鈉離子通道基因SCN1A突變及其相關癲綜合征

許小菁 張月華

1 SCN1A基因與電壓門控Na+通道

SCN1A基因定位于染色體2q24.3，長81 kb，有26個外顯子，編碼Na+通道α1亞單位。SCN1A基因屬電壓門控Na+通道編碼基因家族中的一員，該家族還包括SCN2A、SCN3A、SCN7A和SCN9A基因[3]。

α1亞單位作為主體形成通道孔，由2 000個氨基酸組成，具有典型的4×6結構，由4個高度同源的結構域(D1～D4)組成，每個結構域含有6個跨膜區(qū)域(S1～S6，從左至右)，N端和C端直接朝向細胞內部[4](圖1)。

圖1 Na+通道α1亞單位結構模式圖[4]

S4區(qū)含5～8個帶正電荷的氨基酸殘基，稱為“電壓感受區(qū)”；S5和S6間的結構形成反向平行的β折疊襯于孔道內壁，稱為“門孔區(qū)”，Na+經(jīng)此區(qū)而由細胞膜外流入膜內；D3和D4間的細胞內連接環(huán)為“失活門”。

電壓門控Na+通道存在于大多數(shù)可興奮組織細胞膜上的跨膜大分子糖蛋白，是一種電壓敏感性通道，主要具有觸發(fā)動作電位作用。通道內部傳感結構能夠感知細胞膜上的微小電位變化，同時也能被S4跨膜區(qū)域上的正電荷殘基感知，產生去極化，使S4區(qū)域的堿性殘基朝向細胞外側。這些變化反過來可引發(fā)“活化門”的構象重排，使離子孔傳導時間縮短，僅為數(shù)毫秒，離子孔即關閉成為“失活門”。任何細微的電壓變化對于門控通道的影響都是重大的，進而影響到細胞的興奮性[2]。

2 SCN1A基因突變相關癲綜合征

Escayg等[14]首次報道在GEFS+家系中發(fā)現(xiàn)SCN1A基因突變。5%～10%的GEFS+家系存在SCN1A基因突變[15，16]。迄今為止，在GEFS+家系中已發(fā)現(xiàn)42種SCN1A基因突變類型，且均為錯義突變(http://www.molgen.ua.ac.be/SCN1AMutations/ Mutations/ Default.cf-m)。

GEFS+家系具有遺傳異質性，雖然SCN1A基因是GEFS+最常見的致病基因；但SCN1B和氨基丁酸受體γ2亞單位基因(GABRG2)基因突變也與GEFS+家系發(fā)病有關[12，16，17]；SCN2A和GABRD基因突變也分別見諸于一個GEFS+家系報道[18，19]。

表1 20個SCN1A 基因突變GEFS+家系表型分析[14～16,20～33]

近年的研究結果顯示，約80%的Dravet 綜合征患兒具有SCN1A基因突變，基因突變類型也達300余種，包括截斷突變、錯義、無義、堿基缺失及重復等，其中超過一半的患者因無義和框移突變導致蛋白質的截斷[34]。這些突變廣泛分布于整個亞單位蛋白的C端至N端，包括形成Na+通道孔的重要功能區(qū)(S5～6)[36]。SMEB和SMEI患兒的SCN1A基因突變率無明顯區(qū)別，從分子遺傳學的水平也證實兩者無本質區(qū)別，故兩者均稱為Dravet綜合征。Fujiwara等[37]在10例ICEGTC患兒中發(fā)現(xiàn)7例均為SCN1A基因錯義突變，與SMEI、SMEB中SCN1A基因突變率接近。Harkin等[38]在2例ICEGTC患兒中發(fā)現(xiàn)了1例缺失突變(Q1914fsX1943)。

PCR-DNA測序未發(fā)現(xiàn)SCN1A基因突變的病例，采用多重連接依賴的探針擴增技術(MLPA) 已發(fā)現(xiàn)SCN1A基因的片段缺失或重復，從單一外顯子至整個基因的缺失，在Dravet綜合征中的陽性率為10%～15%[39～42]。采用MLPA的方法還可發(fā)現(xiàn)染色體的微缺失，再通過比較基因組雜交和熒光原位雜交的方法驗證缺失位置和大小，并且能發(fā)現(xiàn)SCN1A鄰近基因的缺失。

有研究[46]發(fā)現(xiàn)，8%的Dravet綜合征病例有SCN9A基因突變， 9例SCN9A基因突變中有6例同時存在SCN1A基因突變，分析認為SCN9A可能是SCN1A的修飾調節(jié)基因。 Depienne等[47]發(fā)現(xiàn)SCN1A基因突變陰性的SEMI患兒中，15%的女性患兒有原鈣黏蛋白基因PCDH19 突變， 該基因定位于染色體Xq22。提示PCDH19 基因與Dravet綜合征女性患者致病高度相關，此外有1例男性SMEI患者為PCDH19 基因突變嵌合體。

SIMFE臨床特點：SIMFE可被歸入CFE。SIMFE臨床表現(xiàn)為1歲以內早發(fā)的多變的部分性發(fā)作；EEG以大量多灶性放電為主，不伴全面性或雙側同步化放電；隨病情發(fā)展出現(xiàn)不同程度的智力發(fā)育落后。已在5例患者中發(fā)現(xiàn)3種SCN1A基因突變(F575fsSX48、F1543S和F1543S)[38]。

CGE臨床特點：可表現(xiàn)為多種發(fā)作形式，智力倒退，EEG有全導棘慢波。已發(fā)現(xiàn)6例CGE患兒SCN1A基因的錯義突變(T226M、A395P、V422E、S626G、M973V和IVS15+1G→T)[38]。

3 SCN1A基因型與表型相關性分析

圖2SCN1A基因突變位點分布(不包括大片段缺失和重復)[2]

4 SCN1A基因突變功能研究

電壓門控Na+通道負責控制神經(jīng)元和其他可興奮細胞動作電位的上升支[4]。Na+通道α亞單位蛋白編碼基因SCN1A的突變可導致Na+通道的功能獲得，如持續(xù)的Na+內流，也可導致Na+通道的功能缺失，如Na+通道密度減少，激活與失活的電壓依賴性改變[62]。

在SCN1A基因突變功能研究中，GEFS+中SCN1A基因突變常導致Na+通道失活[65]。Sugawara等[69]將Dravet綜合征SCN1A基因的錯義突變和無義突變(G979R、N985I、F1831S、R712X，R1407X、R1892X)轉入HEK293細胞，通過膜片鉗功能研究發(fā)現(xiàn)，細胞Na+電流明顯減少，通道功能缺失；其中截斷突變R712X、R1407X和R1892X可引起單倍體功能不足，通道表達量下降，導致通道活性的完全缺失。Dravet綜合征相關的SCN1A基因突變R1648C與F1661S在體外突變模型中功能獲得和缺失同時存在[68]。

5 SCN1A基因突變篩查的意義

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