人类正常细胞核中有22对常染色体与1对性染色体,但一些人细胞核内另有多余的常染色体片段,被称为编外标记染色体(small supernumerary marker chromosome,sSMC).sSMC属于染色体结构异常,因其片段太小,并缺少明显的显带模式,无法通过传统的细胞遗传学显带技术进行识别,需要采用芯片比较基因组杂交或荧光原位杂交等多种分子生物学诊断方法才能确诊.由sSMC导致的染色体异常综合征较多,常见为Pallister-Killian综合征、等臂18p染色体综合征、猫眼综合征、Emanuel综合征.智力障碍人群中sSMC中发生率较高,临床缺乏特异性表现,随着分子细胞遗传学分析技术的提高,sSMC的识别率逐步提高.遗传咨询及产前诊断是减少sSMC发生的重要措施,分子生物学技术是明确sSMC的必要方法.现就sSMC相关18p染色体异常综合征的核型特点、发病机制、临床表现等进行综述.%Humans typically have 22 pairs of autosomal chromosomes in cells,and a pair of sex chromosomes.Some individuals have an extra,autosomal chromosome called a small supernumerary marker chromosome (sSMC).sSMC is a structurally abnormal chromosome fragment.The fragments are too small and no-specific banding pattern to be identified by conventional banding cytogenetic analysis.Array-based comparative genomic hybridization (aCGH),fluorescence in situ hybridization (FISH) or other molecular biological methods are necessary for the diagnosis.This article summarized the karyotype,pathogenesis,and the clinical manifestations of the sSMC-related chromosome 18p abnormalities.The patients with sSMC usually presented with abnormal chromosome syndrome.Some syndromes are relative common,such as Pallister-Killian syndrome,isochromosome 18p syndrome,Cat eye syndromes or Emanuel syndrome.sSMC is considered to be the frequent cause of mental retardation.The patients have no specific symptoms.With the progress of molecular cytogenetics,more sSMC has been identified.Genetic counseling and prenatal diagnosis are important to prevent sSMC.Molecular cytogenetic techniques are necessary to the diagnosis.
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