Section on Gene Structure and Disease, Laboratory of Cell and Molecular Biology, National Institute of Diabetes, Digestive, and Kidney Diseases, National Institutes of Health, Bethesda, Maryland.
Section on Gene Structure and Disease, Laboratory of Cell and Molecular Biology, National Institute of Diabetes, Digestive, and Kidney Diseases, National Institutes of Health, Bethesda, Maryland.
J Mol Diagn. 2017 Nov;19(6):828-835. doi: 10.1016/j.jmoldx.2017.06.008. Epub 2017 Aug 14.
The learning disability fragile X syndrome results from the presence of >200 CGG/CCG repeats in exon 1 of the X-linked gene FMR1. Such alleles arise by expansion from maternally transmitted FMR1 premutation alleles, alleles having 55 to 200 repeats. Expansion risk is directly related to maternal repeat number. However, AGG interruptions to the repeat tract are important modifiers of expansion risk. Thus, the ability to identify such interruptions is crucial for the appropriate genetic counseling of females who are premutation carriers. First-generation triplet-primed PCR assays allow these interruptions to be detected. However, because the triplet primer used has multiple binding sites in the repeat tract, interpreting the results is not straightforward and it is not always possible to unambiguously determine the AGG-interruption status in females because of the difficulties associated with the presence of a second X chromosome. Interpretation is further complicated by any repeat size mosaicism that may be present. We have developed second-generation PCR assays that prime specifically at the interruptions. These assays are simpler to interpret and better able to evaluate this important determinant of expansion risk in females even in those with a mixture of premutation allele sizes.
学习障碍脆性 X 综合征是由 X 连锁基因 FMR1 外显子 1 中 >200 个 CGG/CCG 重复引起的。这种等位基因是通过从母系传递的 FMR1 前突变等位基因（具有 55 至 200 个重复的等位基因）扩展而来的。扩展风险与母系重复数直接相关。然而，重复序列中的 AGG 中断是扩展风险的重要修饰因子。因此，能够识别这些中断对于适当的遗传咨询至关重要，尤其是对于携带前突变等位基因的女性。第一代三核苷酸引物 PCR 检测可检测到这些中断。然而，由于用于三核苷酸引物的重复序列中有多个结合位点，因此解释结果并不简单，并且由于存在第二条 X 染色体的相关困难，并非总是能够明确确定女性的 AGG 中断状态，尤其是当存在重复大小嵌合体时。任何重复大小镶嵌体的存在都会进一步使解释复杂化。我们开发了第二代 PCR 检测，该检测专门在中断处进行引物延伸。这些检测方法更易于解释，并且即使在存在前突变等位基因大小混合的情况下，也能更好地评估女性扩展风险的这一重要决定因素。
