Effect of AGG Interruptions on FMR1 Maternal Transmissions.

Olatz Villate,Nekane Ibarluzea,María Isabel Tejada,Hiart Maortua,Silvia Izquierdo-Álvarez,Ana Belén De La Hoz,Laia Rodriguez-Revenga

doi:10.3389/fmolb.2020.00135

Abstract

There are four classes of CGG repeat alleles in the FMR1 gene: normal alleles have up to 44 repeats; patients with Fragile X Syndrome have more than 200 repeats; those between 55 and 200 CGGs are considered FMR1 premutation alleles, because they are associated with maternal expansions of the number of CGGs in the next generation and finally, alleles between 45 and 54 CGGs are called intermediate or gray zone alleles. In these last categories, the stability depends on the presence of AGG interruptions, which usually occurs between 9 and 10 CGGs. In this context, we have studied retrospectively 66 women with CGG repeats between 45 and 65, and their offspring. In total 87 transmissions were analyzed with triplet repeat primed PCR using AmplideX® FMR1 PCR (Asuragen, Austin, TX, USA) and we found that alleles with CGG repeats between 45 and 58 do not expand in the next generation except two cases with 56 repeats and 0 AGG interruptions. Furthermore, we have found four females with alleles with more than 59 CGG repeats and 2 AGG interruptions that do not expand either. Alleles from 56 CGG repeats without AGGs expand in all cases. In light of these results and those of the literature, we consider that the risk of unstable transmissions should be based on the presence or absence of AGG interruptions and not on the classical cutoffs which define each category of FMR1 alleles. The application of these results in the genetic and reproductive counseling is essential and AGG interruptions should always be studied.

Highlights

The FMR1 gene is the gene responsible for Fragile X Syndrome (FXS) affecting ∼1/3,717 to 1/8,918Caucasian males (Crawford et al, 2001)
In Spanish women with PM, we found similar frequencies to that previously reported for FXPOI (22.61%) (Merino et al, 2016), and for FXTAS with frequencies from 3.27% (Merino et al, 2016) up to
In our study we have found that alleles within the range of 45–59 CGG repeats and two AGG interruptions do not expand in the generation

Summary

Introduction

The FMR1 gene is the gene responsible for Fragile X Syndrome (FXS) affecting ∼1/3,717 to 1/8,918Caucasian males (Crawford et al, 2001). The FMR1 gene is the gene responsible for Fragile X Syndrome (FXS) affecting ∼1/3,717 to 1/8,918. FXS occurs when FMR1 is silenced by methylation or inactivation due to an abnormal expansion of a CGG trinucleotide (>200 repeats and called Full mutation, FM), located in the untranslated sequence at 5′ , before the FMR1 gene’s first exon (Oberle et al, 1991; Verkerk et al, 1991; Yu et al, 1991). It has been established that the normal CGG repeat number is below 45 and alleles in this repeat range are transmitted stably from generation to generation. Those alleles carrying between 45 and CGG repeats are premutation (55–200 repeats, PM) or intermediate alleles (45–54 CGGs, IAs). The American College of Medical Genetics (ACMG) guidelines for defining these ranges are currently followed (Maddalena et al, 2001).

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Methods

Results