Araújo A, Ramos E S
Grupo de Epigenética e Reprodução, Departamento de Genética, Faculdade de Medicina de Ribeirão Preto, Universidade de São Paulo, Ribeirão Preto, SP, Brasil.
Braz J Med Biol Res. 2008 May;41(5):368-72. doi: 10.1590/s0100-879x2008000500004.
The high abortion rate of 45,X embryos indicates that patients with Turner syndrome and 45,X karyotype could be mosaics, in at least one phase of embryo development or cellular lineage, due to the need for the other sex chromosome presence for conceptus to be compatible with life. In cases of structural chromosomal aberrations or hidden mosaicism, conventional cytogenetic techniques can be ineffective and molecular investigation is indicated. Two hundred and fifty patients with Turner syndrome stigmata were studied and 36 who had female genitalia and had been cytogenetically diagnosed as having "pure" 45,X karyotype were selected after 100 metaphases were analyzed in order to exclude mosaicism and the presence of genomic Y-specific sequences (SRY, TSPY, and DAZ) was excluded by PCR. Genomic DNA was extracted from peripheral blood and screened by the human androgen receptor (HUMARA) assay. The HUMARA gene has a polymorphic CAG repeat and, in the presence of a second chromosome with a different HUMARA allele, a second band will be amplified by PCR. Additionally, the CAG repeats contain two methylation-sensitive HpaII enzyme restriction sites, which can be used to verify skewed inactivation. Twenty-five percent (9/36) of the cases showed a cryptic mosaicism involving a second X and approximately 14% (5/36), or 55% (5/9) of the patients with cryptic mosaicism, also presented skewed inactivation. The laboratory identification of the second X chromosome and its inactivation pattern are important for the clinical management (hormone replacement therapy, and inclusion in an oocyte donation program) and prognostic counseling of patients with Turner syndrome.
45,X胚胎的高流产率表明，患有特纳综合征且核型为45,X的患者在胚胎发育或细胞谱系的至少一个阶段可能是嵌合体，因为胚胎需要另一条性染色体的存在才能与生命相容。在结构性染色体畸变或隐匿性嵌合的情况下，传统的细胞遗传学技术可能无效，此时需要进行分子研究。对250例有特纳综合征体征的患者进行了研究，在分析了100个中期分裂相后，选择了36例具有女性生殖器且细胞遗传学诊断为“纯”45,X核型的患者，以排除嵌合体，并通过PCR排除基因组Y特异性序列（SRY、TSPY和DAZ）的存在。从外周血中提取基因组DNA并通过人类雄激素受体（HUMARA）分析进行筛查。HUMARA基因有一个多态性的CAG重复序列，在存在具有不同HUMARA等位基因的第二条染色体时，PCR会扩增出第二条带。此外，CAG重复序列包含两个对甲基化敏感的HpaII酶切位点，可用于验证偏态失活。25%（9/36）的病例显示存在涉及第二条X染色体的隐匿性嵌合，约14%（5/36），即隐匿性嵌合患者中的55%（5/9）也表现出偏态失活。第二条X染色体的实验室鉴定及其失活模式对于特纳综合征患者的临床管理（激素替代疗法以及纳入卵母细胞捐赠计划）和预后咨询非常重要。
