With this study, we aimed to investigate the possibility of using genetic findings with laboratory parameters to predict the severity of Covid-19 and help in patient follow-up and risk assessment to reduce disease morbidity and mortality, since the disease is severe in the presence of advanced age and comorbidities.
In our study, advanced age and comorbidities such as hypertension, diabetes, and coronary heart disease were associated with the severity of the disease and death due to the disease in accordance with previous studies [6,24,25,5,26,4] in Covid-19 patients (Table 1). According to studies, the mortality rate (CFR) (case fatality rate) in all cases was found to be 2.3% [3]. On the other hand, while death was not observed in children under 9 years of age, the mortality rate increased in correlation with comorbidities and advanced age. CFR was elevated among those with preexisting comorbid conditions: 10.5% for cardiovascular disease, 7.3% for diabetes, 6.3% for chronic respiratory disease, 6.0% for hypertension, and 5.6% for cancer [3].
The HLA system plays a key role in the regulation of the immune system in humans [11]. For this reason, although the HLA allele system in Covid-19 patients has been studied by many researchers, there are very different results in this regard. In some studies, low or high HLA alleles in the study groups were found to be associated with the disease or disease severity [27,25,19,15,28]. However, in some studies, low or high alleles between the groups were not significant and not associated with the disease or disease severity [29,25]. This diversity of studies is due to the large polymorphic feature of the HLA system and regional genetic differences or the small number of patients in the study groups [30,1,26].
According to the study by Wang et al. [26], which includes the relationship between Class-I HLA (AC) and Class-II HLA (DPB1, DQA1, DQB1, and DRB1) genes in Covid-19 patients and the severity of the disease, especially HLA-A*11:01, B*. They interpreted that the 51:01 and C*14:02 alleles were significantly higher in severe and critical patients compared to mild and moderate patients, as Class-I HLA alleles were more correlated with disease severity than Class-II HLA alleles. Wang et al. [15] and Khor et al. [27] support the conclusion that Class-I HLA (A-C) alleles are more correlated with disease severity in Covid patients, as in our study results. While Wang et al. [15] found HLA-C*07:29, B*15:27 alleles to be significantly higher in the patient group compared to the control group, Khor et al. [27] found HLA-A*11:01, C*12:02 and B*52:01 alleles from Class-I allele groups to be higher in patients with severe Covid-19 compared to patients with mild Covid-19. We also found in our study that Class-I HLA A*26, A*32, B*27, B*41, B*52, C*5, C*14, and C*16 alleles from parent-1 allele groups and Class-I HLA alleles A*11, A*68, A*25, A*38, B*37, B*58, B*45, B*51 and C*14 alleles from parent-2 allele groups were significantly higher in the patient groups (SG and NSG). (Table 2-4).
In addition, Wang et al. [15] determined that DRB1*14:04, DRB1*01:01, and DQA1*01:01 alleles were higher in the severe group compared to the mild group as a risk allele for the severity of the disease, while DPB1*03:01 and DRB1*12*01 alleles were higher in the mild group compared to the severe group. Therefore, they evaluated the effect of these alleles (DPB1*03:01 and DRB1*12*01) to show their protective effect against the disease.
We evaluated A*26, A*32, B*41, C*14, C*16, DRB1*8 and DRB1*14 alleles from parent-1 detected only in SG and DQB1*4, B*27, B*52, C*5 and A*68, B*37, B*58, C*14, DRB1*16 alleles from parent-1 and parent-2 alleles detected only SG and NSG, respectively, as risk alleles for Covid-19 disease, and A*1, DQB1*4, B*15, and B*54 alleles from parent-2 allele groups detected only in HC group as protective effective alleles. The fact that the risk and protective alleles for Covid-19 disease we found differ from the results of the study conducted by Wang et al. [15] study may be due to the small size of our study group or genetic polymorphisms due to regional differences [1,15].
Again, Wang et al. [15] found the HLA-B*46:01 allele to be higher in the mild patient group than in the severe group, unlike the study results of Nguyen et al., but this difference was not significant.
According to the study by Pisanti et al. [7] with HLA haplotypes in Italian patients with Covid-19, HLA-A*01:01, B*08:01, C*07:01, and DRB1*03:01 alleles were significantly positively correlated with the disease. These alleles are associated with susceptibility to Covid-19, while HLA-A*02:01, B*18:01, C*07:01, and DRB1*11:04 alleles were significantly inversely correlated with the disease. They thought that there might be protective alleles for Covid-19. Therefore, both groups of alleles were associated with the incidence and mortality of Covid-19 in Covid-19 patients.
In addition, Romero-Lopez et al. [31] found a significant negative correlation between the presence of the HLA-DRB1*01 allele in the Mexican population and the death rate due to Covid-19 disease according to their broad-based data analysis in Mexico.
In our study, when allele subtypes were not taken into account, we found the frequencies of alleles found by Pisanti et al. [7] to be high in all three study groups (HC, SG, NSG). However, the frequency differences between the groups were not significant. In addition, according to our study results, although there was not a significant difference between the groups, the alleles that could have susceptibility and protective effects in Covid-19 patients were completely different from the alleles found by Pisanti et al [7]. According to our study results, alleles seen only in the healthy control group were protective alleles, while alleles seen only in the sick or severely ill group were alleles that could cause sensitization.
Correale et al. [30] found a positive correlation between Covid-19 disease and especially HLA-B*44 and C*01 alleles and a negative correlation between B*14, B*18, and B*49 alleles in relation to the spread of Covid-19 disease in Italy. Although the study by Correale et al. [30] is partially similar to our study in terms of alleles detected and their results, both in terms of the frequency of alleles studied and especially from parent-1 allele groups A*26, A*32, B*41, C*14, C*16. and DRB1*14 alleles differ with our study results that A*68 and B*58 alleles from the parent-2 allele group may cause susceptibility to Covid-19.
In addition, study results conducted by Correale et al. [30] were inconsistent with the study by Novelli et al. in Italy [19]. According to the study by Novelli et al. [19], while HLA-DRB1*15:01, DQB1*06:02, and B*27:07 alleles cause susceptibility for Covid-19 disease in the patient group with Covid-19, there was no significant difference between the groups in terms of the HLA-B*58:01 allele. C*06:02 and DRB1*07:01 alleles were negatively correlated with Covid-19 disease [19]. In addition, according to our results, HLA-DRB1*15:01, DQB1*06:02, and B*27:07 alleles were high in our study group as HC, SG, NSG, but the difference between the groups was not significant, whereas the results were significant and high in the patient group in the study by Novelli et al. [19]. The reason for this significant difference could depend on geographical and regional differences, which is also mentioned in the studies done by Nguyen et al. [1] and Correale et al. [30].
According to the results of the study done by Nguyen et al. [1], A*02:02, B*15:03 [11] and C*12:03 from the HLA-A, B and C alleles were very good at binding the Covid-19 peptides, although A*25:01, B*46:01 [11] and C*01:02 alleles were poor at binding the Covid-19 peptides. The conclusion that Covid-19 peptides can be presented mostly by class I HLA alleles was found to be consistent with the results from Wang et al. [26] and our results. In our study, we also found higher Class-I HLA allele frequencies in Covid-19 patients. In addition, Nguyen et al. [1] defined the presence of the HLA-B*46:01 allele in the patient population as a possible risk factor for disease severity [32] and overexpression of the HLA-B*15:03 allele as a protective factor [11,33]. Although these results were similar in terms of the B*15 allele as a protective factor in our study, they were different in terms of the B*46 allele because we could not detect B*46 allele in either parental allele group. This is likely due to the small size of our study population.
According to the study conducted by Iturrieta-Zuazo et al. [24], the affinity of Class I HLA (A, B, C) genes to bind Covid-19 peptides in Covid-19 patients was not correlated with clinical severity. Compared with the light or tight binding of the Covid-19 peptides to the HLA alleles, it was found that there was a significantly higher light and tight binding affinity in mild patients (HLA-C, Mild vs Moderate, excluded) compared to moderate and severe patients. According to the results of the study, the relationship of peptide presentation and binding was not associated with disease severity, unlike the results of Nguyen et al [1]. The distribution of alleles between groups was not different, nor was the distribution between groups according to supertype [24].
This result was different according to study results shown by Wang et al. [26] and Nguyen et al. [1]. According to Wang et al. [26], Class I genes are more important than Class II genes in terms of the severity of the disease in Covid-19 patients. According to Iturrieta-Zuazo et al. [24], the higher detection of Class I HLA genes in the mild group may be due to the high heterozygous distribution of alleles in the mild group. Because of the combination of polymorphisms in the HLA system, heterozygous individuals may be more effective in presenting environmental pathogens to immune system cells than homozygous individuals [34].
Lorente et al. [25], in their study of 72 patients (10 non-survivors and 62 survivors) with Covid-19 and a healthy control group, HLA-A*32 allele were high in the healthy control group, and HLA-B*39 and C*16 alleles were be high in the patient group. However, this difference in frequency between the groups was not significant. In addition, in their analysis dividing the severe patient group into two, as survivors and non-survivors, they found HLA-A*11, C*01, and DQB1*04 alleles to be significantly higher in the non-survivor group [25]. In our study, we detected the HLA-A*32 allele only in SG and the DQB1*4 allele only in the patient and HC group, and our results in this respect differed from the results of Lorente et al. [25].
Despite the similarities and differences in the results of all these studies, there is a close connection between the Covid-19 disease and the patient's genetic structure (HLA allele system).