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

Significance Mutations of SCN1A, a gene encoding the Nav1.1 Na+ channel, can cause familial hemiplegic migraine (FHM-3) or epilepsy. Epileptogenic mutations induce reduction of its function, leading to decreased excitability of GABAergic neurons, but studies of FHM-3 mutations have generated confusing results. We have reported gain-of-function effects, but complete loss of function has been reported for the mutant L1649Q, a paradoxical result. Here we show that L1649Q function can be rescued in conditions that more closely model real conditions in neurons. When partially rescued, L1649Q shows gain of function and can induce neuronal hyperexcitability, consistently with hyperexcitability of GABAergic neurons as the cause of FHM-3. 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 Nav1.1 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 NaV1.1 FHM mutations have generated controversial results. In particular, it has been shown that the NaV1.1-L1649Q mutant is nonfunctional when expressed in a human cell line because of impaired plasma membrane expression, similarly to NaV1.1 mutants that cause severe epilepsy, but we have observed gain-of-function effects for other NaV1.1 FHM mutants. Here we show that NaV1.1-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 NaV1.1 mutants show loss of function also when rescued. Our results are consistent with gain of function as the functional effect of NaV1.1 FHM mutations and hyperexcitability of GABAergic neurons as the pathomechanism of FHM type 3.