illustrates a metaphase and interphase from a short-term whole blood culture, hybridized with an FITC-labeled PNA probe such that telomeres are labeled at each chromosome end and within the interphase nucleus. An initial set of analyses replicated our earlier studies of overall telomere shortening associated with dementia in six adults with DS. In all cases t-tests revealed significantly shorter telomere lengths for the individual with dementia, ps <.001 for metaphase analyses and ps<.02 for interphase analyses (data available but not shown).
Telomeres in metaphase and interphase shown by an FITC-labeled PNA probe with DAPI counterstaining. Arrows indicate chromosome 21.
The age, sex and dementia status of the study participants are given in . Results extending our findings to individuals with MCI-DS are presented in . Each of six pairs of individuals was compared for both metaphase and interphase measures, and in all cases MCI-DS was associated with significantly shorter telomeres than those of their age-matched peers without MCI-DS or dementia. and summarize our studies of individual chromosomes 21, 1, 2, and 16. Here again, both dementia and MCI-DS were associated with telomere shortening in every case, although the p value for the chromosome 1 comparison in Study 1 only reached significance at the one-tailed level. The number of matched pairs for Chromosomes 2 and 16 is not complete at this time due to the departure of a member of our research team. However, the subset of completed individuals should be representative of the larger group.
Age, sex and dementia status of 30 individuals with trisomy 21
Mean light intensity units (×104) are shown from within pairs for whole metaphases and interphases matched specimens from individuals with DS with and without MCI-DS.
Mean light intensity units (×103) for chromo-somes (C) 21 and 1.
Mean light intensity units (×103) for chromosomes (C) 2, and 16
To determine if the effects of dementia or MCI-DS differed among the selected chromosomes, an additional repeated measures multivariate analysis of variance was run using the General Linear Models module within Statistica 8.0 (Statsoft, 2007). Chromosome number (1, 2, 16 and 21) and Group (Dementia/MCI-DS versus Not Demented) were factors, and, as expected, fluorescence signal was greater for the Not Demented Group, F (1,7) = 83.8, p < .00005. In addition, a significant Chromosome by Group interaction was also present, F (3, 5) = 11.48, p < 0.02. Further analysis indicated that this interaction was due to a greater effect of dementia/MCI-DS on telomere shortening for Chromosome 21 compared to the other chromosomes. Given the presence of an extra Chromosome 21 within each cell, this analysis was repeated with the chromosome 21 values divided by 1.5 to determine if trisomy alone was responsible for the differential effect, and in fact the interaction no longer approached significance (with both multivariate and univariate Fs < 1.0). Although fluorescence signals were larger for the two longer chromosomes (1 & 2) compared to the two shorter chromosomes (16 & 21), F (3,5) = 18.6, p < .005, the impact of dementia/MCI-DS on this signal was comparable for the four chromosomes studied. Analyses were repeated omitting the MCI-DS cases and results were essentially unchanged.
Detailed inspection of the results from our individual chromosome studies showed that there was no overlap in the distribution of scores between demented or MCI-DS and non-demented participants in the case of Chromosome 21, t (30) = 8.3, p<.000001. These data are illustrated in . As indicated in and , there was also little to no overlap in the distributions of scores for Chromosomes 2 and 16, and modest overlap for Chromosome 1. Non-parametric group comparisons consistently showed that mean telomere lengths were significantly shorter for the Dementia/MCI-DS group. Wilcoxon Signed Ranks Test significance ranged from .005<ps<.05.
Fig. 2 Chromosome 21 light intensities from 15 individuals with DS only (no dementia, no MCI) vs. those with DS and Dementia DS and MCI. Note there was no overlap in values between the groups labeled Dementia or MCI and the group labeled No Dementia, No MCI (more ...)
To determine if inferred telomere shortening was due to severity of cognitive impairment, fluorescence signals of individuals within the Dementia/MCI-DS group were related to both IQ (obtained prior to the onset of dementia signs and symptoms) and a measure of dementia severity (Silverman et al, 2004
). None of these correlations approached significance, rs < .17, ps > 0.55, suggesting that effects of dementia/MCI-DS on fluorescence signal can be expected to be relatively consistent within this population. The apolipoprotein E (APOE
) ε4 allele has been associated with increased risk for dementia (Schupf et al., 1996
) and with early mortality (Zigman et al, 2005
) in this population. Possible effects of APOE
genotype were also examined by focusing on: (a) overall fluorescence signals within the Dementia/MCI-DS group (chromosomes 1 + 21; 1 + 2 + 16 + 21), (b) overall fluorescent signals within the comparison group, and (c) the difference between the Dementia/MCI and comparison groups. Analyses involving the Dementia/MCI-DS group were conducted twice, once including all cases and once excluding the MCI-DS cases to reduce any confound with stage of progression of Alzheimer’s disease. None of these analyses produced a significant effect.