Association between IQ and FMR1 protein (FMRP) across the spectrum of CGG repeat expansions

Abstract

Fragile X syndrome, the leading heritable form of intellectual disability, is caused by hypermethylation and transcriptional silencing of large (CGG) repeat expansions (> 200 repeats) in the 5' untranslated region of the fragile X mental retardation 1 (FMR1) gene. As a consequence of FMR1 gene silencing, there is little or no production of FMR1 protein (FMRP), an important element in normal synaptic function. Although the absence of FMRP has long been known to be responsible for the cognitive impairment in fragile X syndrome, the relationship between FMRP level and cognitive ability (IQ) is only imprecisely understood. To address this issue, a high-throughput, fluorescence resonance energy transfer (FRET) assay has been used to quantify FMRP levels in dermal fibroblasts, and the relationship between FMRP and IQ measures was assessed by statistical analysis in a cohort of 184 individuals with CGG-repeat lengths spanning normal (< 45 CGGs) to full mutation (> 200 CGGs) repeat ranges in fibroblasts. The principal findings of the current study are twofold: i) For those with normal CGG repeats, IQ is no longer sensitive to further increases in FMRP above an FMRP threshold of ~70% of the mean FMRP level; below this threshold, IQ decreases steeply with further decreases in FMRP; and ii) For the current cohort, a mean IQ of 85 (lower bound for the normal IQ range) is attained for FMRP levels that are only ~35% of the mean FMRP level among normal CGG-repeat controls. The current results should help guide expectations for efforts to induce FMR1 gene activity and for the levels of cognitive function expected for a given range of FMRP levels.

Fig 1. Distribution of FMRP levels within each allele class.

Dashed lines indicate the values of FMRP 1 SD and 2 SD (as indicated) below the relative mean level (= 1.0) of those with normal CGG-repeat alleles (see, also: Fig 2). Individual 1016–15 has a normal CGG repeat (21 CGG) but has a point mutation within the FMRP coding region [80] that prevented detection by the FRET assay (See: S2 Fig).

Fig 2. Dependence of IQ on FMRP levels in cultured dermal fibroblasts, accounting for sex as a covariate.

(A) FSIQ, (B) PIQ, and (C) VIQ were determined using age-appropriate instruments as delineated in the Methods section. FMRP levels were determined using FRET. All FMRP levels were normalized to the mean value of FMRP levels (= 1.0) among individuals with normal CGG repeats, excluding the FMR1 point mutation (1016–15). Data displayed are for both males and females since there is no significant differential dependence of the regression lines on sex (P > 0.05, see S1 Table); separate regression analyses for males and for females are displayed in S3A–S3F Fig. Symbols specify allele classes as indicated. Plus/minus 1 SD for FMRP levels are indicated as red vertical dashed lines. Lower limit of normal IQ (= 85) and borderline IQ (= 70) are indicated as horizontal blue and green dashed lines, respectively. The FMRP level at which the regression line for IQ passes 85 is indicated by a vertical (blue) dashed line.