Apolipoprotein E (APOE) ε4 alleles increase the risk for late-onset Alzheimer disease (LOAD) and decrease the age of onset. Recently, sequencing the APOE region in a small sample of LOAD subjects identified a variable length poly-T repeat sequence in the nearby gene, TOMM40, which may affect age of onset. We genotyped the TOMM40 poly-T repeat using a novel statistical approach to refine the identification of allele length in 892 LOAD subjects and evaluated its effects on age of onset. Because psychosis in LOAD is a heritable phenotype which has shown conflicting associations with APOE genotype, we also evaluated the association of poly-T repeat length with psychosis. Poly-T repeat lengths had a trimodal distribution which differed between APOE genotype groups. After accounting for APOE ε4 there was no association of poly-T repeat length with age of onset. Neither APOE ε4 nor poly-T repeat length was associated with psychosis. Our findings do not support the association of poly-T repeat length with age of onset in LOAD. The clinical implications of this repeat length polymorphism remain to be elucidated.
Late-onset Alzheimer disease (LOAD) is a neurodegenerative illness with substantial heritability (Gatz et al., 1997). Similarly, and not surprisingly, as the risk of LOAD increases in a highly age-dependent manner (Ferri et al., 2005), the age of onset of LOAD is also heritable (Pedersen et al., 2001). The gene with the most strongly established relationship to LOAD risk is apolipoprotein E (APOE), with increased risk of LOAD found in individuals carrying 1 or 2 copies of the ε4 allele ( Farrer et al., 1997). The primary effect of APOE ε4 alleles on LOAD risk appears to be mediated via lowering the age of onset of Alzheimer disease, with a reduction of up to 7–9 years for each ε4 allele ( Reitz and Mayeux, 2009).
The APOE ε4 allele has also been extensively examined for association with the presence of the psychotic phenotype of LOAD (LOAD+Psychosis, LOAD+P) ( Sweet et al., 2003). LOAD+P is heritable ( Bacanu et al., 2005 and Sweet et al., 2010), and identifies a subgroup of LOAD subjects with more severe cognitive impairment and more rapid cognitive decline ( Emanuel et al., 2011 and Ropacki and Jeste, 2005). Unlike age of onset, the association of APOE ε4 allele with psychosis has revealed inconsistent findings, with slightly more negative than positive studies, and some studies showing evidence for a protective effect ( DeMichele-Sweet and Sweet, 2010). Such a pattern could result solely from type I error due to small cohorts with varying approaches to clinical characterization and analysis, however, a variable pattern of association can also arise due to a causal association with genetic variation in linkage disequilibrium with the APOE ε4 allele.
APOE ε4 is defined by a 2 single nucleotide polymorphism (SNP) haplotype in APOE exon 4. SNPs rs429358 and rs7412 each code for either arginine (C) or cysteine (T). APOE ε4 alleles are the CC haplotype (with TT and TC defining ε2 and ε3 alleles, respectively). Recent investigations fine-mapping the region within and surrounding APOE on chromosome 19 identified a set of SNPs within the nearby gene, TOMM40, in linkage disequilibrium with the APOE ε4 allele ( Yu et al., 2007) and affecting APOE expression ( Bekris et al., 2010). This finding, in part, motivated an effort to sequence the APOE and TOMM40 region in subjects with Alzheimer disease (AD), in an effort to identify possible causal variants within the linked region ( Roses et al., 2010). Sequencing identified a variable length poly-T repeat sequence in intron 6 of TOMM40 that was in linkage disequilibrium with APOE ε4. Individuals with APOE ε3/ε4 genotype and long poly-T repeats (defined as ≥ 27) had significantly lower age of onset of LOAD than individuals with APOE ε3/ε4 genotype and short repeats ( Roses et al., 2010).
Ultimately, reconciling the independent effects of APOE ε4 and TOMM40 repeat length polymorphism on age of onset of LOAD will require concurrent genotyping of large numbers of subjects. To address this goal, we developed an approach to high throughput genotyping of the TOMM40 poly-T repeat length polymorphism by starting with polymerase chain reaction (PCR) to generate an initial estimate of allele sizes and then refining these estimates with a statistical model. We evaluated the independent and joint effects of these genetic variants in a large population of 892 Caucasian individuals with LOAD, examining both the age of onset and LOAD+P phenotypes.
