Regional vulnerability in Huntington's disease: fMRI-guided molecular analysis in patients and a mouse model of disease

Abstract

Although the huntingtin gene is expressed in brain throughout life, phenotypically Huntington's disease (HD) begins only in midlife and affects specific brain regions. Here, to investigate regional vulnerability in the disease, we used functional magnetic resonance imaging (fMRI) to translationally link studies in patients with a mouse model of disease. Using fMRI, we mapped cerebral blood volume (CBV) in three groups: HD patients, symptom-free carriers of the huntingtin genetic mutation, and age-matched controls. In contrast to a region in the anterior caudate, in which dysfunction was linked to genotype independent of phenotype, a region in the posterior body of the caudate was differentially associated with disease phenotype. Guided by these observations, we harvested regions from the anterior and posterior body of the caudate in postmortem control and HD human brain tissue. Gene-expression profiling identified two molecules whose expression levels were most strongly correlated with regional vulnerability - protein phosphatase 1 regulatory subunit 7 (PPP1R7) and Wnt inhibitory factor-1 (WIF-1). To verify and potentially extend these findings, we turned to the YAC128 (C57BL/6J) HD transgenic mice. By fMRI we longitudinally mapped CBV in transgenic and wildtype (WT) mice, and over time, abnormally low fMRI signal emerged selectively in the dorsal striatum. A relatively unaffected brain region, primary somatosensory cortex (S1), was used as a control. Both dorsal striatum and S1 were harvested from transgenic and WT mice and molecular analysis confirmed that PPP1R7 deficiency was strongly correlated with the phenotype. Together, converging findings in human HD patients and this HD mouse model suggest a functional pattern of caudate vulnerability and that variation in expression levels of herein identified molecules correlate with this pattern of vulnerability.

Figure 1. Mapping an anatomical phenotype of caudate dysfunction in HD

(A) Voxel-based analysis of rCBV in whole brain maps comparing HD patients to controls reveals greatest rCBV decreases in the caudate. (B) ROIs in the caudate reveals decreased rCBV in the anterior caudate in asymptomatic gene positive subjects (light gray) and HD patients (dark gray) compared to controls (black), while decreased rCBV in the posterior body of the caudate was found only in HD patients. Shown are normalized mean rCBV values and standard deviation (SD). (C) Individual representative examples of rCBV maps in the posterior body of the caudate of a control subject (upper panel), an asymptomatic gene positive subject (middle panel), and a HD patient (lower panel). * denotes p≤0.05. rCBV maps in A and C were color coded, such that warmer colors represent higher rCBV values.

Figure 2. Molecules that track the anatomical phenotype of HD-related caudate dysfunction in humans

The expression profiles of PPP1R7 and WIF1 measured in the anterior and posterior body of the caudate in HD (gray) and control (black) brains. * denotes p≤0.05, ** denotes p≤0.01

Figure 3. Age-dependent dorsal striatum dysfunction in the YAC128 mouse model

(A) rCBV imaged longitudinally over time reveals an accelerated decline in rCBV in the dorsal striatum of YAC128 mice (broken line) compared to WT littermates (solid line). Shown are mean normalized ΔR₂ values (see Methods) and SD vs. age in weeks. (B) Representative rCBV pseudocolor maps of the dorsal striatum shown for a 44 week old YAC128 mouse and a WT littermate. Warmer colors represent higher rCBV values. (C) rCBV imaged longitudinally over time reveals no difference in the ventral striatum of YAC128 mice (broken line) compared to WT littermates (solid line). Shown are mean normalized ΔR₂ values and SD vs. age. (D) Representative rCBV pseudocolor maps of the ventral striatum shown for a 44 week old YAC128 mouse and a WT littermate. Warmer colors represent higher rCBV values

Figure 4. Striatum Atrophy in YAC128 mice

Striatum volumes in YAC128 mice (gray) and WT littermates (black) were determined, demonstrating a significant atrophy only at the 44 weeks time point in YAC128 mice. Shown are mean values in mm³ and SD. * denotes p≤0.05

Figure 5. PPP1R7 deficiency correlates with anatomical phenotype

(A) As shown in individual blots and by group data, PPP1R7 deficiency is observed in the striatum, but not the S1, in 48 weeks old YAC128 mice compared to WT littermates (48 weeks- upper panel). In contrast, 28 weeks old YAC128 mice compared to WT littermates showed no difference in PPP1R7 expression within the striatum (28 weeks- lower panel). (B) As shown in individual blots and by group data, PPP1R7 deficiency is observed in the posterior body of the caudate, but not the S1, in HD human brains compared to controls. Lower panels in A and B show PPP1R7/Tubulin mean ratios and SE. * denotes p≤0.05, ** denotes p≤0.01