SNPs associated with cerebrospinal fluid phospho-tau levels influence rate of decline in Alzheimer's disease

Abstract

Alzheimer's Disease (AD) is a complex and multifactorial disease. While large genome-wide association studies have had some success in identifying novel genetic risk factors for AD, case-control studies are less likely to uncover genetic factors that influence progression of disease. An alternative approach to identifying genetic risk for AD is the use of quantitative traits or endophenotypes. The use of endophenotypes has proven to be an effective strategy, implicating genetic risk factors in several diseases, including anemia, osteoporosis and heart disease. In this study we identify a genetic factor associated with the rate of decline in AD patients and present a methodology for identification of other such factors. We have used an established biomarker for AD, cerebrospinal fluid (CSF) tau phosphorylated at threonine 181 (ptau(181)) levels as an endophenotype for AD, identifying a SNP, rs1868402, in the gene encoding the regulatory sub-unit of protein phosphatase B, associated with CSF ptau(181) levels in two independent CSF series (P(combined) = 1.17 x 10(-05)). We show no association of rs1868402 with risk for AD or age at onset, but detected a very significant association with rate of progression of disease that is consistent in two independent series (P(combined) = 1.17 x 10(-05)). Our analyses suggest that genetic variants associated with CSF ptau(181) levels may have a greater impact on rate of progression, while genetic variants such as APOE4, that are associated with CSF Aβ(42) levels influence risk and onset but not the rate of progression. Our results also suggest that drugs that inhibit or decrease tau phosphorylation may slow cognitive decline in individuals with very mild dementia or delay the appearance of memory problems in elderly individuals with low CSF Aβ(42) levels. Finally, we believe genome-wide association studies of CSF tau/ptau(181) levels should identify novel genetic variants which will likely influence rate of progression of AD.

Figure 1. Rs1868402 is associated with CSF ptau₁₈₁ levels, PPP3R1 mRNA expression levels and tangle counts.

A. Association of rs1868402 with CSF ptau₁₈₁ levels (WU-ADRC-CSF n = 353) was tested by an Analyses of Covariance (ANCOVA) including CDR, age and APOE genotype as covariates. B: Minor allele carriers of rs1868402 have significantly lower PPP3R1 mRNA levels in individuals with AD pathology (n = 82). C: Minor allele carriers of rs1868402 have significantly higher numbers of tangles (n = 82). D: PPP3R1 mRNA expression correlates with tangle counts in individuals with AD pathological changes (n = 82). The p-value is for the correlation between mRNA levels and genotypes.

Figure 2. Survival curves comparing age at onset of LOAD between the different genotypes of rs1868402.

Survival fractions were calculated using the Kaplan-Meier method and significant differences were calculated by Log-rank test. Association with age at onset was calculated in a combined series with samples from WU-ADRC-CC, ADNI-CC and MRC.

Figure 3. Genetic variants associated with CSF ptau₁₈₁ levels are also associated with rate of progression of AD.

Rate of progression is defined as the change in the Clinical Dementia Rating sum of boxes (SB-CDR) score per year. Association of SNPs with progression was calculated using a mixed linear model (PROC MIXED) after controlling for age, sex, APOE, initial CDR, CSF ptau₁₈₁ and Aβ₄₂ levels. A. Minor allele carriers of rs1868402, are associated with higher CSF ptau₁₈₁ levels, and show a 6 fold faster progression than homozygotes for the major allele (CDR-SB/year: 0.58 vs 0.09; p = 0.0026) in individuals from the WU-ADRC-CSF with low CSF Aβ₄₂ levels (<500pg/ml). For this SNP the dominant model was used because it showed the best fit in all the analyses. B. rs3785883 genotypes do not have significantly different progression rates P = 0.057. The genotype frequency distribution for rs3785883 with disease progression is most likely not significant due to the low statistical power. AA carriers show a CDR-SB of 1.01, AG of 0.47 and GG 0.26 (p = 0.057). The additive model was used because it showed the best fit. C. Rs1868402 and rs3785883 show an epistatic interaction. Carriers of the alleles associated with higher CSF ptau₁₈₁ levels (CT+CC for rs1868402 and AA for 3785883) showed a CDR-SB/year of 1.02 vs −0.006 for carriers of alleles associated with lowest CSF ptau₁₈₁ levels. LP indicates lumbar puncture.