A female with typical fragile-X phenotype caused by maternal isodisomy of the entire X chromosome.,Journal of Human Genetics

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A female with typical fragile-X phenotype caused by maternal isodisomy of the entire X chromosome.
Journal of Human Genetics ( IF 2.6 ) Pub Date : 2020-03-06 , DOI: 10.1038/s10038-020-0735-9
Jin-Kyung Kim ₁ , Ji-Eun Jeong ₁ , Jong-Moon Choi ₂ , Gu-Hwan Kim ₃ , Han-Wook Yoo ₃

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Fragile X syndrome (FXS) is the most common inherited cause of intellectual disability, especially in males. Females with FXS tend to be relatively mildly affected because of compensation by a second X chromosome with a normal FMR1 gene. In most cases, FXS is caused by an expansion of the CGG repeats (>200 triplets, full mutation, FM) in the 5'-untranslated region of the FMR1 gene. Premutation alleles (PM, 55-200 repeats), usually lack the clinical features of FXS, are highly unstable when transmitted to offspring and can give rise to FM, especially in female meiosis. We describe a 3-year-old girl with typical FXS, with only a fully expanded FMR1 allele (288 CGG repeats) due to uniparental isodisomy of X chromosome, inherited from mother carrying a premutation allele. The patient's FMR1 methylation region is completely methylated due to full mutation of CGG repeat. This unusual and rare case indicates the importance of a detailed genomic approach to explain nontraditional Mendelian inheritance pattern.

中文翻译：

雌性，具有典型的脆弱X型表型，是由整个X染色体的母体等位线引起的。

脆性X综合征（FXS）是智力残疾的最常见遗传原因，尤其是在男性中。患有FXS的女性倾向于受到相对较轻的影响，因为第二条X染色体具有正常FMR1基因的补偿。在大多数情况下，FXS是由FMR1基因的5'非翻译区中CGG重复序列的扩增（> 200个三联体，完全突变，FM）引起的。突变前等位基因（PM，重复55-200次）通常缺乏FXS的临床特征，在传播给后代时极不稳定，并可能引起FM，尤其是在女性减数分裂中。我们描述了一个具有典型FXS的3岁女孩，由于X染色体的单亲等位线切割，其FMR1等位基因只有一个完全扩展的FMR1等位基因（288个CGG重复序列），继承自携带突变前等位基因的母亲。患者' 由于CGG重复序列的完全突变，其FMR1甲基化区域被完全甲基化。这种罕见的罕见情况表明，采用详细的基因组方法来解释非传统孟德尔遗传模式的重要性。

更新日期：2020-04-24

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