Neuroanatomical anomalies associated with rare AP4E1 mutations in people who stutter

Abstract

Developmental stuttering is a common speech disorder with strong genetic underpinnings. Recently, stuttering has been associated with mutations in genes involved in lysosomal enzyme trafficking. However, how these mutations affect the brains of people who stutter remains largely unknown. In this study, we compared grey matter volume and white matter fractional anisotropy between a unique group of seven subjects who stutter and carry the same rare heterozygous AP4E1 coding mutations and seven unrelated controls without such variants. The carriers of the AP4E1 mutations are members of a large Cameroonian family in which the association between AP4E1 and persistent stuttering was previously identified. Compared to controls, mutation carriers showed reduced grey matter volume in the thalamus, visual areas and the posterior cingulate cortex. Moreover, reduced fractional anisotropy was observed in the corpus callosum, consistent with the results of previous neuroimaging studies of people who stutter with unknown genetic backgrounds. Analysis of gene expression data showed that these structural differences appeared at the locations in which expression of AP4E1 is relatively high. Moreover, the pattern of grey matter volume differences was significantly associated with AP4E1 expression across the left supratentorial regions. This spatial congruency further supports the connection between AP4E1 mutations and the observed structural differences.

Keywords: corpus callosum; fractional anisotropy; gene expression; thalamus; voxel-based morphometry.

Graphical Abstract

Figure 1

Grey and white matter regions showed significant differences between carriers and non-carriers of AP4E1 mutations. (A) T statistics of the between-group differences in GMV and (B) FA are overlaid on a single subject template. Orange indicates that the neuroimaging measures are larger in AP4E1 carriers than controls at uncorrected P < 0.05, while blue indicates the opposite. Areas that exhibited a significant group difference at corrected P < 0.05 are outlined by black lines. The P-values of the significant clusters are listed in Tables 1 and 2. Both voxel-wise group level analyses of GMV and FA were conducted using the GLM with group and sex as factors and age as a covariate. The analysis of GMV also included intracranial volume as a covariate. The box plot in each panel shows the median, minimum, maximum, first and third quartiles of the neuroimaging measure in a cluster exhibiting a significant difference between AP4E1 carriers and controls. (C) Results of TBSS analysis of FA. Individual skeletonized FA was analysed using the same GLM of the voxel-wise FA analysis. Significant FA reductions in the AP4E1 carriers relative to controls are indicated in blue and overlaid on a white matter skeleton (green) and a single subject template. No significant difference was found in the opposite direction. The significant clusters on the skeleton were dilated one voxel to increase their visibility.

Figure 2

Relationship between AP4E1 expression and between-group GMV differences. (A) Magnitudes of GMV differences and AP4E1 expression in the left supratentorial regions defined by a standard atlas (AAL). (B) A scatter plot of regional AP4E1 expression and between-group GMV differences. Each dot represents a supratentorial region defined by a standard atlas (AAL) in the left hemisphere.