In healthy controls, no effects of SNPs in C1236T, G2677T/A and C3435T on Pgp function at the BBB were found in vivo. In contrast, in AD patients, both the presence of T as well as T dose in C1236T, G2677T/A and C3435T were related with increased BPND of (R)-[11C]verapamil, suggesting decreased Pgp function at the BBB.
Two previous PET studies, assessing effects of ABCB1 haplotypes on BBB Pgp function in healthy subjects, also did not find differences in the brain uptake of 11C]verapamil [16, 17]. Before the present study, however, no studies have been performed assessing effects of genetic variations in ABCB1 on in vivo BBB Pgp function in AD patients.
The present study showed a selective effect of the C1236T, G2677T and C3435T SNPs in ABCB1 on BBB Pgp function in AD patients. As, using (R)-11C]verapamil PET, BBB Pgp dysfunction in AD patients has been shown previously , the present data suggest that genetic variations in the ABCB1 gene might affect Pgp function or expression at the BBB, only when Pgp function is already compromised. Therefore, in AD patients, genetic variations in ABCB1 could contribute to disease progression as an additional decrease in BBB Pgp function may lead to an increased rate of accumulation of toxic substances, such as amyloid-beta, in the brain. Alternatively, it is also possible that certain genetic variations in ABCB1 have a more primary role in developing neurodegenerative diseases such as AD or Parkinson's disease (PD). For example, in subjects that were exposed to organochlorine insecticides, polymorphisms in ABCB1, associated with decreased ability to clear xenobiotics from the brain, increased the risk of PD .
In the present study, the prevalences of different variants in C1236T, G2677T and C3435T SNPs in ABCB1 were comparable between AD patients and healthy controls. A previous pilot study in relatively small groups of demented patients and healthy subjects showed no significant differences in the prevalence of C1236T, G2677T/A and C3435T . Nevertheless, a large population-based study would be needed to assess possible differences in prevalence of genetic variations in ABCB1 between healthy controls and AD patients. Only such a larger study could provide insight into the role of ABCB1 SNPs and haplotypes as a possible risk factor for developing AD. In a subset of such a study, BBB Pgp function should be measured to further evaluate the relationship between ABCB1 SNPs and haplotypes, and BBB Pgp function in both healthy controls and AD patients.
The prevalences of different variants in C1236T, G2677T/A and C3435T in this study were comparable to those reported previously [5, 7]. In addition, linkage disequilibrium between the three SNPs, observed in the present study, has also been described before [13–15]. It could very well be that the effects of the different SNPs in ABCB1 that we have found in AD patients are physiologically effectuated through the linkage disequilibrium via the G2677T SNP, which is a non-synonymous variant, thus changing amino acid sequence, while the mutations in C1236T and C3435T are synonymous [5, 6].
At present, measuring Pgp function at the BBB in vivo can only be performed using (R)-[11C]verapamil PET with arterial blood sampling. Consequently, group sizes in the present study are limited, and findings need to be replicated in larger trials, preferably using methods without arterial sampling. Another limitation of the present study is the fact that it is not possible to differentiate between decreased Pgp function due to decreased BBB Pgp transporter expression or decreased Pgp transport functionality with intact transporter expression. This can only be addressed when a PET tracer of Pgp expression becomes available.