PPM - Molecular Genetic Testing of CGG Repeat Expansions in Fragile X Syndrome-Description and References

Description and References

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This document provides an overview of the molecular genetic testing of CGG repeat expansions in the FMR1 gene, specifically focusing on Southern blot analysis and/or polymerase chain reaction (PCR) amplification of FMR1. The module does not cover non-CGG repeat expansion variants and AGG interruptions in the FMR1 gene, and it is intended for clinical laboratory geneticists familiar with FMR1-related disorders and analysis methods.<br /><br />The learning objectives of this module include implementing analysis, interpretation, and reporting based on the most recent technical standards of laboratory testing for fragile X.<br /><br />FMR1 CGG repeat expansion variants are classified into four categories: normal, intermediate or gray zone, premutation, and full mutation. Hypermethylated full mutations are associated with fragile X syndrome in 99% of cases, while premutation alleles can cause a range of neurodegenerative issues and an increased risk of premature ovarian insufficiency in females.<br /><br />Fragile X testing is challenging due to the high GC content, large size repeats, and varying degrees of size and methylation mosaicism. Southern blot methods and triplet repeat–primed PCR are currently considered the gold standards for FMR1 CGG repeat analysis. The American College of Medical Genetics and Genomics (ACMG) has published multiple technical standards and guidelines for fragile X testing.<br /><br />The aim of this module is to promote adherence to the current technical standards published in the Genetics in Medicine journal as the "Laboratory testing for fragile X, 2021 revision: a technical standard of the American College of Medical Genetics and Genomics (ACMG)."<br /><br />The literature references provided include GeneReviews on FMR1 disorders and the ACMG technical standard on laboratory testing for fragile X published in 2021.

Keywords

molecular genetic testing

CGG repeat expansions

FMR1 gene

Southern blot analysis

polymerase chain reaction

FMR1-related disorders

fragile X syndrome

neurodegenerative issues

premature ovarian insufficiency

GC content