In this study of 103 young-onset AD cases with a positive family history of cognitive decline we observed 13 common variants in the SERPINA5 gene at a similar frequency to that previously reported. In addition, we observed one individual with the rare missense variant SERPINA5 p.E228Q (rs140138746). To increase our cohort size, we screened a further 1170 neuropathologically diagnosed AD cases and observed another 5 individuals carrying this variant (< 1%). This revealed an allelic frequency in AD (0.002141) comparable to that observed in the general population (0.002599).
Although the discovery cohort of young-onset AD cases with a positive family history was used to identify SERPINA5 p.E228Q variant carriers, age was not used to select the validation cohort. Despite this, we continued to see a non-significant pattern of a median age of onset of 66 years in carriers, which was 5 years younger than the non-carriers in our cohort. Previous studies have indicated that disease duration in AD dementia ranges between 3 to 10 years [6, 39]. Interestingly, SERPINA5 p.E228Q carriers were found to have a longer-than-average disease duration of 12 years, which approached significance as this was 3 years longer than non-carriers. Additionally, case #2, a HpSp AD subtype, had a disease duration of 12 years, which is much longer than the reported average of 9 years for this subtype [13]. Together, these findings suggest that the SERPINA5 p.E228Q variant may be associated with prolonged disease duration, though more work is needed to understand on the association of SERPINA5 genetic variants and age.
Global neuropathology measures, including Braak tangle stage [4] and Thal amyloid phase [34], did not differ between SERPINA5 p.E228Q carriers and non-carriers. The limbic predominant AD subtype was observed in 33% of the SERPINA5 p.E228Q variants, which is higher than reported in neuropathology-based series (14%) but exactly at the frequency observed in recently validated tau imaging-based classification [13, 37]. We observed a high frequency of Lewy body pathology in 5/6 (83%) of the SERPINA5 p.E228Q carriers. Lewy pathology exists concurrently with AD pathology, occurring most frequently in the olfactory bulb and amygdala, as compared to the substantia nigra that is more commonly seen in Parkinson’s disease [10, 30]. Studies have shown that Lewy body pathology is detected in 60% of AD cases [12] and amygdala predominant Lewy bodies is detected in 18% [35]. The longer disease duration may account for the greater frequency of amygdala predominant Lewy bodies, as it is more commonly observed in severe end-stage AD brains.
It is well established that variants in the APOE gene are strongly associated with risk of sporadic late-onset AD [7]. The strongest risk is associated with the APOE ε4 allele, followed by APOE ε3 and finally by the APOE ε2 allele [36]. Approximately 40% of late-onset AD patients have the APOE ε4 allele, compared to about 14% in the general population [11]. The presence of one copy of the APOE ε4 allele increases the risk of late-onset AD by about 3 times and 2 copies by 12 times [36]. Interestingly, the presence of one or two copies of the APOE ε4 allele is associated with an earlier age of onset by about 10–20 years, as compared to non-carriers [7]. We observed a high frequency (83%) of at least one APOE ε4 allele in our SERPINA5 p.E228Q carriers, which intersects well with the younger age of onset in these cases.
In addition, we sought to assess the SERPINA5 and tau burden across 9 different brain regions, including the entorhinal cortex, limbic regions (amygdala and hippocampus), association cortices (temporal, parietal and frontal) and primary cortices (visual and motor). This allowed us to assess neuropathologic burden in regions routinely used to indicate AD disease progression in the Braak tangle staging scheme [4]. Using digital pathology to quantify immunohistochemical burden, SERPINA5 was overall observed to be lower in the SERPINA5 p.E228Q variant carrier than the non-carrier. To examine neurofibrillary tangle changes, we used two antibodies (AT8 and 2E9) to encompass the dynamic lifespan of tangle maturity [22]. The tau:SERPINA5 ratio was much higher in the carrier compared to the non-carrier. Although unable to test the hypothesis in the current study, our recent data suggests SERPINA5 may be a “tipping point” in the tangle maturation process that could contribute to associated neurodegeneration [8]. The observed lower levels of SERPINA5 immunohistochemical burden in the variant may have allowed for more time for neurofibrillary pathology to accumulate before maturing into a ghost tangle upon the death of the neuron. To examine an area outside of the vulnerable corticolimbic regions, the locus coeruleus was evaluated as this region may have the longest-standing tau accumulation [5]. We observed only moderate neuronal loss in the non-carrier compared to severe neuronal loss in the carrier. As we more commonly observed greater neuronal loss in the carrier, we cautiously speculate that the SERPINA5 p.E228Q variant requires less SERPINA5 to promote more extensive neurofibrillary tangle maturity, leading to the death of the neuron. Taken together with the trend for longer disease duration observed overall, it is interesting to consider that the longer time to accumulate pathology corresponds to greater neuronal loss in the context of similar 2E9 (advanced tangle) burden.
There are several limitations to this study. First, we did not sequence the promotor and regulatory regions of the SERPINA5 gene. It is well established that variants in these regions can affect gene expression, and can contribute to disease [26, 28]. Therefore, it is possible that regulatory region variants may be contributing to the increased expression of SERPINA5 that we observe in AD, although more thorough sequencing must be undertaken before such conclusion can be made. In addition, it will be important to sequence more cases for SERPINA5 variants. Future directions include screening cases of other tauopathies, such as primary age-related tauopathy, corticobasal degeneration, and progressive supranuclear palsy, which will assist in determining if SERPINA5 variants contribute to the development of tau pathology in general. Furthermore, we were unable to carry out statistical analysis for our neuropathologic assessment due to lack of power, and therefore any observations made are descriptive and interpreted with caution.