Clinical mutations in the L1 neural cell adhesion molecule affect cell-surface expression

Mutations in the L1 neural cell adhesion molecule, a transmembrane glycopro tein, cause a spectrum of congenital neurological syndromes, ranging from h ydrocephalus to mental retardation. Many of these mutations are single amin o acid changes that are distributed throughout the various domains of the p rotein. Defective herpes simplex virus vectors were used to express L1 prot ein with the clinical missense mutations R184Q and D598N in the Ig2 and Ig6 extracellular domains, respectively, and S1194L in the cytoplasmic domain. All three mutant proteins were expressed at similar levels in infected cel ls. Neurite outgrowth of cerebellar granule cells was stimulated on astrocy tes expressing wild-type or S1194L L1, whereas those expressing R184Q and D 598N L1 failed to increase neurite length. Live cell immunofluorescent stai ning of L1 demonstrated that most defective vector-infected cells did not e xpress R184Q or D598N L1 on their cell surface. This greatly diminished cel l-surface expression occurred in astrocytes, neurons, and non-neural cells. In contrast to wild-type or S1194L L1, the R184Q and D598N L1 proteins had altered apparent molecular weights and remained completely endoglycosidase H (endoH)-sensitive, suggesting incomplete post-translational processing. We propose that some missense mutations in human L1 impede correct protein trafficking, with functional consequences independent of protein activity. This provides a rationale for how expressed, full-length proteins with sing le amino acid changes could cause clinical phenotypes similar in severity t o knock-out mutants.