The Monique and Jacques Roboh Department of Genetic Research, Hadassah, Hebrew University Medical Center, Jerusalem, Israel.
J Med Genet. 2013 Feb;50(2):118-23. doi: 10.1136/jmedgenet-2012-101223.
Early infantile epileptic encephalopathies usually manifest as severely impaired cognitive and motor development and often result in a devastating permanent global developmental delay and intellectual disability. A large set of genes has been implicated in the aetiology of this heterogeneous group of disorders. Among these, the ion channelopathies play a prominent role. In this study, we investigated the genetic cause of infantile epilepsy in three affected siblings.
Homozygosity mapping in DNA samples followed by exome analysis in one of the patients resulted in the identification of a homozygous mutation, p.L1040P, in the CACNA2D2 gene. This gene encodes the auxiliary α(2)δ2 subunit of high voltage gated calcium channels. The expression of the α(2)δ2-L1040P mutant instead of α(2)δ2 wild-type (WT) in Xenopus laevis oocytes was associated with a notable reduction of current density of both N (Ca(V)2.2) and L (Ca(V)1.2) type calcium channels. Western blot and confocal imaging analyses showed that the α(2)δ2-L1040P mutant was synthesised normally in oocyte but only the α(2)δ2-WT, and not the α(2)δ2-L1040P mutant, increased the expression of α(1B), the pore forming subunit of Ca(V)2.2, at the plasma membrane. The expression of α(2)δ2-WT with Ca(V)2.2 increased the surface expression of α(1B) 2.5-3 fold and accelerated current inactivation, whereas α(2)δ2-L1040P did not produce any of these effects.
L1040P mutation in the CACNA2D2 gene is associated with dysfunction of α(2)δ2, resulting in reduced current density and slow inactivation in neuronal calcium channels. The prolonged calcium entry during depolarisation and changes in surface density of calcium channels caused by deficient α(2)δ2 could underlie the epileptic phenotype. This is the first report of an encephalopathy caused by mutation in the auxiliary α(2)δ subunit of high voltage gated calcium channels in humans, illustrating the importance of this subunit in normal physiology of the human brain.
早发性婴儿癫痫性脑病通常表现为严重的认知和运动发育受损，常导致毁灭性的永久性全面发育迟缓及智力障碍。一大组基因与这组异质性疾病有关。在这些基因中，离子通道病起着突出的作用。在这项研究中，我们调查了 3 名受影响的兄弟姐妹中婴儿癫痫的遗传原因。
对一名患者的 DNA 样本进行纯合子作图，然后进行外显子组分析，导致 CACNA2D2 基因纯合突变 p.L1040P 的鉴定。该基因编码高电压门控钙通道的辅助α(2)δ2 亚基。α(2)δ2-L1040P 突变体而不是野生型（WT）α(2)δ2 在非洲爪蟾卵母细胞中的表达与 N（Ca(V)2.2）和 L（Ca(V)1.2）型钙通道的电流密度显著降低有关。Western blot 和共聚焦成像分析表明，α(2)δ2-L1040P 突变体在卵母细胞中正常合成，但只有 WTα(2)δ2，而不是 α(2)δ2-L1040P 突变体，增加了α(1B)的表达，即 Ca(V)2.2 的孔形成亚基，在质膜上。α(2)δ2-WT 与 Ca(V)2.2 的表达增加了α(1B)的表面表达 2.5-3 倍，并加速了电流失活，而α(2)δ2-L1040P 则没有产生任何这些效应。
CACNA2D2 基因中的 L1040P 突变与α(2)δ2 的功能障碍有关，导致神经元钙通道的电流密度降低和缓慢失活。去极化期间钙内流的延长和钙通道表面密度的变化可能是由α(2)δ2 缺乏引起的，这可能是癫痫表型的基础。这是人类高电压门控钙通道辅助α(2)δ 亚基突变引起的脑病的首例报告，说明了该亚基在人类大脑正常生理中的重要性。
