Hepatitis B, an infectious disease with a high rate of incidence in Asian populations [1], is a major cause of CHB, liver failure, liver cirrhosis, and HCC development, diseases which often result in death [19, 20]. Although the mechanisms underlying the different clinical results of HBV infection have not been fully understood, previous studies have linked a diverse range of factors such as viral strain, gender, age of infection, host immune system, and genetic information of the host, with risk of CHB [21]. When viral infection occurs, several immune-related genes are activated, leading to disease outbreak. According to a GWAS conducted on a Chinese population, a CFB genetic variant had significant association with risk of CHB [10]. As such, this association study was conducted to investigate the genetic effects of CFB genetic variants on CHB susceptibility within a Korean population.
The complement system is composed of over 30 plasma proteins and is activated by microbes or antibodies which attached to microbes or other antigens [22, 23]. This system is an innate immune system that helps operate rapid responses against pathogenic invasions by opsonizing or recruiting inflammatory cells or pathogen lysis [24]. The complement activations occur through three pathways: the classical pathway, the lectin pathway and the alternative pathway. These pathways are worked through a cascade of enzymes reaction [23, 25]. CFB is essential to activate the complement system, particularly the alternative pathway that is against microbe invasion which includes viruses [26].
Previous Chinese studies have identified CFB genetic variants which have genetic effect on CHB risk. The most significant association was identified at rs12614 of CFB (P = 1.28 × 10− 34 − 4.0 × 10− 3) [10, 11]. In this study, rs12614 showed the same direction of genetic effect as found in previous Chinese studies. Moreover, CFB rs12614 was significantly associated with risk of CHB in the HCC (-) CHB and the HCC (+) CHB groups (P = 6.60 × 10− 8 and 3.10 × 10− 6, respectively). However, there was no significant genetic effect on CHB-related HCC progression. Additionally, the rs12614 C > T T allele was more frequently observed in the PC group than the CHB patients with a significance (OR = 0.43, Pcorr = 2.36 × 10− 8). Considering that individuals with the non-synonymous variant (rs12614 T allele) had significantly higher CFB expression than those with the rs12614 C allele in the Chinese study, it can be seen that the rs12614 may affect immune response by influencing the complement system when viral infection occurs [10].
The rs12614 which is located on coding region of CFB, C to T allele change causes the amino acid change, arginine to tryptophan. To predict the effects of the rs12614 amino acid change, we conducted in silico analysis using the PolyPhen-2 program (http://genetics.bwh.harvard.edu/pph2/index.shtml) [27]. The results predicted this amino acid change is predicted to be probably damaging that means this substitution is predicted to be damaging with high confidence (Supplementary Fig. 3A, [27]). In addition, amino acid alignment from the program, arginine at position 32 is highly conserved among species (Supplementary Fig. 3B). As disease-causing substitutions are more likely to occur at positions that is conserved throughout evolution [28], the rs12614 C to T allele substitution may affect CFB functions. Because the alternative pathway is important to against pathogen invasion, an amino acid change in CFB important in the alternative pathway may affect the immune system to against hepatitis B virus invasion.
Some individuals are more susceptible to diseases while others are less susceptible. Identification of the genetic background is key to understand differences in individuals’ disease susceptibility, and that can potentially lead to the targeting of preventive measures at those who are at greatest risk [29]. The results of the conditional analysis conducted on the 10 previously identified markers indicated that rs12614 can be used as a novel causal variant of CHB susceptibility. To elucidate its cumulative genetic effects, we used odd ratios of rs12614 and previously identified 10 CHB markers. Consequently, CHB group showed higher GRSs than the PC group and the higher genetic risk scores range indicated higher odds ratios. These implies CHB patients are more likely to have higher scores than controls.
There is a sampling limitation in this study. While the ideal subjects for the control groups would be the people who are HBsAg (-) and anti-HBc (+) (spontaneously cleared), we used population controls with unknown responses to HBV infection. And some individuals in the control group still have a chance of progression to CHB when exposed to HBV. Although using the population controls in a case-control study may reduce statistical power, it is useful when it is difficult to obtain a sufficient number of disease controls.