Recessive COL6A2 C-globular missense mutations in Ullrich congenital muscular dystrophy: role of the C2a splice variant.
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.
Zhang
Rui-Zhu
RZ
Departments of Dermatology and Cutaneous Biology, Philadelphia, Pennsylvania 19107.
Zou
Yaqun
Y
Division of Neurology, The Children's Hospital of Philadelphia, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104.
Pan
Te-Cheng
TC
Departments of Dermatology and Cutaneous Biology, Philadelphia, Pennsylvania 19107.
Markova
Dessislava
D
Departments of Dermatology and Cutaneous Biology, Philadelphia, Pennsylvania 19107.
Fertala
Andrzej
A
Departments of Dermatology and Cutaneous Biology, Philadelphia, Pennsylvania 19107.
Hu
Ying
Y
Division of Neurology, The Children's Hospital of Philadelphia, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104.
Squarzoni
Stefano
S
Institute of Molecular Genetics-National Research Council, Unit of Bologna, 40136 Bologna, Italy.
Reed
Umbertina Conti
UC
Departamento de Neurologia, Faculdade de Medicina da Universidade de Sao Paulo, O5403-000 Sao Paulo SP, Brazil.
Marie
Suely K N
SKN
Departamento de Neurologia, Faculdade de Medicina da Universidade de Sao Paulo, O5403-000 Sao Paulo SP, Brazil.
Bönnemann
Carsten G
CG
Division of Neurology, The Children's Hospital of Philadelphia, University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania 19104.
Chu
Mon-Li
ML
Departments of Dermatology and Cutaneous Biology, Philadelphia, Pennsylvania 19107; Biochemistry and Molecular Biology, Thomas Jefferson University, Philadelphia, Pennsylvania 19107. Electronic address: mon-li.chu@jefferson.edu.
D000328:Adult; D017398:Alternative Splicing; D000595:Amino Acid Sequence; D001706:Biopsy; D002648:Child; D003094:Collagen / Q000737:chemistry; D024142:Collagen Type VI / Q000235:genetics*; D005260:Female; D005347:Fibroblasts / Q000378:metabolism; D005808:Genes, Recessive; D006720:Homozygote; D006801:Humans; D007477:Ions; D008297:Male; D008969:Molecular Sequence Data; D009136:Muscular Dystrophies / Q000151:congenital* / Q000235:genetics*; D020125:Mutation, Missense; D017386:Sequence Homology, Amino Acid
C504344:COL6A2 protein, human; D024142:Collagen Type VI; D007477:Ions; D003094:Collagen