BIO5 Institute, University of Arizona, Tucson, Arizona, USA.
University of Arizona Genomics Core (UAGC), University of Arizona, Tucson, Arizona, USA.
Epilepsia. 2022 Aug;63(8):1970-1980. doi: 10.1111/epi.17296. Epub 2022 Jun 5.
Family members carrying the same SCN1A variant often exhibit differences in the clinical severity of epilepsy. This variable expressivity suggests that other factors aside from the primary sodium channel variant influence the clinical manifestation. However, identifying such factors has proven challenging in humans.
We perform whole exome sequencing (WES) in a large family in which an SCN1A variant (p.K1372E) is segregating that is associated with a broad spectrum of phenotypes ranging from lack of epilepsy, to febrile seizures and absence seizures, to Dravet syndrome. We assessed the hypothesis that the severity of the SCN1A-related phenotype was affected by alternate alleles at a modifier locus (or loci).
One of our top candidates identified by WES was a second variant in the SCN1A gene (p.L375S) that was shared exclusively by unaffected carriers of the K1372E allele. To test the hypothesized that L375S variant nullifies the loss-of-function effect of K1372E, we transiently expressed Nav1.1 carrying the two variants in HEK293T cells and compared their biophysical properties with the wild-type (WT) variant, and then co-expressed WT with K1372E or L375S with K1372E in equal quantity and tested the functional consequence. The data demonstrated that co-expression of the L375S and K1372E alleles reversed the loss-of-function property brought by the K1372E variant, whereas WT-K1372E co-expression remained partial loss-of-function.
These results support the hypothesis that L375S counteracts the loss-of-function effect of K1372E such that individuals carrying both alleles in trans do not present epilepsy-related symptoms. We demonstrate that monogenic epilepsies with wide expressivity can be modified by additional variants in the disease gene, providing a novel framework for the gene-phenotype relationship in genetic epilepsies.
携带相同 SCN1A 变异的家庭成员在癫痫的临床严重程度上往往存在差异。这种可变表达性表明，除了主要钠离子通道变异外，其他因素也会影响临床表现。然而，在人类中，确定这些因素具有挑战性。
我们对一个大型家族进行了全外显子组测序（WES），该家族中存在 SCN1A 变异（p.K1372E），该变异与从无癫痫发作到热性惊厥和失神发作再到 Dravet 综合征等广泛表型相关。我们评估了以下假设：在修饰基因座（或基因座）上的其他等位基因会影响 SCN1A 相关表型的严重程度。
WES 鉴定的一个候选基因是 SCN1A 基因中的第二个变异（p.L375S），该变异仅在 K1372E 等位基因的无影响携带者中共享。为了测试 L375S 变异是否消除 K1372E 的功能丧失效应，我们在 HEK293T 细胞中瞬时表达携带两种变异的 Nav1.1，并比较它们的生物物理特性与野生型（WT）变异，然后等量共表达 WT 与 K1372E 或 L375S 与 K1372E，并测试功能后果。数据表明，L375S 和 K1372E 等位基因的共表达逆转了 K1372E 变异带来的功能丧失特性，而 WT-K1372E 共表达仍然是部分功能丧失。
这些结果支持 L375S 拮抗 K1372E 功能丧失效应的假设，即携带两种等位基因的个体不会出现与癫痫相关的症状。我们证明，具有广泛表达性的单基因癫痫可以被疾病基因中的其他变异所修饰，为遗传癫痫的基因-表型关系提供了一个新的框架。
