Impaired sequence learning in dystonia mutation carriers: a genotypic effect

Abstract

Abnormalities in motor sequence learning have been observed in non-manifesting carriers of the DYT1 dystonia mutation. Indeed, motor sequence learning deficits in these subjects have been associated with increased cerebellar activation during task performance. In the current study, we determined whether similar changes are also present in clinically manifesting DYT1 carriers as well as in carriers of other primary dystonia mutations such as DYT6. Additionally, we determined whether sequence learning performance and associated brain activation in these subjects correlate with previously described genotype-related abnormalities of cerebellar pathway integrity and striatal D2 dopamine receptor binding. Nineteen DYT1 carriers (10 non-manifesting DYT1: 51.5±15.1 years; nine manifesting DYT1: 46.1±15.1 years) and 12 healthy control subjects (42.8±15.3 years) were scanned with H2(15)O positron emission tomography while performing controlled sequence learning and reference tasks. Eleven DYT6 carriers (four non-manifesting DYT6: 38.0±22.1; seven manifesting DYT6: 35.3±14.2 years) were evaluated during task performance without concurrent imaging. DYT1 and DYT6 carriers also underwent diffusion tensor magnetic resonance imaging for the assessment of tract integrity and 11C-raclopride positron emission tomography to measure caudate/putamen D2 receptor binding. These imaging measures were correlated with sequence learning performance and associated activation responses. Sequence learning deficits of similar magnitude were observed in manifesting and non-manifesting DYT1 carriers. In contrast, learning deficits were not detected in DYT6 carriers, irrespective of clinical penetrance. Affected DYT1 carriers exhibited significant increases in sequence learning-related activation in the left lateral cerebellar cortex and in the right premotor and inferior parietal regions. Increases in premotor cortical activation observed in the mutation carriers correlated with reductions in cerebellar pathway integrity measured using magnetic resonance diffusion tensor imaging and probabilistic tractography. Additionally, the cerebellar tract changes correlated with reductions in dentate nucleus activation recorded during task performance. Sequence learning performance and task-related activation responses did not correlate with striatal D2 receptor binding. In summary, we found that sequence learning deficits and concomitant increases in cerebellar activation are specific features of the DYT1 genotype. The close relationship between reduced cerebellar pathway integrity and increased learning-related activation of the premotor cortex is compatible with the view of DYT1 dystonia as a neurodevelopmental circuit disorder.

Figure 1

Sequence learning performance measures. (A) Motor sequence learning (MSEQ): bar graph of group mean values (±SE) for the learning achieved during task performance. Subject performance was represented by the percentage of correctly hit targets averaged across trials. Significant reductions compared to controls (P < 0.05 Dunnett's test) are denoted by an asterisk. (B) Visual sequence learning (VSEQ): bar graph of group mean values (±SE) for verbal scores (range: 0–8) obtained after each trial block. The standard error for the DYT6 carriers was zero because all subjects reported the observation sequence correctly. NM = non-manifesting; MAN = manifesting.

Figure 2

Within-group analyses of brain activation responses. Top: Healthy control subjects exhibited modality independent sequence learning-related activation responses in the left dorsal premotor (dPMC), anterior cingulate [Brodmann area (BA) 32] and superior parietal cortex, as well as in the medial parietal region (Brodmann area 7) bilaterally (see Supplementary Table 2). Bottom: Manifesting DYT1 carriers showed modality independent bilateral activation responses in the dorsal premotor cortex and inferior and medial parietal regions, as well as in the left posterior cerebellar cortex. [The surface rendering of the statistical map (SPM5 canonical template, T > 3.0)].

Figure 3

Between-group comparison of sequence learning-related activation responses. Brain regions in which task-related activation responses during motor and visual sequence learning were elevated in manifesting (MAN) DYT1 carriers relative to controls (Table 3). SPM(t) maps (left) were superimposed on a single-subject MRI T₁ template. The position of each slice is indicated by x, y or z coordinates in MNI space. Bar diagrams (right) illustrate changes in adjusted regional cerebral blood flow (ΔrCBF) during motor sequence learning (MSEQ-CCW) and during visual sequence learning (VSEQ-AV) in the respective cluster (mean ± SE) for manifesting (MAN, dark grey) and non-manifesting (NM, light grey) DYT1 carriers, and healthy control subjects (white). Increased learning-related activation was present in the right rostral supplementary motor area (pre-SMA) (top), in the right dorsal premotor and inferior parietal regions (middle), and in the left posterior cerebellar cortex (bottom). The clusters were identified using the flexible factorial model in SPM5 for the contrast of the manifesting DYT1 and the healthy control groups. The corresponding regional values for the non-manifesting DYT1 cohort are presented for comparison. The colour stripe represents T-values thresholded at 4.55 (P = 0.05, corrected for multiple comparisons).

Figure 4

Correlations between cerebellar outflow pathway connectivity and regional cerebral blood flow measured during sequence learning. Regions are displayed in which regional cerebral blood flow (rCBF) recorded during visual sequence learning in manifesting DYT1 mutation carriers correlated with measures of left cerebello-thalamic pathway connectivity determined using probabilistic tractography (left). These correlations were validated prospectively (right) using volumes-of-interest centered on the peak voxel and placed on the co-registered motor sequence learning scans acquired in the same subjects. (A) A significant positive correlation was identified between cerebellar tract integrity and regional cerebral blood flow measured in the ipsilateral dentate nucleus (Ncl). (B) Significant negative correlations between these variables were present in the right supplementary motor area extending into the adjacent pre-supplementary motor area (preSMA) region (top), and in the right lateral prefrontal cortex (bottom). The colour stripe represents T-values thresholded at 4.55 (P = 0.05, corrected for multiple comparisons).