Genetic determinants of longitudinal behavioural trajectories in rare conditions: The case of fragile X syndrome

Despite being a monogenic condition, individual variability in the phenotypic profile of fragile X syndrome (FXS) is substantial, with behavioural outcomes differing in severity and frequency. Existing studies have revealed that common variation in 5-HTTLPR (serotonin) and COMT (dopamine) single nucleotide polymorphisms (SNPs) is associated with behavioural variation in FXS when measured cross-sectionally. However, the associations between SNPs and longitudinal behavioural trajectories in FXS remain unknown. This study explored relationships between three SNPs, selected a priori (5-HTTLPR, COMT and monoamine oxidase A (MAOA)), and trajectories of clinically relevant behaviours in 42 males with FXS. Autistic characteristics, property destruction, aggression, stereotyped behaviour, self-injury, repetitive behaviour, and mood/interest and pleasure were measured at two time points across three years via a series of standardised informant questionnaires. DNA was extracted from saliva samples and a combination of PCR and TaqMan genotyping was performed for genetic confirmation of FXS, and COMT, 5-HTTLPR and MAOA analyses. Results revealed that males with FXS with AA COMT genotype were less likely to display persistent stereotyped behaviour compared to AG or GG genotypes. Participants with the S/S 5-HTTLPR genotype displayed a steeper decline in repetitive and stereotyped behaviours compared to the L/S or L/L genotypes. Participants with the three-repeat MAOA genotype demonstrated a steeper decline in communication skills over three years compared to those with four repeats. This study documents the association between common genetic variation and behavioural trajectories in males with FXS. Results suggest specific SNPs play an important role in longitudinal behavioural patterns in FXS. This work may facilitate an understanding of individual trajectories for people with FXS, and, therefore, support future tailored interventions.