Memory and learning are not impaired in presymptomatic individuals with an increased risk of Huntington's disease

Identification of Subtle Verbal Memory Deficits in Premanifest Huntington Disease Using the California Verbal Learning Test

BACKGROUND

Verbal memory impairment in individuals with Huntington disease (HD) is well-documented; however, the nature and extent of verbal memory impairment in individuals with premanifest HD (pre-HD) are less understood.

OBJECTIVE

To evaluate verbal memory function in individuals with pre-HD by comparing their performance on the California Verbal Learning Test to that of individuals with a clinical diagnosis of HD and that of a demographically similar group of adults with no family history of, or genetic risk for, HD, thereby reducing possible complications of psychiatric difficulties commonly experienced by individuals who are at risk for HD but are gene negative.

METHODS

Participant groups included 77 adults with a diagnosis of HD, 23 premanifest gene carriers for HD (pre-HD), and 54 demographically similar, healthy adults. The California Verbal Learning Test-Second Edition (CVLT-II) was used to evaluate the participants' immediate and delayed recall, recognition, learning characteristics, errors, and memory retention.

RESULTS

The pre-HD group performed significantly worse than the healthy group, yet significantly better than the HD group, on Short and Long Delay Recall (Free and Cued) and Recognition Discriminability. On Total Immediate Recall, Learning Slope, Semantic Clustering, and Intrusions, the pre-HD group performed similarly to the healthy group and significantly better than the HD group. None of the groups differed in their performance on Repetitions and a measure of retention.

CONCLUSIONS

Subtle memory deficits can be observed during the premanifest stage of HD with use of a subset of indices from the CVLT-II.

Motor-Language Coupling in Huntington's Disease Families

Traditionally, Huntington’s disease (HD) has been known as a movement disorder, characterized by motor, psychiatric, and cognitive impairments. Recent studies have shown that motor and action–language processes are neurally associated. The cognitive mechanisms underlying this interaction have been investigated through the action compatibility effect (ACE) paradigm, which induces a contextual coupling of ongoing motor actions and verbal processing. The present study is the first to use the ACE paradigm to evaluate action–word processing in HD patients (HDP) and their families. Specifically, we tested three groups: HDP, healthy first-degree relatives (HDR), and non-relative healthy controls. The results showed that ACE was abolished in HDP as well as HDR, but not in controls. Furthermore, we found that the processing deficits were primarily linguistic, given that they did not correlate executive function measurements. Our overall results underscore the role of cortico-basal ganglia circuits in action–word processing and indicate that the ACE task is a sensitive and robust early biomarker of HD and familial vulnerability.

Progression in prediagnostic Huntington disease

OBJECTIVE

To examine rates of decline in individuals at risk for Huntington disease (HD).

METHODS

106 individuals at risk for HD completed a battery of neurocognitive, psychomotor and oculomotor tasks at two visits, approximately 2.5 years apart. Participants were classified as: (1) without the CAG expansion (normal controls, NC; n=68) or (2) with the CAG expansion (CAG+; n=38). The CAG+ group was further subdivided into those near to (near; n=19) or far from (far; n=19) their estimated age of onset. Longitudinal performance in the CAG+ group was evaluated with a repeated measures model with two main effects (time to onset, visit) and their interaction. Analysis of covariance was employed to detect differences in longitudinal performance in the three groups (NC, near and far).

RESULTS

In the CAG+, the interaction term was significant (p < or = 0.02) for four measures (movement time, alternate button tapping, variability of latency for a memory guided task and percentage of errors for a more complex memory guided task), suggesting the rate of decline was more rapid as subjects approached onset. Longitudinal progression in the three groups differed for several variables (p<0.05). In most, the near group had significantly faster progression than NC; however, comparisons of the NC and far groups were less consistent.

CONCLUSIONS

Different patterns of progression were observed during the prediagnostic period. For some measures, CAG+ subjects closer to estimated onset showed a more rapid decline while for other measures the CAG+ group had a constant rate of decline throughout the prediagnostic period that was more rapid than in NC.

A double blind evaluation of cognitive decline in a Norwegian cohort of asymptomatic carriers of Huntington's disease

Previous studies investigating subclinical signs of cognitive decline in presymptomatic carriers of Huntington's disease (HD) have shown conflicting results. The current study examines cognition in 105 at-risk individuals, using a broad neuropsychological test battery and adopting strict inclusion criteria for attaining a homogeneous sample. Results obtained by analyses of variance and effect size calculations indicate no clinical evidence of significant cognitive decline in asymptomatic HD carriers very far from onset of illness compared to noncarriers. Closeness to disease onset amongst gene carriers influenced cognition negatively whereas cytosine-adenine-guanine (CAG) repeat size did not. The findings call for longitudinal follow-up studies using a combination of clinical instruments and experimental paradigms to pinpoint when subtle cognitive deficits occur and within which of the cognitive domains.

Parallel explicit and implicit control of reaching

Background

Human movement can be guided automatically (implicit control) or attentively (explicit control). Explicit control may be engaged when learning a new movement, while implicit control enables simultaneous execution of multiple actions. Explicit and implicit control can often be assigned arbitrarily: we can simultaneously drive a car and tune the radio, seamlessly allocating implicit or explicit control to either action. This flexibility suggests that sensorimotor signals, including those that encode spatially overlapping perception and behavior, can be accurately segregated to explicit and implicit control processes.

Methodology/Principal Findings

We tested human subjects' ability to segregate sensorimotor signals to parallel control processes by requiring dual (explicit and implicit) control of the same reaching movement and testing for interference between these processes. Healthy control subjects were able to engage dual explicit and implicit motor control without degradation of performance compared to explicit or implicit control alone. We then asked whether segregation of explicit and implicit motor control can be selectively disrupted by studying dual-control performance in subjects with no clinically manifest neurologic deficits in the presymptomatic stage of Huntington's disease (HD). These subjects performed successfully under either explicit or implicit control alone, but were impaired in the dual-control condition.

Conclusion/Significance

The human nervous system can exert dual control on a single action, and is therefore able to accurately segregate sensorimotor signals to explicit and implicit control. The impairment observed in the presymptomatic stage of HD points to a possible crucial contribution of the striatum to the segregation of sensorimotor signals to multiple control processes.

Neuropsychological deficits in Huntington's disease gene carriers and correlates of early "conversion"

The authors examined whether the baseline cognitive functioning of 21 clinically normal huntingtin mutation carriers who developed manifest Huntington's disease on follow-up differed from that of 49 mutation carriers who remain asymptomatic over the same period in a longitudinal study. One hundred thirty-four gene-negative offspring of Huntington's disease patients were studied as well. Overall, there were no differences in cognitive test performance among the three groups. However, "converters" who developed signs of Huntington's disease within 8.6 years demonstrated poorer performance on the Wisconsin Card Sorting Test at baseline. People with the Huntington's disease mutation who are carefully examined neurologically and found to be asymptomatic have, at most, very minimal problem-solving impairment, and only if they are within a few years of clinical onset.

Proximity to clinical onset influences motor and cognitive performance in presymptomatic Huntington disease gene carriers

BACKGROUND

Neuroimaging studies of Huntington disease (HD) gene carriers have shown that characteristic striatal atrophy begins long before symptom onset, but findings regarding the presence of preclinical functional deficits are inconsistent.

OBJECTIVE

To further investigate potential motor and cognitive deficits in presymptomatic gene carriers (PSGCs), and relationships between performance and estimated proximity to HD symptom onset.

METHOD

PSGCs and age-matched controls performed motor tasks involving cued sequential button presses, and cognitive tasks involving simple and complex choice responses to visuospatial stimuli.

RESULTS

PSGCs demonstrated similar motor performance speed, and nonsignificantly slower cognitive reaction times, to controls. PSGCs made more errors than controls to stimuli requiring a spatially incongruent response, possibly suggesting some difficulty in inhibiting automatic responses. Movement times for motor conditions where little advance information was provided, and reaction times for low demand cognitive tasks, were positively correlated with PSGCs' estimated probability of symptom onset within 5 years.

CONCLUSIONS

Response speed was slower for those PSGCs estimated to have higher probabilities of close onset. These findings suggest that to provide improved knowledge of how HD begins, knowledge that may be used in clinical trials, future research should further explore relationships between function and proximity to onset.

Verbal episodic memory declines prior to diagnosis in Huntington's disease

Previous studies of verbal episodic memory in pre-diagnostic Huntington's disease (HD) have yielded mixed results; some evidence suggests that memory decline is evident prior to the onset of pronounced neurological signs of HD, whereas other data indicate that memory function remains normal throughout the pre-diagnostic period. This study examines verbal episodic memory in a sample of CAG expanded individuals who have not yet been clinically diagnosed, and who represent a wide range of points along the continuum from health to disease. The Hopkins Verbal Learning Test-Revised (HVLT-R) was administered to 479 participants (428 with the HD CAG expansion and 51 without), and performance was compared to neurobiological indices of disease progression, including a DNA-based estimate of proximity to clinical diagnosis, magnetic resonance imaging (MRI) measures of striatal volume, and neurologist ratings of motor signs. Lower HVLT-R scores were associated with closer proximity to clinical diagnosis and smaller striatal volumes; these relationships were found even in groups with no neurological signs of HD. The CAG expanded groups, including those with only minimal neurological signs, had significantly lower HVLT-R scores than the control group, and performance was worse in sub-groups that had more neurological signs consistent with HD. These findings indicate that verbal episodic memory is affected in early pre-diagnostic HD and may decline as striatal volumes decrease and individuals approach the motor diagnostic threshold.

Cognitive dysfunction precedes neuropathology and motor abnormalities in the YAC128 mouse model of Huntington's disease

Huntington's disease (HD) is an adult-onset neurodegenerative disorder involving motor dysfunction, cognitive deficits, and psychiatric disturbances that result from underlying striatal and cortical dysfunction and neuropathology. The YAC128 mouse model of HD reproduces both the motor deficits and selective degeneration observed in the human disease. However, the presence of cognitive impairment in this model has not been determined. Here, we report mild cognitive deficits in YAC128 mice that precede motor onset and progressively worsen with age. Rotarod testing revealed a motor learning deficit at 2 months of age that progresses such that by 12 months of age, untrained YAC128 mice are unable to learn the rotarod task. Additional support for cognitive dysfunction is evident in a simple swimming test in which YAC128 mice take longer to find the platform than wild-type (WT) controls beginning at 8 months of age. YAC128 mice also have deficits in open-field habituation and in a swimming T-maze test at this age. Strikingly, in the reversal phase of the swimming T-maze test, YAC128 mice take twice as long as WT mice to locate the platform, indicating a difficulty in changing strategy. At 12 months of age, YAC128 mice show decreased prepulse inhibition and habituation to acoustic startle. The clear pattern of cognitive dysfunction in YAC128 mice is similar to the symptoms and progression of cognitive deficits in human HD and provides both the opportunity to examine the relationship between cognitive dysfunction, motor impairment, and neuropathology in HD and to assess whether potential therapies for HD can restore cognitive function.

Cognitive changes in patients with Huntington's disease (HD) and asymptomatic carriers of the HD mutation--a longitudinal follow-up study

OBJECTIVE

Objective information about the onset and progression of cognitive impairment in Huntington's disease (HD) is very important in the light of appropriate outcome measures when conducting clinical trials. Therefore, we evaluated the progression of cognitive functions in HD patients and asymptomatic carriers of the HD mutation (AC) over a 2.5-year period. We also sought to detect the earliest markers of cognitive impairment in AC.

METHODS

A prospective study comparing HD patients, clinically asymptomatic HD mutation-carriers (AC) and non-carriers (NC). These groups were examined three times during a period of 2.5 years. At baseline the study sample consisted of 49 subjects. Forty-two subjects (19 HD patients, 12 AC and 11 NC) completed three assessments. A battery of neuropsychological tests measuring intelligence, attention, memory, language, visuospatial perception, and executive functions was performed.

RESULTS

The performance of HD patients deteriorated on the following cognitive tests: Symbol Digit Modalities Test (SDMT), Stroop Colour and Word, Boston Naming Test (BNT), Object and Space Perception and Trail Making Test-B. Longitudinal comparison of AC and NC revealed that performances on SDMT, Block Span, Digit Span Backwards, Hopkins Verbal Learning Test (learning and delayed recall) and Conditional Associative Learning Test are impaired in AC.

CONCLUSIONS

Tasks measuring mainly attention, object and space perception and executive functions adequately assess the progression of HD disease. Other cognitive functions do not significantly deteriorate. Furthermore, problems in attention, working memory, verbal learning, verbal long-term memory and learning of random associations are the earliest cognitive manifestations in AC.

Future directions in research with presymptomatic individuals carrying the gene for Huntington's disease

Presymptomatic individuals carrying the gene for Huntington's disease (HD) provide researchers with a unique opportunity of learning more about the neuropathophysiology, symptom onset, behavioural functioning, and mediating factors of this fatal disease. In this review, we attempt to demonstrate that research over the last 8 years, since the isolation of the gene, has remained at large controversial. Although we are aware of some of the factors that can influence age at onset and disease progression, we are still unable to determine exactly when an individual will develop HD symptoms, and how fast these symptoms will progress. In an era rapidly advancing with respect to therapeutic intervention that could forestall the onset and progression of HD, systematic research with improved inclusion criteria is paramount. A greater understanding of the time course of the disease would be beneficial not only in monitoring the effectiveness of future treatments, but also in determining the most appropriate time to administer them. Finally, we present various ethical considerations, as well as put forward various recommendations that could assist in better diagnosing preclinical deficits in presymptomatic individuals.

Longitudinal study evaluating neuropsychological changes in so-called asymptomatic carriers of the Huntington's disease mutation after 1 year

OBJECTIVES

To determine (1) whether the battery of neuropsychological tests was sufficiently sensitive to find differences between symptomatic patients with Huntington's disease (HD) and clinically asymptomatic individuals carrying the HD gene (AGC) and individuals without the HD gene (NGC) and (2) whether increasing cognitive impairment is found in AGC as compared with NGC.

METHODS

A case-control, single-blind study comparing subjects with clinically manifest HD (n=21), AGC (n=12) or NGC (n=11) and a 1-year follow-up of AGC and NGC. Genotype for the HD gene was determined by molecular testing. A large battery of neuropsychological tests measuring several cognitive domains was performed.

RESULTS

On most neuropsychological tasks, HD patients perform significantly worse than AGC and NGC. At baseline and follow-up examination, compared with NGC, AGC had lower scores on the symbol digit modalities test. Scores on a block span task declined more rapidly among AGC than among NGC.

CONCLUSION

Cognitive impairments in HD patients are found when compared with clinically asymptomatic individuals carrying the HD mutation. Furthermore, our results suggest that subtle cognitive deficits are present in asymptomatic persons who have inherited the HD gene.

Subtle changes among presymptomatic carriers of the Huntington's disease gene

OBJECTIVES

To compare the neurological and psychometric characteristics of presymptomatic gene carriers and non-gene carriers who are at risk for developing Huntington's disease so as to characterise early signs of disease and to identify markers of neurological function that could be used to assess the impact of experimental therapies on the progression of disease, even among those who are clinically presymptomatic.

METHODS

A sample of people at risk for Huntington's disease was genotyped and evaluated using subscales of the Wechsler adult intelligence scale-revised (WAIS-R), a quantified neurological rating scale, and computerised physiological measures including speed of movement and reaction time.

RESULTS

Genotyping and clinical examination determined that 171 participants were presymptomatic gene carriers (PSGCs) and 414 participants were non-gene carriers (NGCs). The PSGCs performed significantly worse when compared with the NGCs on the digit symbol, picture arrangement, and arithmetic subscales of the WAIS-R (p<0.02) and for the physiological measures: button tapping, auditory reaction time, visual reaction time with decision, and movement time with and without decision (p<0.05). Although no PSGCs had sufficient neurological findings to warrant a diagnosis of Huntington's disease on clinical examination, the PSGCs had more frequent possible or definite abnormality for oculomotor function, chorea, muscle stretch reflexes, gait, and station stability, and rapid alternating movements (p</=0.02).

CONCLUSIONS

Among Huntington's disease gene carriers, subtle cognitive and motor deficits precede the onset of sufficient neurological abnormality to warrant a clinical diagnosis of Huntington's disease.