【佳學(xué)基因檢測(cè)】癲癇基因檢測(cè)與遺傳咨詢

癲癇基因檢測(cè)遺傳咨詢導(dǎo)讀：

盡管基因檢測(cè)從未像現(xiàn)在這樣廣泛應(yīng)用于癲癇的患者和家庭成員。但佳學(xué)基因認(rèn)為，開始檢測(cè)之前，應(yīng)考慮許多關(guān)于癲癇基因檢測(cè)社會(huì)、倫理、心理問題。每種基因檢測(cè)，包括單基因測(cè)序、染色體微陣列基因檢測(cè)、基因檢測(cè)包、醫(yī)學(xué)全外顯子、全外顯子組或全基因組測(cè)序基因檢測(cè)——都有各自需要考慮的因素。由訓(xùn)練有素的遺傳學(xué)專業(yè)人員提供的遺傳咨詢有助于患者和家庭應(yīng)對(duì)這些復(fù)雜、必需考慮的因素。關(guān)于遺傳咨詢對(duì)癲癇患者及其家庭成員的影響，一般受檢者知之甚少。但是在其他病的基因檢測(cè)中，正確的遺傳咨詢已被證明在基因檢測(cè)及疾病知識(shí)普及、正確理解疾病風(fēng)險(xiǎn)、減少不同形式的焦慮方面具有很大的幫助作用。國(guó)際抗癲癇聯(lián)盟（ILAE）建議將遺傳咨詢作為所有嬰兒癲癇發(fā)作患者、Dravet綜合征和其他嬰兒癲癇性腦病的遺傳評(píng)估的基本內(nèi)容。同樣地，賊近制定的智力和發(fā)育殘疾成年人癲癇治療建議將基因檢測(cè)作為該患者群體治療的標(biāo)準(zhǔn)組成部分。盡管有這些建議，基因檢測(cè)及遺傳咨詢服務(wù)在臨床神經(jīng)病學(xué)實(shí)踐中的應(yīng)用往往不一致，到目前為止，臨床醫(yī)生沒有正式的實(shí)踐指南可供依賴。賊近對(duì)美國(guó)神經(jīng)學(xué)家的一項(xiàng)調(diào)查發(fā)現(xiàn)，大多數(shù)接受調(diào)查的兒童神經(jīng)學(xué)家會(huì)要求對(duì)一名18歲的嬰兒痙攣史患者進(jìn)行診斷性基因檢測(cè)，該病發(fā)展為L(zhǎng)ennox-Gastaut綜合征。然而，大多數(shù)接受調(diào)查的成年神經(jīng)學(xué)家表示，他們不會(huì)為同一個(gè)假想患者安排診斷性基因檢測(cè)。這種差異突出了為癲癇患者提供基因檢測(cè)及遺傳服務(wù)的不一致性。

遺傳咨詢對(duì)癲癇患者有幾個(gè)好處。首先，基因咨詢可以幫助個(gè)人和家庭在對(duì)基因檢測(cè)和遺傳風(fēng)險(xiǎn)充分了解后做出基于知識(shí)和賊新科技進(jìn)步的決定，也就是基因檢測(cè)的知情決定。其次，它有助于了解基因檢測(cè)的可能結(jié)果以及這些結(jié)果對(duì)診斷和治療的意義。第三，它有助于個(gè)人了解未來子女和其他家庭成員的再次發(fā)生這種病的風(fēng)險(xiǎn)，并幫助患者從基因檢測(cè)中找到任何次要或意外的發(fā)現(xiàn)。第四，它可以使患者接受更為正確的治療。佳學(xué)基因分享一些遺傳性癲癇的例子以及常見的、已建立的和/或與遺傳咨詢和臨床診斷與治療相關(guān)的潛在基因。

癲癇基因檢測(cè)中出現(xiàn)的意義不確定、意義未明的變異和新基因

（VUS）是一種對(duì)于檢測(cè)機(jī)構(gòu)、臨床醫(yī)生來說，其致病性和在疾病中的作用尚不清楚。在臨床基因檢測(cè)采用下一代測(cè)序技術(shù)（NGS)之前，由于基因檢測(cè)只涉及到已知的意義明確的基因位點(diǎn)，意義未明的基因變異位點(diǎn)（VUS)相對(duì)少見。但現(xiàn)在大約有10%的癲癇患者進(jìn)行了全外顯子組測(cè)序，所檢測(cè)的位點(diǎn)及其變異大多數(shù)沒有在基因檢測(cè)數(shù)據(jù)庫(kù)列出。即使采用癲癇基因檢測(cè)包進(jìn)行基因檢測(cè)，這是一種為了降低檢測(cè)價(jià)格，大幅度減少檢測(cè)范圍的一種基因檢測(cè)，在采用數(shù)據(jù)庫(kù)比對(duì)而不是基因解碼技術(shù)進(jìn)行分析的機(jī)構(gòu)中，也會(huì)有高達(dá)40%的基因檢測(cè)報(bào)告出具的是意義未明基因檢測(cè)結(jié)果（VUS）。根據(jù)美國(guó)醫(yī)學(xué)遺傳學(xué)和基因組學(xué)學(xué)院（ACMG），基因檢測(cè)結(jié)果是臨床意義未明時(shí)，不能用于指導(dǎo)臨床診斷和治療的決策依據(jù)。在基因檢測(cè)后，獲得提是臨床意義未明的結(jié)果，可能會(huì)引起患者、家庭成員和臨床醫(yī)生的焦慮，因?yàn)樗麄冊(cè)诮忉尰驒z測(cè)結(jié)果方面經(jīng)驗(yàn)較少。遺傳咨詢，賊好在基因檢測(cè)前后提供，可以幫助患者和家屬為可能的臨床意義未明的結(jié)果做心理準(zhǔn)備，并了解不同性質(zhì)的變異結(jié)果可能意味著什么和它不意味著什么。值得指出的是盡管基因解碼技術(shù)被認(rèn)為是一種可以大幅度提升檢測(cè)陽(yáng)性率、降低意義未明結(jié)果的新型基因密碼破解技術(shù)。它在數(shù)據(jù)庫(kù)比對(duì)的基礎(chǔ)上，增加了基于結(jié)構(gòu)與功能的分析方法，原則上可以檢出一部分未報(bào)道的新型變異，但是也仍有一部分意義未明的基因檢測(cè)報(bào)告。因此，不管采用什么基因檢測(cè)技術(shù)和分析技術(shù)，在檢測(cè)前進(jìn)行遺傳咨詢都是有幫助的。

如果在癲癇患者中沒有發(fā)現(xiàn)致病基因突變，只發(fā)現(xiàn)了意義未明的突變，則在其他家庭成員（通常是父母雙方）中檢測(cè)該基因突變的存在有可能有助于進(jìn)一步評(píng)估該變異的致病性。對(duì)于與嚴(yán)重、早發(fā)常染色體顯性或X連鎖癲癇綜合征相關(guān)的基因變異，父母測(cè)試可能會(huì)確認(rèn)該變異是新發(fā)的，這意味著在父母中找不到該變異，并提供了該變異具有致病性的額外證據(jù)。然而，如果顯示該變異是從健康的父母那里遺傳的，則該變異可能被認(rèn)為不太可能導(dǎo)致疾病。對(duì)于與常染色體隱性遺傳疾病來說的兩個(gè)雜合或者是純合變異，對(duì)父母進(jìn)行測(cè)試助于確定變異產(chǎn)生的方式和時(shí)間。對(duì)于家族性癲癇，在其他家族成員中進(jìn)行基因檢測(cè)可以明確被標(biāo)記為意義未明的基因突變是否在家族內(nèi)存在疾病表現(xiàn)與基因型存在共分離的現(xiàn)象。在基因解碼中這種具有更高分析能力的基因科技術(shù)，臨床表型和基因型的共分離現(xiàn)象是數(shù)據(jù)庫(kù)比對(duì)之處的另一種收集基因突變是否具有致病性的證據(jù)。

此外，全外顯子組或全基因組測(cè)序可以確定沒有明確或尚未建立的基因-疾病關(guān)系的基因序列，即所謂的新基因或候選基因。這些檢測(cè)結(jié)果在以數(shù)據(jù)庫(kù)比對(duì)基礎(chǔ)分析方法的檢測(cè)機(jī)構(gòu)中也會(huì)被標(biāo)記為意義不確定的結(jié)果。這些結(jié)果由于通常不用于臨床決策，但也可能給患者、家屬和臨床醫(yī)生帶來相當(dāng)大的不確定感覺。為了進(jìn)一步研究基因與疾病關(guān)聯(lián)證據(jù)有限的基因突變的致病性，通常需要進(jìn)行額外的研究，包括對(duì)具有相似臨床癥狀的患者時(shí)行致病基因鑒定、對(duì)其他家族成員進(jìn)行基因檢測(cè)，以為基因表型和癥狀的分訑收集證據(jù)。

癲癇基因檢測(cè)與癲癇的反復(fù)風(fēng)險(xiǎn)評(píng)估

遺傳咨詢還可以幫助癲癇患者家庭了解未來兒童和其他家庭成員的反復(fù)風(fēng)險(xiǎn)。許多嚴(yán)重的早發(fā)性癲癇，包括癲癇性腦病，都是新發(fā)突變的結(jié)果。在這種情況下，父母不含有導(dǎo)致癲癇發(fā)生的基因突變，因此遺傳到其他家庭成員的分險(xiǎn)極低。通常，對(duì)于已確認(rèn)的新發(fā)變異，未來妊娠也就是生育下一胎再次罹患同樣疾病的風(fēng)險(xiǎn)<1%。然而，先進(jìn)的基因的基因檢測(cè)機(jī)構(gòu)，采用改進(jìn)的下一代測(cè)序技術(shù)，在沒有出現(xiàn)疾病表征的、或者只受影響的父母中，能夠鑒別出存在具有新發(fā)突變的性腺或體細(xì)胞鑲嵌，這大大增加了下一胎疾病風(fēng)險(xiǎn)的靈敏度。根據(jù)《癲癇疾病發(fā)生的遺傳變異多樣性》，“遺傳鑲嵌”是指由具有不同基因型的細(xì)胞組成的細(xì)胞群體。曾在遺傳鑲嵌的個(gè)體，所有器官或者是某些器官變異型和野生型細(xì)胞的混合而成的。如果變異率低，由于致病性基因突變細(xì)胞未包含基因檢測(cè)所使用的細(xì)胞或組織類型中，采用血液DNA測(cè)序可能會(huì)錯(cuò)過存在的突變。在這些情況下，可能需要對(duì)頭發(fā)、皮膚或唾液等其他組織進(jìn)行測(cè)序?；蚪獯a做的研究發(fā)現(xiàn)，Dravet綜合征患者中大約10%的SCN1A新變突變實(shí)際上是父母的體細(xì)胞鑲嵌。在這些情況下，無法提供正確的關(guān)于后代是否反復(fù)的判斷，但可能接近50%。新出現(xiàn)的數(shù)據(jù)也表明，盡管大多數(shù)嚴(yán)重早發(fā)性癲癇是散發(fā)性的，但在一些較大的患者系列中，高達(dá)20%的基因解碼分析確認(rèn)癲癇性腦病為常染色體隱性遺傳。如果沒有基因解碼、基因檢測(cè)的指導(dǎo)并采用可干預(yù)的生育方式，再生育的每一個(gè)孩子的反復(fù)風(fēng)險(xiǎn)25%。

癲癇基因檢測(cè)額外發(fā)現(xiàn)

隨著診斷性全外顯子組和基因組測(cè)序在臨床實(shí)踐中變得越來越普遍，通過診斷性基因測(cè)試在一些患者中發(fā)現(xiàn)了更多的額外的發(fā)現(xiàn)。這些額外發(fā)現(xiàn)是與主要檢測(cè)指征無關(guān)的遺傳發(fā)現(xiàn)，但對(duì)患者的健康具有醫(yī)學(xué)價(jià)值。ACMG已鑒定出59個(gè)醫(yī)學(xué)上可干預(yù)的基因，并建議基因檢測(cè)報(bào)告在這些基因中發(fā)現(xiàn)的可能致病性和/或致病性基因突變。據(jù)估計(jì)，接受全外顯子組測(cè)序并進(jìn)行基因解碼分析基因檢測(cè)中，約有1-3%會(huì)有醫(yī)學(xué)上可干預(yù)的額外。建議患者在開始基因組診斷測(cè)試之前了解接收額外發(fā)現(xiàn)選項(xiàng)，這可能會(huì)多增加一些少量的費(fèi)用。理想情況下基因檢測(cè)結(jié)構(gòu)通過知情同意程序，讓患者選擇在報(bào)告中接受或不接受此類信息。此外，如果檢測(cè)是采用三人全外顯子基因檢測(cè)方案，由于全外顯子組或全基因組測(cè)序是在父母-子女三人組的基礎(chǔ)上進(jìn)行的，因此測(cè)試可能會(huì)發(fā)現(xiàn)有關(guān)家庭關(guān)系的意外結(jié)果，基因突變的來源，甚至?xí)l(fā)現(xiàn)或非親子關(guān)系。提出基因檢測(cè)的這種可能性通常是基因檢測(cè)開始前知情同意的一部分內(nèi)容。

癲癇基因檢測(cè)的社會(huì)心理學(xué)影響

接受基因診斷可能會(huì)改變家庭的生活。許多具有癲癇病的兒童或年輕成人的家庭已經(jīng)進(jìn)行了多年的“癲癇病診斷之旅”，這可能會(huì)帶來身體上的困難、經(jīng)濟(jì)上的負(fù)擔(dān)和情感上的枯竭。盡管許多家庭對(duì)獲得基因檢測(cè)的結(jié)果后感到輕松，但接受基因診斷可能會(huì)導(dǎo)致內(nèi)疚、焦慮、沮喪或孤獨(dú)感。因?yàn)樵S多基因?qū)е碌陌d癇是罕見的疾病，可用信息有限，與具有相同診斷結(jié)果的其他家庭聯(lián)系的能力有限。此外，基因測(cè)試可能無法給出賊終答案。接受全外顯子組測(cè)序基因檢測(cè)的癲癇患者中約有50-70%得到陰性結(jié)果。負(fù)面結(jié)果可能令人失望，而遺傳咨詢可以幫助家庭預(yù)測(cè)基因檢測(cè)的可能結(jié)果，并幫助他們?cè)谑盏交蛟\斷或不確定結(jié)果后確定應(yīng)對(duì)的方式。

遺傳性癲癇舉例及其基因檢測(cè)

遺傳性全身性癲癇（GGE）

兒童和青少年失神癲癇（CAE、JAE）、青少年肌陣攣性癲癇（JME）和覺醒時(shí)全身強(qiáng)直陣攣發(fā)作癲癇（EGMA）呈現(xiàn)出是典型的GGE臨床表征。這些癲癇亞型在發(fā)作開始時(shí)間、發(fā)作類型和腦電圖（腦電圖）形式方面有清晰的特征。如存在廣義棘波和多棘波復(fù)合形式。CAE中的失神癲癇發(fā)作通常出瑞在3至10歲之間，持續(xù)時(shí)間短，通常約10秒，每天發(fā)作高達(dá)100次。在青春期，這些患者很少出現(xiàn)全身強(qiáng)直陣攣發(fā)作。JAE中的失神性癲癇發(fā)作基本相似，但頻率較低，在青春期開始發(fā)病，青春期的全身強(qiáng)直陣攣發(fā)作更頻繁。肌陣攣性抽搐，尤其是上肢的抽搐，沒有意識(shí)喪失，是JME的臨床特征。該病也是在青春期出現(xiàn)，發(fā)作通常在醒時(shí)發(fā)生，因前一晚缺少睡眠或飲酒而引引起。大約75%的患者出現(xiàn)全身強(qiáng)直陣攣發(fā)作。在青春期，癲癇在覺醒時(shí)出現(xiàn)全身強(qiáng)直陣攣發(fā)作。癲癇發(fā)作通常發(fā)生在患者醒來后兩小時(shí)內(nèi)，與白天無關(guān)。在個(gè)體患者或家庭患者所有臨床綜合征會(huì)出現(xiàn)中重疊。大腦成像不明顯。

基因解碼研究表明，GGE的不同亞型的遺傳方式復(fù)雜，只有少數(shù)罕見的大家族呈現(xiàn)明顯的常染色體顯性遺傳。致病基因鑒定基因解碼在編碼GABAA受體α1亞基的基因發(fā)現(xiàn)先進(jìn)個(gè)突變，引起家族性JME的發(fā)生。第二個(gè)突變?cè)贑AE患者中發(fā)現(xiàn)。基因解碼研究人員為了驗(yàn)證致病基因突變所導(dǎo)致的功能性變化，當(dāng)在爪蟾卵母細(xì)胞或哺乳動(dòng)物細(xì)胞中表達(dá)時(shí)突變的基因序列進(jìn)，α1亞基突變導(dǎo)致GABAA受體功能顯著喪失。這使得基因基因解碼為癲癇的基因檢測(cè)提供了堅(jiān)實(shí)的證據(jù)。

《癲癇發(fā)生的遺傳學(xué)基礎(chǔ)》把微缺失列為GGE產(chǎn)生的風(fēng)險(xiǎn)因素之一。癲癇的致病基因鑒定基因解碼在1.0-2.5%的GGE患者發(fā)現(xiàn)染色體微缺失基因突變，這些微缺失基因突變存在于染色體15q13.3、15q11或16p13上存在微缺失。微缺失基因突變特別是在具有GGE表型和發(fā)育問題或智力殘疾的患者中比較明顯。

泛發(fā)性癲癇的另一種形式是泛發(fā)性（遺傳性）癲癇伴熱性驚厥綜合征（GEFS+）?！栋d癇的各種亞型及其基因檢測(cè)結(jié)果的異同》中GEFS+用來指兒童期發(fā)作的常染色體顯性綜合征，包括發(fā)熱性驚厥和多種非熱性癲癇發(fā)作類型，如同一家系中的全身強(qiáng)直陣攣發(fā)作、失神性癲癇、無張力或肌陣攣性發(fā)作性癲癇。極少病例中出現(xiàn)部分癲癇發(fā)作。有家族史，但家族成員癲癇形式可能不同。雖然GEFS+范疇內(nèi)的癲癇大多是良性的，但少數(shù)家族成員會(huì)出現(xiàn)更嚴(yán)重的癲癇癥狀和發(fā)育問題，類似于肌陣攣性無張力發(fā)作（MAE）或Dravet綜合征癲癇。這使得遺傳咨詢變得重要但困難。致病基因鑒定基因解碼在為這一類癲癇的基因檢測(cè)提供的一個(gè)基因突變位點(diǎn)是編碼電壓門控鈉離子通道β1亞基的SCN1B基因。這一基因缺陷是在大型GEFS+家族中發(fā)現(xiàn)的]。基因解碼研究的功能和結(jié)構(gòu)解析，從而可以明確疾病的發(fā)病機(jī)理，并為新藥研究和治療提供依據(jù)。在這一類病例中，明確了致病基因編碼鈉通道α。如果只檢測(cè)SCNA1,只有10%的GEFS+患者會(huì)出陽(yáng)性基因檢測(cè)結(jié)果。GGE中一種重要的、與治療相關(guān)但罕見的情況是患者出現(xiàn)葡萄糖轉(zhuǎn)運(yùn)蛋白1型基因SLC2A1的突變，這一基因突變使得攜帶有突變的孩子在4歲之前出現(xiàn)開始的早發(fā)失神發(fā)作（EOAE），很少出現(xiàn)經(jīng)典CAE。

GGE患者的基因檢測(cè)：在單一家族中，該疾病組中僅描述了少數(shù)基因的明顯效應(yīng)。基因檢測(cè)應(yīng)包含這些基因。在多種治療效果不佳的癲癇患者，尤其應(yīng)當(dāng)選擇基因覆蓋范圍大的致病基因鑒定基因解碼。如果發(fā)現(xiàn)患者是因?yàn)镾LC2A1突變而引起，可以從生酮飲食中獲益。而如果基因檢測(cè)發(fā)現(xiàn)是SCN1A突變引起的，則應(yīng)避免使用鈉通道阻滯劑。陣列CGH可以檢測(cè)患者是否存在染色體微缺突變，在于存在智力殘疾的患者，尤其應(yīng)當(dāng)考慮這一檢測(cè)方案。

遺傳性局灶性癲癇

3.6.2 Genetic focal epilepsies.
Familial epilepsies in early childhood comprise the syndromes of benign familial neonatal, infantile-neonatal and infantile seizures (BFNS, BFNIS, BFIS). They are characterized by clusters of seizures in the first days or months of life, up to one year of age, resolving spontaneously after weeks to months. Seizures might present as focal or generalized. Interictal EEGs are usually normal. The rare ictal EEG recordings showed focal and generalized discharges. The risk of seizure recurrence later in life is about 15%. Although psychomotor development is usually normal, an increasing number of cases with intellectual disability has been described [52]. In up to 50% of BFIS cases a movement disorder, presenting as paroxysmal kinesigenic dyskinesia (PKD), can occur in school age. It is characterized by involuntary short lasting dyskinesias induced by fast voluntary movements. The combination of both diseases is called ICCA (infantile convulsions and choreoathetosis [53]. Both syndromes respond very well on different anticonvulsive drugs. Mutations in KCNQ2 and KCNQ3 have been identified to cause BFNS [9-11]. KCNQ2 and KCNQ3 channels give rise to the M-current, a slowly activating potassium current which can be suppressed by the activation of muscarinic acetylcholine receptors [54]. Co-expression of heteromeric wild type and mutant KCNQ2/3 channels usually revealed a reduction of the resulting potassium current of about 20-30% which is apparently sufficient to cause BFNS [55]. Mutations in the SCN2A gene encoding one of the α-subunits of voltage-gated sodium channels expressed in mammalian brain are found in BFNIS [56]. The first functional investigations revealed small gain-of-function effects of some mutations predicting an increased neuronal excitability [57,58]. For BFIS, PKD and ICCA, two different loci have been described. In up to 80% of patients, mutations were found in PRRT2 coding for the proline-rich transmembrane domain 2 which is the major gene in these syndromes [59]. Recently, a novel mutation was described in the potassium channel subtype gene SCN8A associated to ICCA [60]. Mutations in SCN8A were also found in severe epileptic encephalopathy (see below) and isolated intellectual disability [61]. In all these syndromes the penetrance is high, up to 80% [62,63].

Patients with mutations in DEPDC5 (Dep domain-containing protein 5) present with a broad spectrum of focal epilepsy syndromes spanning from benign Rolandic epilepsy [64] up to the severe familial focal epilepsy with variable foci (FFEVF). DEPDC5 is part of the GATOR1 complex which acts as an inhibitor of the mTORC1 complex [65,66]. mTORC1 regulates several cellular functions like cell growth, migration and proliferation [67]. In the last years, mutations were found in several genes relevant in the mTOR signaling pathway associated to focal cortical dysplasias which frequently lead to focal epilepsies. Mutations were found in DEPDC5, MTOR, NPRL2/3, PIK3CA and TSC1/TSC2 as germline and somatic mutations. These findings have therapeutical implications since a therapy with mTOR inhibitors (e.g. rapamycin) improve seizures in animal models and patients (for review see [68]). Rolandic epilepsy, also called benign epilepsy of childhood with centrotemporal spike (BECTS), typically presents at the age of 5-6 years with nocturnal focal seizures of the face and vocal tract [69]. It is considered benign since the epilepsy resolves with puberty and most patients show normal psychomotor development. FFEVF is characterized by focal seizures arising from different cortical areas combined with intellectual disability. The onset ranges from infancy to adulthood [70,71]. Mesial (familial mesial temporal lobe epilepsy, FMTLE) [72] or lateral temporal lobe epilepsy (autosomal dominant epilepsy with auditory features, ADLTE) [73] can also be associated to DEPDC5 mutations as well as ADNFLE (autosomal dominant nocturnal frontal lobe epilepsy, see below). FMTLE and ADLTE syndromes are characterized by onset in infancy up to adulthood and benign outcome. In addition to germline mutations of DEPDC5 resulting in these familial epilepsy syndromes, somatic mutations in DEPDC5 as well as other genes of the mTOR pathway were identified in brain specimens of focal cortical dysplasias [73].
The first mutations described for ADLTE were found in LGI1 which was initially described as a tumor gene [74]. LGI1 is important in the regulation of postnatal glutamatergic synapse development and can therefore indirectly influence synaptic processing [75]. Up to 50% of patients with ADLTE are positive for alterations in this gene [76].
For ADNFLE, a first mutation was identified in CHRNA4 encoding the α4-subunit of a neuronal nicotinic acetylcholine receptor (nAChR) as the first ion channel mutation found in an inherited form of epilepsy [7]. Later, mutations in the CHRNB2 gene encoding the β2- subunit of neuronal nAChR and CHRNA2, encoding the nAChR α2-subunit were detected [77,78]. All these mutations reside in the pore-forming M2 transmembrane segments. Different effects on gating of heteromeric α4β2 channels leading either to a gain-of-function or a loss-of-function were reported when most of the known mutations were functionally expressed in Xenopus oocytes or human embryonic kidney (HEK) cells. The exact pathomechanism is not fully understood, but an increased acetylcholine sensitivity could be the main common gating defect of the mutations [77,78]. Only 5-10% of families are positive for nAChR subtypes [79].
Genetic testing in focal epilepsy patients: Genetic testing can be performed in patients with benign and early onset epilepsies belonging to the spectrum of BFNS, BFIS or BFIS since genetic confirmation of the diagnosis allows for genetic counseling, may have implications on therapeutic decisions in difficult-to-treat cases (for example more severe early-onset epilepsies with KCNQ2 or SCN2A mutations profit from sodium channel blockers [80] and prevents further unnecessary and stressful diagnostic procedures. Depending on the age of onset, sequencing of the respective genes (KCNQ2/3, SCN2A, PRRT2, better panel sequencing if possible) should be initiated. For all other forms of (familial) focal epilepsies described above, genetic variants are detected only in a small percentage of families and are even less frequent in single patients. A gene panel analysis including the above-mentioned genes might be useful as positive results would have implications for genetic counseling in families. In case of panels are not available, sequencing of LGI1 in ADTLE and DEPDC5 in various other forms of familial focal epilepsies might generate positive results.
3.6.3 Early Infantile Epileptic Encephalopathies (EIEE) and severe epilepsies of infancy. The term EIEE comprises a large group of epilepsies with the common features of early onset epilepsy (before the age of 3 years) and developmental problems such as psychomotor delay or regression [1]. Within this group of early onset epilepsies, the term “epileptic encephalopathy” is often used in the broader sense of severe epilepsies accompanied by developmental problems. However, according to the definition given by the ILAE, the term “epileptic encephalopathy” should be reserved to situations in which the epilepsy itself causes ongoing cognitive deficits. Recent studies revealed that the activity of the epilepsy is often unrelated to the severity level of the developmental disturbance. In these cases, the main pathophysiological component might be the genetic defect itself leading to epilepsy as well as to disturbances of brain development. Table 1 summarizes the main forms of EE, the most common underlying genes and their relevance for genetic counselling and/or treatment. The different EE syndromes are defined by the onset of seizures, semiology (seizure types) and EEG characteristics. The spectrum includes well known entities such as Ohtahara syndrome, West syndrome, Dravet syndrome and Lennox-Gastaut syndrome as well as less defined clinical pictures (unclassified EE). Figure 2 shows an overview of the main syndromes and the respective genes sorted by the age of onset.
Ohtahara syndrome (OS) is characterized by frequent tonic spasms starting in the first days and weeks of life which are highly pharmaco-resistant. Other seizure types can occur such as focal motor, hemiclonic or generalized tonic-clonic seizures. The EEG shows a characteristic burst suppression pattern (Figure 3). Most patients have developmental problems such as severe global developmental delay and intellectual disability and the mortality rate is high. In 75% of patients, the epilepsy converts into West syndrome, and about 12% of patients present with Lennox-Gastaut syndrome later in life [81].
West syndrome (infantile spasms, IS) starts between 3 and 12 months of age with clustered and frequent infantile spasms, developmental delay, and the characteristic EEG pattern of hypsarrhythmia. It is defined by high amplitude slow waves combined with multifocal irregular epileptic potentials (Figure 3). The criteria (i) early onset, (ii) other seizures types than spasms and (iii) a recurrence of seizures after seizure freedom are negative predicting factors. In contrast, normal MRI and fast responsiveness to therapy are predictive factors for positive outcome [82]. First line therapeutic options are vigabatrin and steroids.
All these syndromes are genetically heterogeneous since several genes were described for each of them (Table 1). OS and IS are genetically overlapping as mutations in genes like ARX, SCN2A, STXBP1 and CDKL5 were found in both syndromes [83,84]. ARX (aristalessrelated homeobox protein) is a transcription factor involved in brain development [85]. ARX mutations are also found in patients with a lissencephaly, a severe disturbance of cortical integrity [86]. Several genes affected in early onset epilepsies and epileptic encephalopathies are involved in the (pre-)synaptic vesicle cycle. Examples are STXBP1 (encoding for syntaxin binding protein 1), STX1B and DNM1 [for overview see 87]. CDKL5 is involved in RNA processing and interacts with MeCP2 by mediating MeCP2 phosphorylation [88,89). Malignant migrating partial seizures in infancy (MMPSI) is a highly pharmaco-resistant epilepsy syndrome starting before 6 months of age. Polymorphic seizures with migrating ictal EEG discharges during the seizures are characteristic for the syndrome. A plateau or regression in psychomotor development is a defining attribute [90]. An important gene for MMPSI is KCNT1 [91] which is coding for a sodium-activated potassium channel. The mutations lead to a gain of function which can be reversed by the potassium channel blocker quinidine that has been reported to be useful in single patients [92,93].
Dravet syndrome (previously known as severe myoclonic epilepsy of infancy, SMEI) is characterized by prolonged and frequent febrile seizures in the first year of life. Later on, patients develop afebrile hemi- or generalized clonic or tonic-clonic seizures, myoclonic seizures and absences, as well as simple and complex partial seizures. Frequently, obtundation status occurs. Cognitive deterioration appears in early childhood. The epilepsy is resistant to pharmacotherapy in most cases [94]. The major gene in this syndrome as well as the most common epilepsy gene in general is SCN1A, encoding a sodium channel alphasubunit [95]. Functional analysis using heterologous expression systems revealed a predominant loss of function mechanism in inhibitory neurons leading to system hyperexcitability [96]. Genetic testing results are relevant to treatment decisions as sodium channel blockers can aggravate seizure frequency in some Dravet patients [97] while specific orphan drugs such as stiripentol can be used. Although SCN1A mutations occur de novo in most patients, mosaic status in parents is possible and should be ruled out carefully (see above and 37].
Patients with MAE (Doose syndrome) present between ages 1 to 3 years with generalized tonic-clonic, myoclonic and atonic seizures as well as absences. In many patients, the genetic background of the disease remains unknown. However, mutations in the GABA transporter gene SLC6A1 are found in about 4% of cases [98]. Mutations in SLC2A1 are rare but identification of these patients allows for a specific therapy, i.e. the ketogenic diet [99]. Lennox-Gastaut syndrome (LGS) is characterized by polymorphic seizures combined with developmental delay or regression with onset between 3 to 5 years of age. Frequently, the disease starts as Ohtahara or West syndrome and evolves into Lennox-Gastaux syndrome. Seizure types characterizing LGS are atypical absences, tonic, atonic or generalized tonicclonic seizures including (tonic) drop attacks with high risk for injuries. EEG characteristics are slow spike wave complexes and polyspikes (Figure 3). LGS is very heterogeneous since causing mutations were described in a bunch of genes such as SCN1A, STXBP1, SCN2A and CDKL5, especially when the disease starts in early childhood (see above) [100].
Common metabolic forms of early infantile epileptic encephalopathies comprise Glut1 deficiency and pyridoxine-dependent epilepsies. Glut1-Deficiency syndrome starts in the first few months of age with clusters of dyscognitive seizures and non-convulsive status epilepticus predominantly in fasting state e.g. just before to breakfast. The children development a severe psychomotor retardation, dystonic features and ataxia (Seidner) [101]. The spectrum of glucose transporter type 1, the glucose transporter of the blood-brainbarrier, associated syndromes is broad since also paroxysmal exercise induced movement disorder (PED), childhood absence epilepsy (CAE) and early onset absence epilepsy (EOAE) starting before 3 years of age were found to be caused by mutations in SLC2A1 coding for Glut1 [15,51,102]. With the ketogenic diet, which bypasses the defect in glucose metabolism, a specific therapy is available.
Vitamin B6 (pyridoxine) dependent epilepsies typically start in the first days of life or are even recognized as intrauterine seizures during pregnancy. The seizures are highly pharmacoresistant. Burst suppression patterns and hypsarrhythmia in EEG have been described (Mills) [97]. Affected children show global developmental delay. Sometimes, a history of perinatal problems such as premature delivery or asphyxia pretends symptomatic epilepsy. Biallelic mutations in ALDH7A1 (coding for the alpha-aminoadipic semialdehyde dehydrogenase antiquitin) and more rarely PNPO (coding for pyridoxamine phosphate oxidase) involved in the pyridoxin metabolism are responsible for these metabolic epilepsies and an early therapy with pyridoxine (in antiquitin deficiency) or pyridoxal-5-phosphate (in pyridoxamine phosphate-oxidase deficiency) should be started [103].
There are many children who present with non-specific phenotypes rendering targeted genetic testing impossible. In this context, two additional genes should be mentioned since the detection of mutations in these genes might have implications on therapy. Mutations were found in in the potassium channel gene KCNA2 in a Dravet-like phenotype [104] and mutations in the NMDA glutamate receptor genes GRIN2A and GRIN2B are found in patients with non-specific epileptic encephalopathies or epilepsy-aphasia-spectrum disorders. Pathological effects of gain-of-function mutations might be specifically blocked by memantine [18,105].
Last but not least CNVs were described in several forms of EE in up to 5% of cases including specific phenotypes such as LGS as well as unclassified EE [106].
Genetic testing in patients with EIEE and severe epilepsies in infancy: For all subtypes of EE genetic testing is highly recommended since a positive result avoids further diagnostics, allows for prognostic estimations and might have implications on therapeutic decisions. In this group of epilepsies, many private and de novo mutations are found. Few genes follow autosomal-recessive inheritance. Due to phenotypic and genotypic heterogeneity, a gene panel approach combined with a CNV analysis is recommended. In few cases, e.g. in typical Dravet syndrome or pyridoxine-responsive epilepsy, a targeted gene analysis will be effective. Prior to initiation of genetic testing as well as in cases with positive results, genetic counselling should be offered to parents.
Genetic testing is recommended in all forms of epileptic encephalopathies since it has important influence in patient`s management. The genetic result helps to define a diagnosis, can spare further diagnostics, give advice for prognosis and genetic counseling and may influence therapy decisions. Prior to genetic testing a detailed genetic counseling is necessary to prevent negative socio-psychological effects in the affected families and unnecessary health costs. To date, only in a few of the common forms of generalized and focal epilepsies genetic testing should be performed as a routine diagnostic step, since major genes and consequences for clinical management are missing in most cases.