This study showed that the predictive factors for aspiration pneumonia may differ between young and elderly stroke patients, with SNP influences manifested mainly in the former group. Among the various SNPs analyzed, only the APOE genotypes showed a positive association with the minor C rs429358 allele, increasing the risk of aspiration pneumonia in young post-stroke dysphagia patients. With the inclusion of this SNP, a multivariable model that included worse baseline mRS and MBSImP© scores predicted the risk of aspiration pneumonia in the young group with high accuracy (AUROC = 0.82 (95% CI: 0.74 – 0.90)). In contrast, in the elderly group, a model that included only the clinical factors with worse initial GUSS and PAS scores was shown to predict the risk of aspiration pneumonia (AUROC = 0.89 (95% CI: 0.83 – 0.95)).
The well-known factors that can increase the risk of aspiration pneumonia after stroke are increased age, a higher degree of cognitive impairment, low levels of substance P or the use of angiotensin-converting enzyme inhibitors, increased severity of aspiration and dysphagia, and increased levels of post-stroke disability [5–7, 27–29]. Our study's results were consistent with those of previous studies, reflected by the lower GUSS and higher PAS and NIHSS scores in the elderly group and the increased severity of the oral stage of impairment in the younger age stroke group with higher initial mRS scores. Of significant interest was that in the young age group, the multiple inheritance analysis showed that certain SNPs, specifically rs4252961, were also associated with an increased risk of aspiration pneumonia.
One of the unexpected findings was that APOE was the SNP associated with aspiration pneumonia, which is discrepant with previous findings that reported that the major allele of rs4252961 in IL1RN was associated with infection risk . APOE is encoded by three alleles (ɛ2, ɛ3, and ɛ4) on chromosome 19q13, and is produced by the liver, brain, spleen, kidneys, lungs, and muscle tissue . By binding to lipoprotein particles and cellular receptors, APOE controls triglyceride homeostasis and lipoprotein clearance in tissues including the lungs . This regulation is affected by APOE polymorphism, and APOE ɛ4 polymorphism is known to increase the risk of atherosclerosis and neurodegenerative disorders, including Alzheimer’s dementia. This allele can also lead to higher mortality in dementia patients and is known to be associated with younger stroke onset [32–35].
Previous studies have discussed the role of APOE genotypes in infection especially in respiratory disease. APOE acts on macrophages and T cells, regulating the innate immune system. In one study, when injected with bacterial endotoxin, those with APOE ɛ4 showed higher hyperthermia and plasma tumor necrosis factor-alpha (TNF-α) levels . APOE ɛ4 has also been shown to be related to the development of sepsis , and enhanced the attachment of Chlamydia pneumonia elementary bodies to host cells . Furthermore, in a recent study, those with APOE ɛ4 showed a higher risk of severe COVID-19 infections .
Another point of interest from our study was that SNP showed different effects in young versus elderly stroke patients. Only young adults with a minor C rs429358 allele showed an increased risk of aspiration pneumonia in multiple inheritance models, including both additive and dominant models, showing that APOE ɛ4 increased the risk of aspiration pneumonia with an OR of 4.53, even after multiple Bonferroni corrections. When included in the multivariable analysis model, this minor C rs429358 allele remained an important risk factor for aspiration pneumonia. However, the effects of this SNP on infection were only observed for aspiration pneumonia. Minor APOE alleles seemed not to exert a similar influence on urinary or gastrointestinal tract infections. The predilection of APOE for pneumonia is consistent with past studies advocating that the role of APOE ɛ4 in lung disease may be linked to the increased risk of pulmonary complications [37, 38].
In contrast, no APOE allele seemed to play a role in increasing the risk of aspiration pneumonia in the elderly group. Since old age itself is a vital risk factor for aspiration pneumonia [4, 5], genetic polymorphism influences may have been overridden by this age factor and thus, be less influenced when compared to the young age group. In fact, 44.6% of the young and 61.2% of the elderly patients had aspiration pneumonia history, with more aspiration pneumonia in the elderly group regardless of APOE ɛ4 presence.
Some limitations of the study need to be considered. First, in this study, we compared the groups based on the presence of APOE ɛ4, instead of evaluating each APOE genotype. Comparison between those with APOE ɛ2/ɛ4, APOE ɛ3/ɛ4, and APOE ɛ4/ɛ4 was not feasible due to the limited number of participants in the APOE ɛ2/ɛ4 and APOE ɛ4/ɛ4 groups. There were only two participants with APOE ɛ2/ɛ4 and APOE ɛ4/ɛ4 each, with most of the participants having APOE ɛ3/ɛ4 polymorphism. The two participants with APOE ɛ2/ɛ4 did not experience aspiration pneumonia, whereas two APOE ɛ4/ɛ4 participants had aspiration pneumonia during the follow-up period, which may support our results that APOE ɛ4 may act as a risk factor for aspiration pneumonia after stroke. Secondly, recruitment was done in only two university-affiliated hospitals, leading to possible selection bias. Also, the study was conducted only on post-stroke patients. Therefore, the results of the study may not be generalizable to all patients with aspiration pneumonia related to deglutition impairment caused by other neurodegenerative disorders. In the future, a multi-center prospective study with a larger number of participants of each genotype with different etiologies is needed to further support our findings.
Nevertheless, the results of this study are noteworthy since according to our knowledge, this is one of the first studies to explore the effects of genetic polymorphisms on post-stroke aspiration pneumonia. Many studies have attempted to identify factors based on clinical information [39, 40], and neuroanatomical correlates [41, 42] to help determine those at an increased risk of aspiration pneumonia. Our results may provide information on the complexities of understanding how the immune responds following stroke and how certain SNPs may modulate the risk of infection.
In summary, this study suggests that APOE ɛ4 may be a risk factor for post-stroke aspiration pneumonia in patients younger than 65 years old. Considering that aspiration pneumonia may increase post-stroke mortality, this new role of APOE may be clinically relevant. Genetic polymorphisms may be taken into consideration in predicting post-stroke aspiration pneumonia.