This is the first study exploring the association between NGFR polymorphisms and the risk of AD by using 5 htSNPs. We found that NGFR rs734194 was significantly associated with a decreased risk of AD, but this was not observed in the only previous study in an Italian population . Possible reasons for the inconsistent findings between the Italian study  and ours include differences in sample size (Cozza et al. vs. ours, sporadic AD: 151 vs. 264, controls: 97 vs. 389), case selection (not available vs. incident cases), race (Italian vs. Chinese), study time period (not available vs. 2007 to 2010), mean age (AD: 65 vs. 79, controls: 64 vs. 73), and SNPs selected (4 functional SNPs vs. 5 htSNPs). This is also the only Asian study up to date. In addition, no significant association was observed for NGFR rs2072445 (in intron), rs2072446 (in exon), and rs741072 [in 3' untranslated region (UTR)], which is consistent with the findings of the Italian study . rs741073 has not been explored for AD risk previously and was not associated with AD risk in our study. Although 5 htSNPs are in strong linkage disequilibrium (LD; i.e., high pairwise D' as shown in dark gray, Figure 1) and located within the same haplotype block, the pairwise correlation (r2) between rs734194 (SNP3) and any other SNP is quite low (SNP1:0.02, SNP2: 0.04, SNP4: 0.20, SNP5: 0.10, Figure 1).
Cholinergic hypothesis [3, 4] has been used to elucidate the role of NGFR in AD pathogenesis because of selective loss of BFCN observed in AD patients. That is, elevated expression of NGFR and decreased TrkA may activate neuron apoptosis [29, 30]. In addition, the binding of Aβ [31–33] and proNGF [9, 34] to NGFR also induce neuron apoptosis. rs734194 is located on 3' UTR and thus plays an important role in regulating the mRNA stability and translational efficiency. Therefore, variations in rs734194 may reduce the expression of NGFR or the binding of NGF, Aβ, or proNGF to NGFR, which inactivates the neuron apoptotic signaling and leads to decreased risk of AD. It is also possible that the variations of rs734194 decrease the secretion of NGFR on BFCN and thus reduce the interaction of NGFR with Aβ and proNGF, which lower the neurotoxicity and apoptosis of BFCN. All together, these mechanisms may explain the protective effect of NGFR rs734194 on the risk of AD observed in this study.
We found that Hap1 GCGCG was significantly associated with a decreased risk of AD. rs734194 is the only SNP carrying the variant allele in Hap1. Therefore, the significant association of Hap1 and AD may be attributable to rs734194. It is also possible that other rare polymorphisms not analyzed here are responsible for the association observed. Our finding was not comparable to the Italian study  because fewer SNPs were selected and no significant association was observed for NGFR haplotypes in that study.
This study found that type 2 DM significantly modified the association between NGFR polymorphisms and the risk of AD in ApoE ε4 non-carriers. It is possible that type 2 DM modifies the association between NGFR and AD via the following mechanisms: (1) hyperglycemia [35–37], (2) altered insulin level and sensitivity in the brain [13, 38–40], and (3) diabetes-related vascular diseases, e.g., hypertension and arterial disease . In addition, ApoE ε4 status affects cholesterol metabolism and may act together with DM to modulate the risk of AD [42–44]. The significant effect modification was only observed in ApoE ε4 non-carriers, which may be due to the counteracting effect between ApoE ε4 allele (increase AD risk) and variant of NGFR (protective effect) on AD risk.
This study has some strengths. No study has investigated the role of NGFR polymorphisms on the risk of AD using a set of representative htSNPs and their corresponding haplotypes. In this study, 5 htSNPs were selected via a systematic approach and captured over 85% of genetic information in NGFR (estimated by tagSNP program). In contrast, the only prior study  assessed 4 NGFR SNPs, which capture only 14% of genetic information in the gene. Second, the sample size of our study is larger than the Italian study (Cozza et al. vs. ours, sporadic AD: 151 vs. 264, controls: 97 vs. 389). In addition, this study has over 90% power to detect an OR of 0.43 for the main effect and 78% power to detect an OR of 0.28 for the interaction between NFGR and type 2 DM on the risk of AD. Third, no study has assessed this association in Chinese population and identified NGFR SNPs representative for this ethnic group. Last, the use of brain image increased the validity of AD ascertainment and reduced misclassification of disease subtypes.
This study has some limitations. DM status was self-reported and thus may be biased. However, in our questionnaire, this information was further confirmed by asking if there was a previous diagnosis or taking medications for type 2 DM after seeing a doctor. Because DM is a major disease, participants' recall of DM diagnosis and their awareness of DM should be relatively accurate [45–47]. As a whole, the chance of recall bias was low.
In summary, this study found that NGFR htSNPs and haplotypes were associated with AD risk. Type 2 DM significantly modified the association between NGFR polymorphisms and the risk of AD in ApoE ε4 non-carriers. Although these findings did not reach statistical significance after correction for multiple tests, it is possible that the NFGR polymorphisms were associated with familial AD. This is because NGFR rs2072446 was associated with a decreased risk of familial AD in the Italian study . Most of sporadic AD cases are ApoE ε4 non-carriers (60%) observed in this and other studies, therefore, our findings shed light on the importance of identifying genetic markers in ApoE ε4 non-carriers. Future large studies are warranted to confirm our findings.