Non-CGG trinucleotide repeat expansions as pathogenic genetic mutations in Fragile X Syndrome

Purpose Fragile X syndrome (FXS) is a hereditary genetic condition, caused by the expansion of the trinucleotide CGG repeated over 200 times (full mutation) in the 5’UTR (untranslated region) regulatory region of the FMR1 gene, which leads to the absence of FMRP protein. Although the clinical standard genetic confirmation for FXS diagnosis is limited to the repeats, the use of gene sequencing techniques allowed the identification of genetic variants that occur throughout the entire FMR1 gene, including protein-coding and 3’UTR gene regions. These mutations may also cause the inactivation of FMR1 gene, leading to the FXS phenotypes in individuals with CGG repeat expansions at a normal level (5-44 repeats) or at the premutation level (between 55 and 200 repeats), and not necessarily diagnosed with FXS.

To investigate how widespread the genetic mutations occurring throughout the FMR1 gene locus are, we performed a Systematic Literature Review (SLR) to identify and synthesize a catalog of disease-causing mutations in the gene that are related to cause FXS or correlated conditions.

After a detailed literature analysis, we found 44 single nucleotide variants (SNV) at the locus of the FMR1 gene associated with developmental delay and/or intellectual disability, also including characteristics of FXS. Deletions involving the FMR1 gene that remove several other genes were also found to be associated with FXS phenotype and ovarian problems, besides cases of mosaicisms with deletions and a case of germline mosaicism. Moreover, several of the mutations found, although occurring in distinct parts of FMR1 gene, can alter the aminoacid sequence of FMRP protein.

Our critical review presents several non-CGG repeat mutations in FMR1 gene that are directly involved in the phenotypes found in Fragile X Syndrome, indicating that genetic screening for this neurodevelopmental condition should not be restricted to the CGG repeats. Competing Interest Statement The authors have declared no competing interest. Funding Statement This work was supported by Pontificia Universidade Catolica do Parana - PUCPR (Grant number PAMA22014, Parana, Brazil), Fundacao de Amparo a Pesquisa do Estado de Sao Paulo - FAPESP (Grant number 2017/07053-3, Sao Paulo, Brazil), Fundacao Araucaria - FA (Grant number #FA092016, FA, Parana, Brazil), National Council for Scientific and Technological Development - CNPq (Grant numbers #402773/2022-5 and #311438/2022-9, Brazil), and Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior - CAPES (Grant number 001, Brazil). Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes Data Availability All data produced in the present work are contained in the manuscript.