The effects of PPARγ on the regulation of the TOMM40-APOE-C1 genes cluster.

Chromosome 19q13.32 is a gene rich region, and has been implicated in multiple human phenotypes in adulthood including lipids traits, Alzheimer's disease, and longevity. Peroxisome Proliferator Activated Receptor Gamma (PPARγ) is a ligand-activated nuclear transcription factor that plays a role in human complex traits that are also genetically associated with the chromosome 19q13.32 region. Here, we study the effects of PPARγ on the regional expression regulation of the genes clustered within chromosome 19q13.32, specifically TOMM40, APOE, and APOC1, applying two complementary approaches. Using the short hairpin RNA (shRNA) method in the HepG2 cell-line we knocked down PPARγ expression and measured the effect on mRNA expression. We discovered PPARγ knock down increased the levels of TOMM40-, APOE-, and APOC1-mRNAs, with the highest increase in expression observed for APOE-mRNA. To complement the PPARγ knockdown findings we also examined the effects of low doses of PPARγ agonists (nM range) on mRNA expression of these genes. Low (nM) concentrations of pioglitazone (Pio) decreased transcription of TOMM40, APOE, and APOC1 genes, with the lowest mRNA levels for each gene observed at 1.5nM. Similar to the effect of PPARγ knockdown, the strongest response to pioglitazone was also observed for APOE-mRNA, and rosiglitazone (Rosi), another PPARγ agonist, produced results that were consistent with these. In conclusion, our results further established a role for PPARγ in regional transcriptional regulation of chr19q13.32, underpinning the association between PPARγ, the chr19q13.32 genes cluster, and human complex traits and disease.

The biological foundation of the genetic association of TOMM40 with late-onset Alzheimer's disease.

A variable-length poly-T variant in intron 6 of the TOMM40 gene, rs10524523, is associated with risk and age-of-onset of sporadic (late-onset) Alzheimer's disease. In Caucasians, the three predominant alleles at this locus are Short (S), Long (L) or Very long (VL). On an APOE ε3/3 background, the S/VL and VL/VL genotypes are more protective than S/S. The '523 poly-T has regulatory properties, in that the VL poly-T results in higher expression than the S poly-T in luciferase expression systems. The aim of the current work was to identify effects on cellular bioenergetics of increased TOM40 protein expression. MitoTracker Green fluorescence and autophagic vesicle staining was the same in control and over-expressing cells, but TOM40 over-expression was associated with increased expression of TOM20, a preprotein receptor of the TOM complex, the mitochondrial chaperone HSPA9, and PDHE1a, and increased activities of the oxidative phosphorylation complexes I and IV and of the TCA member α-ketoglutaric acid dehydrogenase. Consistent with the complex I findings, respiration was more sensitive to inhibition by rotenone in control cells than in the TOM40 over-expressing cells. In the absence of inhibitors, total cellular ATP, the mitochondrial membrane potential, and respiration were elevated in the over-expressing cells. Spare respiratory capacity was greater in the TOM40 over-expressing cells than in the controls. TOM40 over-expression blocked Ab-elicited decreases in the mitochondrial membrane potential, cellular ATP levels, and cellular viability in the control cells. These data suggest elevated expression of TOM40 may be protective of mitochondrial function.

The Alu neurodegeneration hypothesis: A primate-specific mechanism for neuronal transcription noise, mitochondrial dysfunction, and manifestation of neurodegenerative disease.

It is hypothesized that retrotransposons have played a fundamental role in primate evolution and that enhanced neurologic retrotransposon activity in humans may underlie the origin of higher cognitive function. As a potential consequence of this enhanced activity, it is likely that neurons are susceptible to deleterious retrotransposon pathways that can disrupt mitochondrial function. An example is observed in the TOMM40 gene, encoding a ß-barrel protein critical for mitochondrial preprotein transport. Primate-specific Alu retrotransposons have repeatedly inserted into TOMM40 introns, and at least one variant associated with late-onset Alzheimer's disease originated from an Alu insertion event. We provide evidence of enriched Alu content in mitochondrial genes and postulate that Alus can disrupt mitochondrial populations in neurons, thereby setting the stage for progressive neurologic dysfunction. This Alu neurodegeneration hypothesis is compatible with decades of research and offers a plausible mechanism for the disruption of neuronal mitochondrial homeostasis, ultimately cascading into neurodegenerative disease.

Family history and TOMM40 '523 interactive associations with memory in middle-aged and Alzheimer's disease cohorts.

INTRODUCTION: Family history (FH) of Alzheimer's disease (AD) affects mitochondrial function and may modulate effects of translocase of the outer mitochondrial membrane 40 kDa (TOMM40) rs10524523 ('523) poly-T length on memory decline. METHODS: For 912 nonapolipoprotein ε4 middle-aged adults and 365 aged adults across the AD spectrum, linear mixed models gauged FH and TOMM40 '523 interactions on memory and global cognition between baseline and up to 10 years later. A cerebrospinal fluid mitochondrial function biomarker was also assessed. RESULTS: For FH negative participants, gene-dose preservation of memory and global cognition was seen for "very long" versus "short" carriers. For FH positive, an opposite gene-dose decline was seen for very long versus short carriers. Maternal FH was a stronger predictor in aged, but not middle-aged, participants. Similar gene-dose effects were seen for the mitochondrial biomarker aspartate aminotransferase. DISCUSSION: These results may clarify conflicting findings on TOMM40 poly-T length and AD-related decline.

The SSV Evaluation System: A Tool to Prioritize Short Structural Variants for Studies of Possible Regulatory and Causal Variants.

Short structural variants (SSVs) are short genomic variants (<50 bp) other than SNPs. It has been suggested that SSVs contribute to many human complex traits. However, high-throughput analysis of SSVs presents numerous technical challenges. In order to facilitate the discovery and assessment of SSVs, we have developed a prototype bioinformatics tool, "SSV evaluation system," which is a searchable, annotated database of SSVs in the human genome, with associated customizable scoring software that is used to evaluate and prioritize SSVs that are most likely to have significant biological effects and impact on disease risk. This new bioinformatics tool is a component in a larger strategy that we have been using to discover potentially important SSVs within candidate genomic regions that have been identified in genome-wide association studies, with the goal to prioritize potential functional/causal SSVs and focus the follow-up experiments on a relatively small list of strong candidate SSVs. We describe our strategy and discuss how we have used the SSV evaluation system to discover candidate causal variants related to complex neurodegenerative diseases. We present the SSV evaluation system as a powerful tool to guide genetic investigations aiming to uncover SSVs that underlie human complex diseases including neurodegenerative diseases in aging.

The genetic contributions of SNCA and LRRK2 genes to Lewy Body pathology in Alzheimer's disease.

The molecular genetic basis that leads to Lewy Body (LB) pathology in 15-20% of Alzheimer disease cases (LBV/AD) was largely unknown. Alpha-synuclein (SNCA) and Leucine-rich repeat kinase2 (LRRK2) have been implicated in the pathogenesis of Parkinson's disease (PD), the prototype of LB spectrum disorders. We tested the association of SNCA variants with LB pathology in AD. We then stratified the SNCA association analyses by LRRK2 genotype. We also investigated the expression regulation of SNCA and LRRK2 in relation to LB pathology. We evaluated the differences in SNCA-mRNA and LRRK2-mRNA levels as a function of LB pathology in the temporal cortex (TC) from autopsy-confirmed LBV/AD cases and AD controls. We further investigated the cis-effect of the LB pathology-associated genetic variants within the SNCA and LRRK2 loci on the mRNA expression of these genes. SNCA SNPs rs3857059 and rs2583988 showed significant associations with increased risk for LB pathology. When the analyses were stratified by LRRK2-rs1491923 genotype, the associations became stronger for both SNPs and an association was also observed with rs2619363. Expression analysis demonstrated that SNCA- and LRRK2-mRNA levels were significantly higher in TC from LBV/AD brains compared with AD controls. Furthermore, SNCA-mRNA expression level in the TC was associated with rs3857059; homozygotes for the minor allele showed significant higher expression. LRRK2-transcript levels were increased in carriers of rs1491923 minor allele. Our findings demonstrated that SNCA contributes to LB pathology in AD patients, possibly via interaction with LRRK2, and suggested that expression regulation of these genes may be the molecular basis underlying the observed LB associations.

New Genetic Approaches to AD: Lessons from APOE-TOMM40 Phylogenetics.

Clinical trials for Alzheimer's disease are now focusing on the earliest stages of the disease with the goal of delaying dementia onset. There is great utility in using genetic variants to identify individuals at high age-dependent risk when the goal is to begin treatment before the development of any cognitive symptoms. Genetic variants identified through large-scale genome-wide association studies have not substantially improved the accuracy provided by APOE genotype to identify people at high risk of late-onset Alzheimer's disease (LOAD). We describe novel approaches, focused on molecular phylogenetics, to finding genetic variants that predict age at LOAD onset with sufficient accuracy and precision to be useful. We highlight the discovery of a polymorphism in TOMM40 that, in addition to APOE, may improve risk prediction and review how TOMM40 genetic variants may impact the develop of LOAD independently from APOE. The analysis methods described in this review may be useful for other genetically complex human diseases.

Understanding the genetics of APOE and TOMM40 and role of mitochondrial structure and function in clinical pharmacology of Alzheimer's disease.

The methodology of Genome-Wide Association Screening (GWAS) has been applied for more than a decade. Translation to clinical utility has been limited, especially in Alzheimer's Disease (AD). It has become standard practice in the analyses of more than two dozen AD GWAS studies to exclude the apolipoprotein E (APOE) region because of its extraordinary statistical support, unique thus far in complex human diseases. New genes associated with AD are proposed frequently based on SNPs associated with odds ratio (OR) < 1.2. Most of these SNPs are not located within the associated gene exons or introns but are located variable distances away. Often pathologic hypotheses for these genes are presented, with little or no experimental support. By eliminating the analyses of the APOE-TOMM40 linkage disequilibrium region, the relationship and data of several genes that are co-located in that LD region have been largely ignored. Early negative interpretations limited the interest of understanding the genetic data derived from GWAS, particularly regarding the TOMM40 gene. This commentary describes the history and problem(s) in interpretation of the genetic interrogation of the "APOE" region and provides insight into a metabolic mitochondrial basis for the etiology of AD using both APOE and TOMM40 genetics.

A cytosine-thymine (CT)-rich haplotype in intron 4 of SNCA confers risk for Lewy body pathology in Alzheimer's disease and affects SNCA expression.

INTRODUCTION: We recently showed that tagging single-nucleotide polymorphisms across the SNCA locus were significantly associated with increased risk for Lewy body (LB) pathology in Alzheimer's disease (AD) cases. However, the actual genetic variant(s) that underlie the observed associations remain elusive. METHODS: We used a bioinformatics algorithm to catalog structural variants in a region of SNCA intron 4, followed by phased sequencing. We performed a genetic association analysis in autopsy series of LB variant of Alzheimer's disease (LBV/AD) cases compared with AD-only controls. We investigated the biological functions by expression analysis using temporal-cortex samples. RESULTS: We identified four distinct haplotypes within a highly polymorphic low-complexity cytosine-thymine (CT)-rich region. We showed that a specific haplotype conferred risk to develop LBV/AD. We demonstrated that the CT-rich site acts as an enhancer element, where the risk haplotype was significantly associated with elevated levels of SNCA messenger RNA. DISCUSSION: We have discovered a novel haplotype in a CT-rich region in SNCA that contributes to LB pathology in AD patients, possibly via cis-regulation of the gene expression.

Community engagement in diverse populations for Alzheimer disease prevention trials.

The recruitment of asymptomatic volunteers has been identified as a critical factor that is delaying the development and validation of preventive therapies for Alzheimer disease (AD). Typical recruitment strategies involve the use of convenience samples or soliciting participation of older adults with a family history of AD from clinics and outreach efforts. However, high-risk groups, such as ethnic/racial minorities, are traditionally less likely to be recruited for AD prevention studies, thus limiting the ability to generalize findings for a significant proportion of the aging population. A community-engagement approach was used to create a registry of 2311 research-ready, healthy adult volunteers who reflect the ethnically diverse local community. Furthermore, the registry's actual commitment to research was examined, through demonstrated participation rates in a clinical study. The approach had varying levels of success in establishing a large, diverse pool of individuals who are interested in participating in pharmacological prevention trials and meet the criteria for primary prevention research trials designed to delay the onset of AD. Our efforts suggest that entry criteria for the clinical trials need to be carefully considered to be inclusive of African Americans, and that sustained effort is needed to engage African Americans in pharmacological prevention approaches.

A comparison of neuropsychological performance between US and Russia: preparing for a global clinical trial.

BACKGROUND: Understanding regional differences in cognitive performance is important for interpretation of data from large multinational clinical trials. METHODS: Data from Durham and Cabarrus Counties in North Carolina, USA and Tomsk, Russia (n = 2972) were evaluated. The Montreal Cognitive Assessment (MoCA), Trail Making Test Part B (Trails B), Consortium to Establish a Registry for Alzheimer's Disease Word List Memory Test (WLM) delayed recall, and self-report Alzheimer's Disease Cooperative Studies Mail-In Cognitive Function Screening Instrument (MCFSI) were administered at each site. Multilevel modeling measured the variance explained by site and predictors of cognitive performance. RESULTS: Site differences accounted for 11% of the variation in the MoCA, 1.6% in Trails B, 1.7% in WLM, and 0.8% in MCFSI scores. Prior memory testing was significantly associated with WLM. Diabetes and stroke were significantly associated with Trails B and MCFSI. CONCLUSIONS: Sources of variation include cultural differences, health conditions, and exposure to test stimuli. Findings highlight the importance of local norms to interpret test performance.

The cis-regulatory effect of an Alzheimer's disease-associated poly-T locus on expression of TOMM40 and apolipoprotein E genes.

BACKGROUND: We investigated the genomic region spanning the Translocase of the Outer Mitochondrial Membrane 40-kD (TOMM40) and Apolipoprotein E (APOE) genes, that has been associated with the risk and age of onset of late-onset Alzheimer's disease (LOAD) to determine whether a highly polymorphic, intronic poly-T within this region (rs10524523; hereafter, 523) affects expression of the APOE and TOMM40 genes. Alleles of this locus are classified as S, short; L, long; and VL, very long based on the number of T residues. METHODS: We evaluated differences in APOE messenger RNA (mRNA) and TOMM40 mRNA levels as a function of the 523 genotype in two brain regions from APOE ε3/ε3 white autopsy-confirmed LOAD cases and normal controls. We further investigated the effect of the 523 locus in its native genomic context using a luciferase expression system. RESULTS: The expression of both genes was significantly increased with disease. Mean expression of APOE and TOMM40 mRNA levels were higher in VL homozygotes compared with S homozygotes in the temporal and occipital cortexes from normal and LOAD cases. Results of a luciferase reporter system were consistent with the human brain mRNA analysis; the 523 VL poly-T resulted in significantly higher expression than the S poly-T. Although the effect of poly-T length on reporter expression was the same in HepG2 hepatoma and SH-SY5Y neuroblastoma cells, the magnitude of the effect was greater in the neuroblastoma than in the hepatoma cells, which implies tissue-specific modulation of the 523 poly-T. CONCLUSIONS: These results suggest that the 523 locus may contribute to LOAD susceptibility by modulating the expression of TOMM40 and/or APOE transcription.

African-American TOMM40'523-APOE haplotypes are admixture of West African and Caucasian alleles.

BACKGROUND: Several studies have demonstrated a lower apolipoprotein E4 (APOE ε4) allele frequency in African-Americans, but yet an increased age-related prevalence of AD. An algorithm for prevention clinical trials incorporating TOMM40'523 (Translocase of Outer Mitochondria Membrane) and APOE depends on accurate TOMM40'523-APOE haplotypes. METHODS: We have compared the APOE and TOMM40'523 phased haplotype frequencies of a 9.5 kb TOMM40/APOE genomic region in West African, Caucasian, and African-American cohorts. RESULTS: African-American haplotype frequency scans of poly-T lengths connected in phase with either APOE ε4 or APOE ε3 differ from both West Africans and Caucasians and represent admixture of several distinct West African and Caucasian haplotypes. A new West African TOMM40'523 haplotype, with APOE ε4 connected to a short TOMM40'523 allele, is observed in African-Americans but not Caucasians. CONCLUSION: These data have therapeutic implications for the age of onset risk algorithm estimates and the design of a prevention trial for African-Americans or other mixed ethnic populations.

A single-nucleotide polymorphism tagging set for human drug metabolism and transport.

Interindividual variability in drug response, ranging from no therapeutic benefit to life-threatening adverse reactions, is influenced by variation in genes that control the absorption, distribution, metabolism and excretion of drugs. We genotyped 904 single-nucleotide polymorphisms (SNPs) from 55 such genes in two population samples (European and Japanese) and identified a set of tagging SNPs that represents the common variation in these genes, both known and unknown. Extensive empirical evaluations, including a direct assessment of association with candidate functional SNPs in a new, larger population sample, validated the performance of these tagging SNPs and confirmed their utility for linkage-disequilibrium mapping in pharmacogenetics. The analyses also suggest that rare variation is not amenable to tagging strategies.

TOMM40 and APOE: Requirements for replication studies of association with age of disease onset and enrichment of a clinical trial.

A number of recent studies have not replicated the association of the translocase of the outer mitochondrial membrane pore subunit (TOMM40) rs10524523 polymorphism, which is in linkage disequilibrium with apolipoprotein E (APOE), with age of onset of Alzheimer's disease (AD). This perspective describes the differences between these later studies and the original experiments. We highlight the necessity for using standardized and informative assessment tools and processes when determining the age of development of AD or AD symptoms, and also stress that this clinical phenotype is best measured reliably in prospective studies during which subjects are monitored over time. This is true when assessing potential biomarkers for age of onset and when assessing the therapeutic potential of medicines that may delay the onset or progression of this disease.

The knockout mouse project.

Mouse knockout technology provides a powerful means of elucidating gene function in vivo, and a publicly available genome-wide collection of mouse knockouts would be significantly enabling for biomedical discovery. To date, published knockouts exist for only about 10% of mouse genes. Furthermore, many of these are limited in utility because they have not been made or phenotyped in standardized ways, and many are not freely available to researchers. It is time to harness new technologies and efficiencies of production to mount a high-throughput international effort to produce and phenotype knockouts for all mouse genes, and place these resources into the public domain.

Pleiotropy and allelic heterogeneity in the TOMM40-APOE genomic region related to clinical and metabolic features of hepatitis C infection.

Hepatitis C virus (HCV) modulates host lipid metabolism as part of its lifecycle and is dependent upon VLDL for co-assembly and secretion. HCV dyslipidemia is associated with steatosis, insulin resistance, IL28B genotype and disease progression. Apolipoprotein E (ApoE) is an important lipid transport protein, a key constituent of VLDL, and is involved in immunomodulation. Our aims were to determine the role of APOE regional polymorphisms on host lipids, IL28B genotype and disease severity in chronic HCV (CHC) patients. The study cohort included 732 CHC patients with available DNA for genotype determination of four polymorphisms in the chromosome 19 region that encompasses the TOMM40, APOE and APOC1 genes. Serum lipid analysis and apolipoproteins levels were measured using an immunoturbidimetric assay. APOE rs7412 polymorphism (capturing the ε2 isoform) was significantly associated with serum ApoE levels in both Caucasians and African-American patients (p = 2.3 × 10(-11)) and explained 7 % of variance in serum ApoE. Among IL28B-CC patients (n = 196), the rs429358 (defines ε4 isoform) and TOMM40 '523' S polymorphisms were associated with 12 % of variance in ApoB levels. Patients homozygous for the APOE ε3 isoform had a greater than twofold increased odds of F2-F4 fibrosis (p = 1.8 × 10(-5)), independent of serum lipid and lipoprotein levels. There were no associations between APOE polymorphisms and serum HDL-C, APO-CIII and triglycerides. In CHC patients, genetic heterogeneity in the APOE/TOMM40 genomic region is significantly associated with variation in serum ApoE and ApoB levels, and also with fibrosis suggesting a pleiotropic attribute of this genomic region.

A genome-wide association study in chronic obstructive pulmonary disease (COPD): identification of two major susceptibility loci.

There is considerable variability in the susceptibility of smokers to develop chronic obstructive pulmonary disease (COPD). The only known genetic risk factor is severe deficiency of alpha(1)-antitrypsin, which is present in 1-2% of individuals with COPD. We conducted a genome-wide association study (GWAS) in a homogenous case-control cohort from Bergen, Norway (823 COPD cases and 810 smoking controls) and evaluated the top 100 single nucleotide polymorphisms (SNPs) in the family-based International COPD Genetics Network (ICGN; 1891 Caucasian individuals from 606 pedigrees) study. The polymorphisms that showed replication were further evaluated in 389 subjects from the US National Emphysema Treatment Trial (NETT) and 472 controls from the Normative Aging Study (NAS) and then in a fourth cohort of 949 individuals from 127 extended pedigrees from the Boston Early-Onset COPD population. Logistic regression models with adjustments of covariates were used to analyze the case-control populations. Family-based association analyses were conducted for a diagnosis of COPD and lung function in the family populations. Two SNPs at the alpha-nicotinic acetylcholine receptor (CHRNA 3/5) locus were identified in the genome-wide association study. They showed unambiguous replication in the ICGN family-based analysis and in the NETT case-control analysis with combined p-values of 1.48 x 10(-10), (rs8034191) and 5.74 x 10(-10) (rs1051730). Furthermore, these SNPs were significantly associated with lung function in both the ICGN and Boston Early-Onset COPD populations. The C allele of the rs8034191 SNP was estimated to have a population attributable risk for COPD of 12.2%. The association of hedgehog interacting protein (HHIP) locus on chromosome 4 was also consistently replicated, but did not reach genome-wide significance levels. Genome-wide significant association of the HHIP locus with lung function was identified in the Framingham Heart study (Wilk et al., companion article in this issue of PLoS Genetics; doi:10.1371/journal.pgen.1000429). The CHRNA 3/5 and the HHIP loci make a significant contribution to the risk of COPD. CHRNA3/5 is the same locus that has been implicated in the risk of lung cancer.

A genome-wide investigation of SNPs and CNVs in schizophrenia.

We report a genome-wide assessment of single nucleotide polymorphisms (SNPs) and copy number variants (CNVs) in schizophrenia. We investigated SNPs using 871 patients and 863 controls, following up the top hits in four independent cohorts comprising 1,460 patients and 12,995 controls, all of European origin. We found no genome-wide significant associations, nor could we provide support for any previously reported candidate gene or genome-wide associations. We went on to examine CNVs using a subset of 1,013 cases and 1,084 controls of European ancestry, and a further set of 60 cases and 64 controls of African ancestry. We found that eight cases and zero controls carried deletions greater than 2 Mb, of which two, at 8p22 and 16p13.11-p12.4, are newly reported here. A further evaluation of 1,378 controls identified no deletions greater than 2 Mb, suggesting a high prior probability of disease involvement when such deletions are observed in cases. We also provide further evidence for some smaller, previously reported, schizophrenia-associated CNVs, such as those in NRXN1 and APBA2. We could not provide strong support for the hypothesis that schizophrenia patients have a significantly greater "load" of large (>100 kb), rare CNVs, nor could we find common CNVs that associate with schizophrenia. Finally, we did not provide support for the suggestion that schizophrenia-associated CNVs may preferentially disrupt genes in neurodevelopmental pathways. Collectively, these analyses provide the first integrated study of SNPs and CNVs in schizophrenia and support the emerging view that rare deleterious variants may be more important in schizophrenia predisposition than common polymorphisms. While our analyses do not suggest that implicated CNVs impinge on particular key pathways, we do support the contribution of specific genomic regions in schizophrenia, presumably due to recurrent mutation. On balance, these data suggest that very few schizophrenia patients share identical genomic causation, potentially complicating efforts to personalize treatment regimens.