Joó József Gábor, Beke Artúr, Tóth-Pál Ernö, Hargitai Béata, Szigeti Zsanett, Papp Csaba, Papp Zoltán
1st Department of Obstetrics and Gynecology, Faculty of General Medicine, Semmelweis University Medical School, Budapest, Hungary.
J Reprod Med. 2006 Mar;51(3):209-12.
Together with chromosome 19, chromosome 20 belongs to group F, the group of small metacentric chromosomes. Trisomy 20 mosaicism is one of the most frequent chromosomal mosaicisms, representing approximately 16% of prenatally diagnosed cases. In nonmosaic trisomy 20, the usual findings are severe and manifold. Only 3 cases in the literature involved fetuses surviving past the first trimester.
In case 1, a 42-year-old woman presented in her sixth pregnancy; she had had 4 vaginal deliveries of term infants and a miscarriage. Both her familial and personal genetic histories were unremarkable. Genetic amniocentesis was performed in the 18th gestational week for advanced maternal age. Sample analysis revealed a normal, male karyotype in 27 mitoses, while 4 were trisomy 20 (46,XY [27]/47,XY, +20 [4]). In the 37th gestational week a live, immature, male infant weighing 1,730 g was delivered. Chromosomal investigation of the newborn's blood sample did not reveal trisomy 20 but a normal male karyotype. In case 2, a healthy 37-year-old nullipara underwent amniocentesis at the 18th week of pregnancy for advanced maternal age. Amniotic fluid cell karyotype revealed trisomy 20 (47,XX, +20). Ultrasonography performed simultaneously with genetic amniocentesis showed slightly shortened fetal long bones, detectable narrowing of the cranium in the region of the frontal bone, lateral ventricles of 10 mm in width bilaterally, echogenic bowel and polyhydramnios. Abortion was induced in the 23rd week of pregnancy, and a 490-g female fetus was delivered.
Based on these 2 well-documented, prenatally diagnosed cases, as far as genetic counseling is concerned, nonmosaic trisomy 20 is much less challenging than its mosaic form since the prognosis is uniformly poor in the former.
20号染色体与19号染色体同属F组,即小中着丝粒染色体组。20号染色体三体性嵌合体是最常见的染色体嵌合体之一,约占产前诊断病例的16%。在非嵌合型20号染色体三体中,常见的表现严重且多样。文献中仅有3例胎儿在孕早期后存活。
病例1,一名42岁女性,此次为其第六次妊娠;她曾顺产4个足月儿,并有过一次流产。其家族遗传史和个人遗传史均无异常。因孕妇年龄偏大,在孕18周时进行了遗传羊膜腔穿刺术。样本分析显示,27个有丝分裂细胞的核型正常,为男性,而4个为20号染色体三体(46,XY [27]/47,XY, +20 [4])。孕37周时,产下一名体重1730克的存活、不成熟男婴。对新生儿血样进行染色体检查未发现20号染色体三体,而是正常的男性核型。病例2,一名健康的37岁未育女性,因孕妇年龄偏大,在孕18周时进行了羊膜腔穿刺术。羊水细胞染色体核型显示为20号染色体三体(47,XX, +20)。与遗传羊膜腔穿刺术同时进行的超声检查显示,胎儿长骨略短,额骨区域颅骨可检测到变窄,双侧侧脑室宽10毫米,肠回声增强,羊水过多。孕23周时引产,产下一名体重490克的女胎。
基于这2例记录完整的产前诊断病例,就遗传咨询而言,非嵌合型20号染色体三体比其嵌合型的挑战性小得多,因为前者的预后普遍较差。