Pathomechanistic characterization of two exonic L1CAM variants located in trans in an obligate carrier of X-linked hydrocephalus.
Mutations in the gene encoding the neural cell adhesion molecule L1CAM cause several neurological disorders collectively referred to as L1 syndrome. We report here a family case of X-linked hydrocephalus in which an obligate female carrier has two exonic L1CAM missense mutations in trans substituting amino acids in the first (p.W635C) or second (p.V768I) fibronectin-type III domains. We performed various biochemical and cell biological in vitro assays to evaluate the pathogenicity of these variants. Mutant L1-W635C protein accumulates in the endoplasmic reticulum (ER), is not transported into axons, and fails to promote L1CAM-mediated cell-cell adhesion as well as neurite growth. Immunoprecipitation experiments show that L1-W635C associates with the molecular ER chaperone calnexin and is modified by poly-ubiquitination. The mutant L1-V768I protein localizes at the cell surface, is not retained in the ER, and promotes neurite growth similar to wild-type L1CAM. However, the p.V768I mutation impairs L1CAM-mediated cell-cell adhesion albeit less severe than L1-W635C. These data indicate that p.W635C is a novel loss-of-function L1 syndrome mutation. The p.V768I mutation may represent a non-pathogenic variant or a variant associated with low penetrance. The poly-ubiquitination of L1-W635C and its association with the ER chaperone calnexin provide further insights into the molecular mechanisms underlying defective cell surface trafficking of L1CAM in L1 syndrome.
Institute of Anatomy and Cell Biology, Center for Neurosciences, University of Freiburg, Freiburg, Germany.
Mariola
M
Marx
Simone
S
Diestel
Muriel
M
Bozon
Laura
L
Keglowich
Nathalie
N
Drouot
Elisabeth
E
Bouché
Thierry
T
Frebourg
Marie
M
Minz
Pascale
P
Saugier-Veber
Valérie
V
Castellani
Michael K E
MK
Schäfer
D000328:Adult; D002460:Cell Line; D002535:Cerebral Aqueduct / Q000002:abnormalities / Q000378:metabolism / Q000473:pathology; D004252:DNA Mutational Analysis; D005091:Exons; D005260:Female; D040181:Genetic Diseases, X-Linked / Q000235:genetics* / Q000378:metabolism / Q000473:pathology; D014644:Genetic Variation; D006801:Humans; D006849:Hydrocephalus / Q000235:genetics* / Q000378:metabolism / Q000473:pathology; D008297:Male; D008875:Middle Aged; D009154:Mutation; D039842:Neural Cell Adhesion Molecule L1 / Q000235:genetics*; D009474:Neurons / Q000166:cytology / Q000502:physiology; D010375:Pedigree
D039842:Neural Cell Adhesion Molecule L1