胎儿拷贝数变异的产前诊断与遗传咨询

Prenatal diagnosis and genetic counseling of fetal copy number variation

胎儿拷贝数变异(CNV)是指长度>1 kb的DNA片段的拷贝数增加或者减少,也包括亚显微水平的染色体片段缺失和重复。CNV所导致的胎儿疾病,被统称为微缺失/微重复综合征(MMS)。通过染色体微阵列分析(CMA)或者CNV测序等技术,可以准确、快速检测出CNV、染色体数目异常,以及嵌合比例≥30%嵌合体,可使MMS检出率相较于染色体核型分析提高1.0%~1.7%。由于在胎儿期多数疾病尚无明显和特征性的影像学异常表现,对胎儿致病性CNV(pCNV)进行判断及其遗传咨询,成为目前产前诊断领域的难点。对于胎儿pCNV判断原则包括:①不能仅凭借CNV片段长度判断pCNV,还需要结合其所处位置、所包含基因突变数量、所包含基因是否具有剂量效应进行评估,一般应进一步检测亲代染色体核型、亲代CNV和结合亲代临床表型等综合评估;②片段长度<1 Mb且亲代中携带相同CNV的胎儿CNV致病作用有限,而对于≥1 Mb的新发突变CNV,则意味着其致病风险更高;③同等大小CNV比较时,缺失型CNV的临床影响比重复型更大。可借助ClinGen网站CNV评分系统,快速判断胎儿pCNV,若CNV评分≥0.99,则判定为pCNV;评分为0.9~0.98,则为疑似pCNV(LpCNV);评分为-0.89~0.89,则为临床意义未明(VUS)CNV;评分为-0.98~-0.9,则为疑似良性(LB)CNV;评分≤-0.99,则为良性CNV。正常人群中广泛存在着良性CNV,部分pCNV的临床表现可能并不严重,仅当确诊胎儿具有可导致死亡或严重出生缺陷的pCNV时,临床应建议孕妇及时终止妊娠,防止重大出生缺陷患儿出生。

Fetal copy number variation(CNV)refers to increase or decrease in copy number of DNA fragments with more than 1 kb in length,and it also includes the deletion and duplication of chromosome fragments at submicroscopic level.Diseases caused by fetal CNV are collectively referred as microdeletion and microduplication syndromes(MMS).Through technologies such as chromosome microarray analysis(CMA)and CNV sequencing,CNV and numerical chromosome abnormalities can be detected accurately and quickly,as well as chimera and mosaic individuals with mosaic ratio≥30%.CMA or/and CNV sequencing can increase the detection rate of MMS by 1.0%-1.7%compared with karyotype analysis.Since there are no obvious and characteristic imaging abnormalities in most diseases during fetal period,judgment of fetal pathogenicity CNV(pCNV)and genetic counseling have become main challenges in current prenatal diagnosis service.Therefore,relevant principles for pCNV sequencing are as follows.①Judgement of pCNV fragment is not only based on its fragment size,but also needs to be evaluated based on fragment′s location,the number of gene mutations,and whether gene has a dose effect.It is often necessary to further test parental chromosome karyotype,parental CNV and combined with clinical phenotype of parents,thus conducting a comprehensive evaluation of pCNV.②A fetal CNV with fragment<1 Mb in length and inherited from one parental generation has limited pathogenicity,while a CNV with fragment≥1 Mb length and being a new mutation means that it bears higher pathogenic risk.③When CNV of the same size are compared with each other,clinical impact of chromosome deletion is greater than that of duplicates.CNV scoring system on the ClinGen website can be used to quickly determine fetal pCNV:CNV with a score≥0.99 are judged as pCNV,those with a score between 0.9 to 0.98 are likely pCNV(LpCNV),CNV scored between-0.89 to 0.89 are regarded as variants of uncertain significance(VUS)CNV,and those with a score between-0.98 to-0.9 are determined as likely benign(LB)CNV,and the rest with a score≤-0.99 are benign CNV.Benign CNV is widespread in normal population.Clinical manifestations of some pCNV may not be serious.Only when it is confirmed that fetal pCNV can cause death or serious birth defects,clinicians should advice pregnant women terminate pregnancy in time to prevent major birth defects.