The genetics of dementia in late life

Apolipoprotein E genotype and cognitive impairment in community-dwelling black older adults

OBJECTIVE

The relationship between the epsilon 4 allele of the apolipoprotein E gene (APOE-epsilon4) located on chromosome 19 and Alzheimer's disease is well documented among Caucasian populations. However, the findings of research addressing the link between APOE polymorphism and neurocognitive functioning in populations of African origin from around the world have been equivocal. Therefore, the current study explored the relation of APOE-epsilon4 with cognitive impairment in a sample of community-dwelling English-speaking elderly blacks.

METHODS

All participants (N = 57) were recruited consecutively from a community memory-screening program at a University affiliated Memory Disorders Clinic and evaluated using standardized assessment procedures. Cognitive impairment was classified according to an age and education adjusted Mini-Mental State Exam score of less than 24 as well as poorer functioning on a measure of delayed verbal memory.

RESULTS

Increased risk for global cognitive dysfunction (OR = 9.5, 95 percent CI = 2.3-55.3, p = .004) and poorer verbal recall performance (beta = -.36, p = .006) were linked with the APOE epsilon4 allele after controlling for the potentially confounding effects of age, education, and gender.

CONCLUSIONS

This investigation supports the role of APOE polymorphism in determining neurocognitive impairment among black elders residing in the community.

Multiple-threshold models for genetic influences on age of onset for Alzheimer disease: findings in Swedish twins

Twin studies of dementia have typically used relatively simple 2 x 2 contingency tables with one threshold to estimate the relative importance of genetic variance for liability to disease. These designs are inadequate for addressing issues of age at onset, censoring of data, and distinguishing shared environmental effects from age effects. Meyer and Breitner [1998: Am J Med Genet 81:92-97] applied a multiple-threshold model to the NAS-NRC Twin Panel (average age of onset, 63.5 years) and report that additive genetic effects and shared environmental effects account for 37% and 35% of the variation, respectively, in age of onset for Alzheimer disease. We apply a modified version of their model to the Study of Dementia in Swedish Twins (average age of onset, 75 years) and find that genetic effects account for 57%-78% of the variance, whereas shared environmental effects are of no importance. Heritability is lower when thresholds are freely estimated rather than fixed to the population prevalences. We interpret the findings to suggest that models with free thresholds confound influences on longevity with influences for the disease. Multiple-threshold models, however, do not confound age effects with shared environmental influences.

Fine-scale mapping of a locus for severe bipolar mood disorder on chromosome 18p11.3 in the Costa Rican population

We have searched for genes predisposing to bipolar disorder (BP) by studying individuals with the most extreme form of the affected phenotype, BP-I, ascertained from the genetically isolated population of the Central Valley of Costa Rica (CVCR). The results of a previous linkage analysis on two extended CVCR BP-I pedigrees, CR001 and CR004, and of linkage disequilibrium (LD) analyses of a CVCR population sample of BP-I patients implicated a candidate region on 18p11.3. We further investigated this region by creating a physical map and developing 4 new microsatellite and 26 single-nucleotide polymorphism markers for typing in the pedigree and population samples. We report the results of fine-scale association analyses in the population sample, as well as evaluation of haplotypes in pedigree CR001. Our results suggest a candidate region containing six genes but also highlight the complexities of LD mapping of common disorders.