# Personalized Follow Up and Genetic Diagnosis Update of FMR1-Related Conditions: A Change in Diagnosis, Prognosis and Expectations

## TL;DR

This paper discusses how updated genetic testing and personalized follow-up can change the diagnosis and understanding of Fragile X syndrome, especially in cases with milder symptoms.

## Contribution

The study emphasizes the importance of revisiting old molecular diagnoses using improved genetic tests to better understand milder phenotypes in Fragile X syndrome.

## Key findings

- Revisiting molecular diagnoses with RT-PCR and methylation analysis can lead to more accurate diagnoses in Fragile X syndrome.
- Mosaic conditions and XCI patterns may explain milder symptoms in some patients.
- Personalized follow-up can change prognosis and expectations for patients with Fragile X syndrome.

## Abstract

Fragile X syndrome (FXS, OMIM#300624) is the most common inherited cause of X-linked intellectual disability and behavior difficulties. In 99% of cases, it is caused by the pathological expansion (>200 repeats, full mutation -FM) of the CGG trinucleotide located at the 5′ UTR of the FMR1 (Fragile X Messenger Ribonucleoprotein 1) gene, leading to the lack of production of the FMRP. Clinical manifestations are well known in boys but are sometimes overlooked in girls, who may remain underdiagnosed. Premutation (PM) populations (55–200 repeats) may present other medical issues, such as FXPOI or FXTAS. Mosaic conditions, such as a combination of PM and FM lines in the same patient, may lead to milder phenotypes. With the improvement of genetic testing, information regarding the exact number of CGG triplet repeats and methylation status could help explain milder phenotypes in patients who may produce some FMRP. Chromosome X preferential inactivation (XCI) in FXS women can also play a role in clinical severity. We present four non-related families who were followed up in our FXS clinic. Some of their members showed FM on Southern blot, but had milder symptoms than expected. To rule out size mosaicism, a RT-PCR was performed, giving a different and more consistent molecular diagnosis. When mosaicism was not present, methylation status was performed, excluding full methylation. For females, XCI showed preferential inactivation in one case. Revisiting old molecular diagnoses should be considered in clinical practice, especially for patients with a milder phenotype than expected from their molecular reports. This personalized follow up may change their former diagnosis, prognosis, and expectations.