EMQN best practice guidelines for the molecular genetic testing and reporting of fragile X syndrome and other fragile X-associated disorders

Abstract

Different mutations occurring in the unstable CGG repeat in 5' untranslated region of FMR1 gene are responsible for three fragile X-associated disorders. An expansion of over ∼200 CGG repeats when associated with abnormal methylation and inactivation of the promoter is the mutation termed ‘full mutation' and is responsible for fragile X syndrome (FXS), a neurodevelopmental disorder described as the most common cause of inherited intellectual impairment. The term ‘abnormal methylation' is used here to distinguish the DNA methylation induced by the expanded repeat from the ‘normal methylation' occurring on the inactive X chromosomes in females with normal, premutation, and full mutation alleles. All male and roughly half of the female full mutation carriers have FXS. Another anomaly termed ‘premutation' is characterized by the presence of 55 to ∼200 CGGs without abnormal methylation, and is the cause of two other diseases with incomplete penetrance. One is fragile X-associated primary ovarian insufficiency (FXPOI), which is characterized by a large spectrum of ovarian dysfunction phenotypes and possible early menopause as the end stage. The other is fragile X-associated tremor/ataxia syndrome (FXTAS), which is a late onset neurodegenerative disorder affecting males and females. Because of the particular pattern and transmission of the CGG repeat, appropriate molecular testing and reporting is very important for the optimal genetic counselling in the three fragile X-associated disorders. Here, we describe best practice guidelines for genetic analysis and reporting in FXS, FXPOI, and FXTAS, including carrier and prenatal testing.

Neurogenética en el Perú, ejemplo de investigación traslacional

“…El síndrome X frágil, la causa más común de discapacidad intelectual heredada, es causado por la expansión del triplete CGG en el gen FMR1 en el cromosoma X (18) . La técnica estándar de oro para el diagnóstico molecular de esta enfermedad, no disponible en el Perú, está basada en Southern Blot.…”

Identification of Males with Cryptic Fragile X Alleles by Methylation-Specific Quantitative Melt Analysis

“…Methylation mosaicism or PM/FM CGG size mosaicism have been previously reported in 12% to 41% of FM males identified (14,15 ) with the conventional 2-step testing protocol (16 ). The phenotype of these males is milder than in FM males with a completely methylated FMR1 promoter, presumably because a proportion of cells in these individuals express FMRP (17 ).…”

“…The current conventional 2-step testing protocol for FXS uses a relatively low-cost first-line CGG-sizing PCR assay to exclude individuals with a normal or GZ allele from reflexed FM testing with methylation-sensitive Southern blot (mSouthern) or a triplet primed PCR test (16,22,23 ). Because the standard CGG sizing PCR does not amplify alleles with Ͼ130 repeats, only PM positives, "no product" samples (e.g., males with FM) or females "apparently" homozygous for the same NS allele are reflexed for second-line testing.…”

Leitlinien zur molekulargenetischen Diagnostik: Fragiles-X Syndrom und andere FMR1-assoziierte Syndrome

“…Die zugrundeliegende molekulare Veränderung betrifft das X-chromosomale Gen FMR1 (fragile X mental retardation 1). Dieses enthält in der nichttranslatierten Region (5'UTR) des ersten Exons einen variablen repetitiven Abschnitt mit tandemartig wiederholten CGG-Tripletts, der als CGG-Repeat bezeichnet wird.…”