impaired by the R876S substitution. The E624K substitution altered the electrostatic potential of the region surrounding the metal ion-dependent adhesion site, resulting in a collagen VI network containing thick fibrils and spots with densely packed microfibrils. The R876S mutation prevented the chain from assembling into triple-helical collagen VI molecules. The minute amount of collagen VI secreted by the R876S fibroblasts was solely composed of a faster migrating chain corresponding to the C2a splice variant with an alternative C2 subdomain. In transfected cells, the C2a splice variant was able to assemble into short microfibrils. Together, the results suggest that the C2a splice variant may functionally compensate for the loss of the normal COL6A2 chain when mutations occur in the C2 subdomain.
先天性肌营养不良症（UCMD）是一种致残和危及生命的疾病，由编码胶原 VI 的基因的隐性或显性突变引起。虽然大多数隐性 UCMD 病例在 COL6A1、COL6A2 或 COL6A3 中存在移码或无义突变，但也出现了 COL6A2 C-球形区的隐性结构突变。然而，潜在的分子机制仍然难以捉摸。在这里，我们在两名 UCMD 患者中鉴定出 COL6A2 E624K 突变（C1 亚结构域）和 COL6A2 R876S 突变（C2 亚结构域）的纯合子。使用来自患者的成纤维细胞和稳定转染突变构建体的细胞来研究突变的后果。与基于这两个患者的临床严重程度的预期相反，E624K 突变对胶原 VI 的分泌和组装仅有中度影响，但 R876S 取代严重受损。E624K 取代改变了金属离子依赖性粘附位点周围区域的静电势，导致含有厚原纤维和微纤维紧密堆积的点状胶原 VI 网络。R876S 突变阻止了链组装成三螺旋胶原 VI 分子。由 R876S 成纤维细胞分泌的微量胶原 VI 仅由与具有替代 C2 亚结构域的 C2a 剪接变体对应的迁移更快的链组成。在转染细胞中，C2a 剪接变体能够组装成短微纤维。总之，这些结果表明，当 C2 亚结构域发生突变时，C2a 剪接变体可能在功能上补偿正常 COL6A2 链的缺失。
