The Frequency and Clinical Impact of Synonymous HTT Loss-of-Interruption and Duplication-of-interruption Variants in a Diverse HD Cohort.

PURPOSE: To determine the frequency and clinical impact of loss-of-interruption (LOI) and duplication-of-interruption (DOI) modifier variants of the HTT CAG and CCG repeat in a cohort of individuals with Huntington disease (HD). METHODS: We screened symptomatic HD participants from the UBC HD Biobank and five research sites for sequence variants. Following variant identification, we examined the clinical impact and frequency in the reduced penetrance range. RESULTS: Participants with CAG-CCG LOI and CCG LOI variants have a similar magnitude of earlier onset of HD, by 12.5 years. The sequence variants exhibit ancestry-specific differences. Participants with the CAG-CCG LOI variant also have a faster progression of TMS by 1.9 units per year. Symptomatic participants with the CAG-CCG LOI variant show enrichment in the reduced penetrance range. The CAG-CCG LOI variant explains the onset of two symptomatic HD participants with diagnostic repeats below the pathogenetic range. CONCLUSION: Our findings have significant clinical implications for participants with the CAG-CCG LOI variant who receive inaccurate diagnoses near diagnostic cut-off ranges. Improved diagnostic testing approaches and clinical management are needed for these individuals. We present the largest and most diverse HTT CAG and CCG sequence variant cohort and emphasize their importance in clinical presentation in HD.

Globus pallidus degeneration and clinicopathological features of Huntington disease.

OBJECTIVE: Numerous studies have focused on striatal neurodegeneration in Huntington disease (HD). In comparison, the globus pallidus (GP), a main striatal output nucleus, has received less focus in HD research. This study characterizes the pattern of neurodegeneration in 3 subdivisions of the human GP, and its relation to clinical symptomatology. METHODS: Stereology was used to measure regional atrophy, neuronal loss, and soma neuronal atrophy in 3 components of the GP-the external segment (GPe), internal segment (GPi), and ventral pallidum (VP)-in 8 HD cases compared with 7 matched control cases. The findings in the HD patients were compared with HD striatal neuropathological grade, and symptom scores of motor impairment, chorea, cognition, and mood. RESULTS: Relative to controls, in the HD patients the GPe showed a 54% overall volume decline, 60% neuron loss, and 34% reduced soma volume. Similarly, the VP was reduced in volume by 31%, with 48% neuron loss and 64% reduced soma volume. In contrast, the GPi was less affected, with a 38% reduction in overall volume only. The extent of GP neurodegeneration correlated with increasing striatal neuropathological grade. Decreasing GPe and VP volumes were associated with poorer cognition and increasing motor impairments, but not chorea. In contrast, decreasing GPi volumes were associated with decreasing levels of irritability. INTERPRETATION: The HD gene mutation produces variable degrees of GP segment degeneration, highlighting the differential vulnerability of striato-GP target projections. The relationship established between clinical symptom scores and pallidal degeneration provides a novel contribution to understanding the clinicopathological associations in HD. Ann Neurol 2016;80:185-201.

Symptom heterogeneity in Huntington's disease correlates with neuronal degeneration in the cerebral cortex.

BACKGROUND: Huntington's disease (HD) is characterised by variable symptoms and neuropathology of the basal ganglia and cortex. Previously, we have shown that the pattern of pyramidal cell loss in 8 different cortical regions correlates with the phenotypic variability in HD. In the primary motor and anterior cingulate cortices, the pattern of interneuron degeneration correlates with pyramidal cell death and variable HD symptom profiles. OBJECTIVES: This study aimed to examine the pattern of interneuron degeneration in 3 further regions of the HD cortex (primary sensory, superior frontal, superior parietal cortices) to determine whether HD neuropathogenesis was characterised by a general fundamental pattern of cortical interneuron loss, and explore the relationship between cortical interneuron loss with previously determined pyramidal cell loss and clinical heterogeneity. METHODS: Stereological counting was used to quantify 3 sub-populations of calcium-binding protein containing interneurons in 3 cortical human brain regions of 14 HD and 13 control cases as used in our previous studies (Nana et al., 2014; Kim et al., 2014). The HD cases were grouped according to their predominant symptom profile ("motor", "mood", "mixed"). RESULTS: The present results demonstrated a heterogeneous loss of interneurons across the 3 cortical regions which, when compared with our previous studies, mirrored the pattern of pyramidal cell loss in the same cortical areas. Most interestingly, the pattern of neuronal loss in these regions correlated with the variable HD symptom profiles. CONCLUSION: The overall findings in our present and previous cortical studies establish a clear correlative pattern of variable cortical neuronal degeneration in HD pathogenesis, which mirrors the heterogeneity of HD symptom phenotypes.

Cortical interneuron loss and symptom heterogeneity in Huntington disease.

OBJECTIVE: The cellular basis of variable symptoms in Huntington disease (HD) is unclear. One important possibility is that degeneration of the interneurons in the cerebral cortex, which play a critical role in modulating cortical output to the basal ganglia, might play a significant role in the development of variable symptomatology in HD. This study aimed to examine whether symptom variability in HD is specifically associated with variable degeneration of cortical interneurons. METHODS: We undertook a double-blind study using stereological cell counting methods to quantify the 3 major types of γ-aminobutyric acidergic interneurons (calbindin-D28k, calretinin, parvalbumin) in 13 HD cases of variable motor/mood symptomatology and 15 matched control cases in the primary motor and anterior cingulate cortices. RESULTS: In the primary motor cortex, there was a significant loss (57% reduction) of only calbindin interneurons (p=0.022) in HD cases dominated by motor symptoms, but no significant interneuron loss in cases with a dominant mood phenotype. In contrast, the anterior cingulate cortex showed a major significant loss in all 3 interneuron populations, with 71% loss of calbindin (p=0.001), 60% loss of calretinin (p=0.001), and 80% loss of parvalbumin interneurons (p=0.005) in HD cases with major mood disorder, and no interneuron loss was observed in cases with major motor dysfunction. INTERPRETATION: These findings suggest that region-specific degeneration of cortical interneurons is a key component in understanding the neural basis of symptom heterogeneity in HD.

Striatal parvalbuminergic neurons are lost in Huntington's disease: implications for dystonia.

Although dystonia represents a major source of motor disability in Huntington's disease (HD), its pathophysiology remains unknown. Because recent animal studies indicate that loss of parvalbuminergic (PARV+) striatal interneurons can cause dystonia, we investigated if loss of PARV+ striatal interneurons occurs during human HD progression, and thus might contribute to dystonia in HD. We used immunolabeling to detect PARV+ interneurons in fixed sections, and corrected for disease-related striatal atrophy by expressing PARV+ interneuron counts in ratio to interneurons co-containing somatostatin and neuropeptide Y (whose numbers are unaffected in HD). At all symptomatic HD grades, PARV+ interneurons were reduced to less than 26% of normal abundance in rostral caudate. In putamen rostral to the level of globus pallidus, loss of PARV+ interneurons was more gradual, not dropping off to less than 20% of control until grade 2. Loss of PARV+ interneurons was even more gradual in motor putamen at globus pallidus levels, with no loss at grade 1, and steady grade-wise decline thereafter. A large decrease in striatal PARV+ interneurons, thus, occurs in HD with advancing disease grade, with regional variation in the loss per grade. Given the findings of animal studies and the grade-wise loss of PARV+ striatal interneurons in motor striatum in parallel with the grade-wise appearance and worsening of dystonia, our results raise the possibility that loss of PARV+ striatal interneurons is a contributor to dystonia in HD.

Selective neurodegeneration, neuropathology and symptom profiles in Huntington's disease.

Huntington's disease (HD) is an autosomal dominant inherited neurodegenerative disease caused by a CAG repeat expansion in exon 1 of the Huntington gene (HD) also known as IT15. Despite the disease being caused by dysfunction ofa single gene, expressed as an expanded polyglutamine in the huntingtin protein, there is a major variability in the symptom profile of patients with Huntington's disease as well as great variability in the neuropathology. The symptoms vary throughout the course of the disease and vary greatly between cases. These symptoms present as varying degrees of involuntary movements, mood, personality changes, cognitive changes and dementia. To determine whether there is a morphological basis for this symptom variability, recent studies have investigated the cellular and neurochemical changes in the striatum and cerebral cortex in the human brain to determine whether there is a link between the pathology in these regions and the symptomatology shown by individual cases. These studies together revealed that cases showing mainly mood symptom profiles correlated with marked degeneration in the striosomal compartment of the striatum, or in the anterior cingulate gyrus of the cerebral cortex. In contrast, in cases with mainly motor symptoms neurodegeneration was especially marked in the primary motor cortex with variable degeneration in both the striosomes and matrix compartments of the striatum. These studies suggest that the variable degeneration of the striatum and cerebral cortex correlates with the variable profiles of Huntington's disease.

Cell loss in the motor and cingulate cortex correlates with symptomatology in Huntington's disease.

Huntington's disease is an autosomal dominant inherited neurodegenerative disease with motor symptoms that are variably co-expressed with mood and cognitive symptoms, and in which variable neuronal degeneration is also observed in the basal ganglia and the cerebral cortex. We have recently shown that the variable symptomatology in Huntington's disease correlates with the variable compartmental pattern of GABAA receptor and cell loss in the striatum. To determine whether the phenotypic variability in Huntington's disease is also related to variable neuronal degeneration in the cerebral cortex, we undertook a double-blind study using unbiased stereological cell counting methods to determine the pattern of cell loss in the primary motor and anterior cingulate cortices in the brains of 12 cases of Huntington's disease and 15 controls, and collected detailed data on the clinical symptomatology of the patients with Huntington's disease from family members and clinical records. The results showed a significant association between: (i) pronounced motor dysfunction and cell loss in the primary motor cortex; and (ii) major mood symptomatology and cell loss in the anterior cingulate cortex. This association held for both total neuronal loss (neuronal N staining) and pyramidal cell loss (SMI32 staining), and also correlated with marked dystrophic changes in the remaining cortical neurons. There was also an association between cortical cell loss and striatal neuropathological grade, but no significant association with CAG repeat length in the Huntington's disease gene. These findings suggest that the heterogeneity in clinical symptomatology that characterizes Huntington's disease is associated with variation in the extent of cell loss in the corresponding functional regions of the cerebral cortex whereby motor dysfunction correlates with primary motor cortex cell loss and mood symptomatology is associated with cell loss in the cingulate cortex.

A retrospective study of the impact of lifestyle on age at onset of Huntington disease.

In transgenic mouse models of Huntington disease (HD) environmental enrichment significantly delays disease onset. A questionnaire-based survey of 154 adults with diagnosed HD (mean 4.2 years postdiagnosis) and a known IT15 CAG repeat length, explored whether premorbid lifestyle may relate to age-at-onset (AO). Participants were drawn from HD outpatient clinics in Australia and New Zealand. Premorbid physical, intellectual, and passive activity levels were used to generate scores in the categories of leisure, nonleisure (education, occupation and domestic duties) and total lifestyle. AO was associated with increased CAG repeat length as expected (r = -0.72, P < 0.001), but also with a lifestyle that included higher levels of passive activity (r = -0.38, P < 0.001). Multiple linear regression modeling showed lifestyle passivity to be a variable independent of CAG repeat length in predicting AO (R(2) = 0.54, b = -0.22, P = 0.005). Comparison of the mean AO across tertiles of lifestyle passivity scores showed onset 4.6 years (95% CI = 1.3-7.9) later in the least compared with the most passive tertile. CAG repeat length was also shown to predict lifestyle passivity (R(2) = 0.12, b = 1.08, P < 0.0005). Neither intellectual nor physical activity showed significant relationships to AO or CAG repeat length in this cohort. Our study leads to two conclusions: that a passive lifestyle may be a preclinical expression of HD, and that it actually contributes to the earlier onset of symptoms. Overcoming the tendency to be passive may substantially delay onset of HD. (c) 2010 Movement Disorder Society.

Striosomes and mood dysfunction in Huntington's disease.

Variable phenotype is common in neurological disorders with single-gene inheritance patterns. In Huntington's disease, mood and cognitive symptoms are variably co-expressed with motor symptoms. There is also variable degeneration of neurons in the two major neurochemical compartments of the striatum, the striosomes and the extrastriosomal matrix. To determine whether the phenotypic variability in Huntington's disease is related to this compartmental organization, we carried out a double-blind study in which we used GABA(A) receptor immunohistochemistry to analyse the status of striosomes and matrix in the brains of 35 Huntington's disease cases and 13 control cases, and collected detailed data on the clinical symptomatology expressed by the patients from family members and records. We report here a significant association between pronounced mood dysfunction in Huntington's disease patients and differential loss of the GABA(A) receptor marker in striosomes of the striatum. This association held for both clinical onset and end-stage assessments of symptoms. The cases with accentuated striosome abnormality further exhibited later onset age, lower disease grade and lower CAG repeat length in the HD gene. We found no independent association, however, between CAG repeat length or age of onset and mood dysfunction. We suggest that variation in clinical symptomatology in Huntington's disease is associated with variation in the relative abnormality of GABA(A) receptor expression in the striosome and matrix compartments of the striatum, and that striosome-related circuits may modulate mood functioning.

Aggregate distribution in frontal and motor cortex in Huntington's disease brain.

Insoluble protein aggregates have been considered a pathological hallmark of Huntington's disease and other polyglutamine disorders. In this study the number of aggregates was assessed in the superior frontal gyrus and motor cortex of seven Huntington's disease patients and was compared with the symptoms (motor/mood) these patients displayed during the course of the disease. Regardless of the pattern of symptoms present in the patients, there was a consistently higher number of nuclear and non-nuclear aggregates in the superior frontal gyrus than in the motor cortex. This suggests that there is a consistent regional difference in the density of aggregates and that this consistency is not reflected in the variable symptomatology between cases.

Widespread heterogeneous neuronal loss across the cerebral cortex in Huntington's disease.

Huntington's disease is an autosomal dominant neurodegenerative disease characterized by neuronal degeneration in the basal ganglia and cerebral cortex, and a variable symptom profile. Although progressive striatal degeneration is known to occur and is related to symptom profile, little is known about the cellular basis of symptom heterogeneity across the entire cerebral cortex. To investigate this, we have undertaken a double blind study using unbiased stereological cell counting techniques to determine the pattern of cell loss in six representative cortical regions from the frontal, parietal, temporal, and occipital lobes in the brains of 14 Huntington's disease cases and 15 controls. The results clearly demonstrate a widespread loss of total neurons and pyramidal cells across all cortical regions studied, except for the primary visual cortex. Importantly, the results show that cell loss is remarkably variable both within and between Huntington's disease cases. The results also show that neuronal loss in the primary sensory and secondary visual cortices relate to Huntington's disease motor symptom profiles, and neuronal loss across the associational cortices in the frontal, parietal and temporal lobes is related to both Huntington's disease motor and to mood symptom profiles. This finding considerably extends a previous study (Thu et al., Brain, 2010; 133:1094-1110) which showed that neuronal loss in the primary motor cortex was related specifically to the motor symptom profiles while neuronal loss in the anterior cingulate cortex was related specifically to mood symptom profiles. The extent of cortical cell loss in the current study was generally related to the striatal neuropathological grade, but not to CAG repeat length on the HTT gene. Overall our findings show that Huntington's disease is characterized by a heterogeneous pattern of neuronal cell loss across the entire cerebrum which varies with symptom profile.