TOMM40 '523' poly-T repeat length is a determinant of longitudinal cognitive decline in Parkinson's disease

Managing cognitive impairment in Parkinson's disease: an update of the literature

INTRODUCTION

Cognitive impairment in Parkinson's disease (PD) substantially affects patient outcomes, function, and quality of life. PD-related cognitive dysfunction is often heterogeneous in clinical presentation and rates of progression. As cognitive changes occur in many people with PD, it is essential to evaluate cognition, provide education, and implement management strategies for cognitive symptoms.

AREAS COVERED

This article describes the symptomatology, epidemiology, risk factors, and pathobiology of cognitive impairment in PD. Additionally, the article provides an overview of evidence-based management and other therapeutic and coping strategies for cognitive impairment and dementia in PD. Comment is offered on challenges and opportunities for trials and emerging therapeutics targeting cognitive symptoms or decline.

EXPERT OPINION

While our understanding of cognitive dysfunction in PD has grown, effective and safe therapeutics are still needed to not only treat cognitive impairment and dementia symptomatically but also slow down or prevent cognitive decline. Further research is needed to elucidate the pathobiology of PD cognitive impairment, develop validated biomarkers reflecting cognitive change, and ultimately, integrate clinical and biological frameworks. Consensus regarding cognitive evaluations, definitions, and criteria of cognitive impairment, evaluating functional abilities in the context of cognitive impairment, and determining optimal outcome measures for clinical trials remain unmet needs.

TOMM40 may mediate GFAP, neurofilament light Protein, pTau181, and brain morphometry in aging

A growing amount of data has implicated the TOMM40 gene in the risk for Alzheimer's disease (AD), neurodegeneration, and accelerated aging. No studies have investigated the relationship of TOMM40 rs2075650 ('650) on the structural complexity of the brain or plasma markers of neurodegeneration. We used a comprehensive approach to quantify the impact of TOMM40 '650 on brain morphology and multiple cortical attributes in cognitively unimpaired (CU) individuals. We also tested whether the presence of the risk allele, G, of TOMM40 '650 was associated with plasma markers of amyloid, tau, and neurodegeneration and if there were interactions with age and sex, controlling for the effects of APOE ε4. We found that the TOMM40 '650 G-allele was associated with decreased sulcal depth, increased gyrification index, and decreased gray matter volume. NfL, GFAP, and pTau181 had independent and age-associated increases in individuals with a G-allele. Our data suggest that TOMM40 '650 is associated with aging-related plasma biomarkers and brain structure variation in temporal-limbic circuits.

Alzheimer's disease cortical morphological phenotypes are associated with TOMM40'523-APOE haplotypes

Both the APOE ε4 and TOMM40 rs10524523 ("523") genes have been associated with risk for Alzheimer's disease (AD) and neuroimaging biomarkers of AD. No studies have investigated the relationship of TOMM40'523-APOE ε4 on the structural complexity of the brain in AD individuals. We quantified brain morphology and multiple cortical attributes in individuals with mild cognitive impairment (MCI) and AD, then tested whether APOE ε4 or TOMM40 poly-T genotypes were related to AD morphological biomarkers in cognitively unimpaired (CU) and MCI/AD individuals. We identified several AD-specific phenotypes in brain morphology and found that TOMM40 poly-T short alleles are associated with early, AD-specific brain morphological differences in healthy aging. We observed decreased cortical thickness, sulcal depth, and fractal dimension in CU individuals with the poly-T short alleles. Moreover, in MCI/AD participants, the APOE ε4 (TOMM40 L) individuals had a higher rate of gene-related morphological markers indicative of AD. Our data suggest that TOMM40'523 is associated with early brain structure variations in the precuneus, temporal, and limbic cortices.

Genotypic Effects of the TOMM40'523 Variant and APOE on Longitudinal Cognitive Change over 4 Years: The TOMMORROW Study

BACKGROUND

The 523 poly-T length polymorphism (rs10524523) in TOMM40 has been reported to influence longitudinal cognitive test performance within APOE ε3/3 carriers. The results from prior studies are inconsistent. It is also unclear whether specific APOE and TOMM40 genotypes contribute to heterogeneity in longitudinal cognitive performance during the preclinical stages of AD.

OBJECTIVES

To determine the effects of these genes on longitudinal cognitive change in early preclinical stages of AD, we used the clinical trial data from the recently concluded TOMMORROW study to examine the effects of APOE and TOMM40 genotypes on neuropsychological test performance.

DESIGN

A phase 3, double-blind, placebo-controlled, randomized clinical trial.

SETTING

Academic affiliated and private research clinics in Australia, Germany, Switzerland, the UK, and the USA.

PARTICIPANTS

Cognitively normal older adults aged 65 to 83.

INTERVENTION

Pioglitazone tablet.

MEASUREMENTS

Participants from the TOMMORROW trial were stratified based on APOE genotype (APOE ε3/3, APOE ε3/4, APOE ε4/4). APOE ε3/3 carriers were further stratified by TOMM40'523 genotype. The final analysis dataset consists of 1,330 APOE ε3/3 carriers and 7,001 visits. Linear mixed models were used to compare the rates of decline in cognition across APOE groups and the APOE ε3/3 carriers with different TOMM40'523 genotypes.

RESULTS

APOE ε3/4 and APOE ε4/4 genotypes compared with the APOE ε3/3 genotype were associated with worse performance on measures of global cognition, episodic memory, and expressive language. Further, over the four years of observation, the APOE ε3/3 carriers with the TOMM40'523-S/S genotype showed better global cognition and accelerated rates of cognitive decline on tests of global cognition, executive function, and attentional processing compared to APOE ε3/3 carriers with TOMM40'523-S/VL and VL/VL genotypes and compared to the APOE ε3/4 and APOE ε4/4 carriers.

CONCLUSIONS

We suggest that both APOE and TOMM40 genotypes may independently contribute to cognitive heterogeneity in the pre-MCI stages of AD. Controlling for this genetic variability will be important in clinical trials designed to slow the rate of cognitive decline and/or prevent symptom onset in preclinical AD.

Short structural variants as informative genetic markers for ALS disease risk and progression

There is considerable variability in disease progression for patients with amyotrophic lateral sclerosis (ALS) including the age of disease onset, site of disease onset, and survival time. There is growing evidence that short structural variations (SSVs) residing in frequently overlooked genomic regions can contribute to complex disease mechanisms and can explain, in part, the phenotypic variability in ALS patients. Here, we discuss SSVs recently characterized by our laboratory and how these discoveries integrate into the current literature on ALS, particularly in the context of application to future clinical trials. These markers may help to identify and differentiate patients for clinical trials that have a similar ALS disease mechanism(s), thereby reducing the impact of participant heterogeneity. As evidence accumulates for the genetic markers discovered in SQSTM1, SCAF4, and STMN2, we hope to improve the outcomes of future ALS clinical trials.