The present study evaluated the influence of distinct dimensions of stress, and the independent effects of FMR1 CGG repeat length (up to but excluding the full mutation), on self-reported cognitive functioning (i.e., executive function and memory). Importantly, CGG repeat length accounted for small, but statistically significant elevations in executive function difficulty and memory problems, above and beyond stress, age and level of education. To date, this study represents the largest sample in which the association between cognitive function and FMR1 CGG repeat length has been studied. By taking a continuous approach to evaluating FMR1-related effects on cognitive function, and by assessing mothers of non-disabled children as well as children with a diverse range of disability conditions, this study advances understanding of how both environmental and genetic factors influence self-reported cognitive functioning.
Historically, examination of behavioral phenotypes associated with FMR1-related variability (e.g., CGG repeat length) have largely focused on individuals with full mutation fragile X syndrome or the premutation, with some exceptions (16, 17, 22, 66). Many prior assessments of cognition associated with the FMR1 gene involved group comparisons, typically between PM carriers and those with modal numbers of CGG repeats (4, 6, 7, 22). With consideration of the continuous nature of the CGG repeat range, as in the present study, the interpretation of the relationship between FMR1-related variation and phenotypic expression can be advanced.
Our findings revealed linear effects of CGG repeat length on executive functioning and curvilinear effects of CGG repeat length on memory problems. Higher incidence of memory problems was evident at approximately 80 CGG repeats, suggesting that repeats in the premutation range may be driving this effect. The divergent results observed between limitations in executive functioning and memory problems suggest that these constructs may represent distinct aspects of cognition. Higher scores on the BRIEF-A reflect day-to-day difficulties with a broad array of tasks other than memory problems, including difficulties sitting still and waiting, the propensity to make untactful remarks, and the tendency to complete tasks in a hurried manner.
One question that remains is why increasing numbers of CGG repeats across the full range are associated with problems with cognitive functioning. As noted previously, there is evidence to suggest that executive dysfunction is more common among PM carriers and possibly among those in the intermediate range (4, 6, 17, 67, 68). However, our findings are in contrast with other research literature suggesting that FMR1 translation may be most efficient at ~ 30 CGGs, with less efficient translation of FMRP at both higher and lower repeat numbers (9, 69). Replication of the current findings is necessary, and research examining the basic biological functions of the FMR1 CGG repeat is needed to fully understand these effects.
In this study, we observed associations between multiple measures of stress and cognitive functioning, both objective and subjective, with each measure of stress providing unique insights about how stress is associated with cognitive functioning. For example, the life events that were endorsed from the past year encompassed a variety of events that are not necessarily “negative”, such as the birth of a child, increased income, and moving to a new home. Luhmann and colleagues (31) conducted a meta-analysis on relationships between life events and subjective well-being, including cognitive well-being. Similar to our study, they found that cognitive well-being varied in response to the presence of life events, both positive and negative, which may simply be an indication that life change is stressful and can affect cognition.
Though parenting status was a significant predictor of higher self-reported problems with executive function and memory, it predicted less variance than perceived stress and life events. The conditions of the children represented in the sample varied considerably. Whereas some mothers had children with developmental conditions typically present at birth or in early childhood (e.g., Down syndrome), other conditions are later in onset (e.g., schizophrenia). Furthermore, while some conditions were rare, most conditions were relatively common, as the majority of affected children had conditions such as anxiety, depression, or ADHD. The differences in duration and severity of stress exposure to these varied conditions may have influenced the magnitude of variance in cognitive function predicted by parenting a child with a developmental or mental health condition. These factors warrant further attention in future work.
In the present analysis, age and education each significantly contributed to variance in cognitive function in addition to stress and CGG repeat effects, as suggested in prior research (47, 48). Prior work suggests that age-related cognitive problems are most pronounced for individuals with lower levels of education (49). The present research suggests that studies of the relationships between variation in the FMR1 gene and behavioral phenotypes should continue to consider additional individual and environmental factors to accurately evaluate FMR1-related influences.
Study Strengths, Limitations, And Future Directions
This study had several notable strengths. First, the availability of DNA and FMR1 CGG repeat assays across the full range of CGG repeats (up to the full mutation) enabled robust examination of the effects of FMR1-related variability on self-reported cognitive function. Second, we had a large sample size, which drew, in large part, from a population-based sample of participants. Third, this study was strengthened by consideration of multiple measures of stress. Additionally, the inclusion of child disabilities was broad, further contributing to the generalizability of study findings.
This study also had some limitations. Although the sample was diverse with regards to age and the range of FMR1-related variation, the participants in the sample were racially and ethnically homogenous. Additionally, many prior reports of associations between cognitive function and FMR1 expansions have included direct-assessment measures, whereas the present study relied on self-report. The study’s large sample size precluded direct testing of > 1200 individuals. It has been suggested that PM carriers may over-report symptoms not evidenced on neurological exam (70). However, there is also extant literature to suggest significant associations between cognitive function and CGG repeat length using both direct-assessment and self-report measures across the CGG range (22, 71), suggesting the validity of self-reported results. Hunter and colleagues (22) have discussed the possibility that individuals who participate in research may be less likely to have cognitive difficulties. Thus, while it is possible that the present findings could reflect potential ascertainment bias in recruited individuals (who are less likely to experience cognitive difficulties), the high response rate from our sample recruitment (77.4%) is indicative of a sample that is largely representative of the population from which participants were drawn.
Another limitation of the present study is that the only FMR1-related biomarker available for the study participants was CGG repeat number. Inclusion of activation ratio, mRNA, and FMRP levels would greatly enhance understanding of the processes investigated here. Additionally, interpretation of these findings can only be extrapolated to females. Given prior work suggestive of age-related cognitive differences in male PM carriers, most obvious in those with motor-related symptoms or signs of FXTAS, it may be that executive function difficulties and memory problems manifest differently in males and females across the CGG range. Future work should examine cognitive function across the CGG range in males, particularly with consideration as to how external stressors may affect profiles, as there may be particular stressors that may be more salient for males than females. This is an important consideration, particularly for male PM carriers, as stress could play an important role in the onset of FXTAS symptoms (72), but further work is needed to investigate this possibility.
Finally, the participants in the present study were recruited using multiple methods, including drawing from a 20,000-person population-based biobank and via a national sample of premutation carriers who were identified clinically after a child was diagnosed with FXS. Although this approach made it possible to include participants with repeats ranging from 18 to 123 CGGs, in future research, it would be advantageous to use a single method of recruitment across diverse samples, but that would require access to much larger population biobanks.