Nonfunctional NaV1.1 familial hemiplegic migraine mutant transformed into gain of function by partial rescue of folding defects

Familial hemiplegic migraine (FHM) is a rare subtype of migraine with aura. Mutations causing FHM type 3 have been identified in SCN1A, the gene encoding the Na(V)1.1 Na+ channel, which is also a major target of epileptogenic mutations and is particularly important for the excitability of GABAergic neurons. However, functional studies of Na(V)1.1 FHM mutations have generated controversial results. In particular, it has been shown that the Na(V)1.1-L1649Q mutant is nonfunctional when expressed in a human cell line because of impaired plasma membrane expression, similarly to Na(V)1.1 mutants that cause severe epilepsy, but we have observed gain-off-unction effects for other Na(V)1.1 FHM mutants. Here we show that Na(V)1.1-L1649Q is nonfunctional because of folding defects that are rescuable by incubation at lower temperatures or coexpression of interacting proteins, and that a partial rescue is sufficient for inducing an overall gain of function because of the modifications in gating properties. Strikingly, when expressed in neurons, the mutant was partially rescued and was a constitutive gain of function. A computational model showed that 35% rescue can be sufficient for inducing gain of function. Interestingly, previously described folding-defective epileptogenic Na(V)1.1 mutants show loss of function also when rescued. Our results are consistent with gain of function as the functional effect of Na(V)1.1 FHM mutations and hyperexcitability of GABAergic neurons as the pathomechanism of FHM type 3.