Hepatitis B virus (HBV) is a small, non-cytopathic virus with a circular partially double-strand DNA genome of approximately 3.2 Kb [1]. The genome has 4 overlapping genes: PreS/S, PreC/C, X, and P. The Pre S/S gene encodes for the three envelope proteins, large, middle, and small or HBsAg. The PreC/C gene encodes for the nucleocapsid protein and HBeAg. The X gene encodes for the transactivating protein X and the P gene encodes for polymerase with reverse transcriptase (RT) and RNase H activity [1]. The HBsAg contains the major epitopes for induction of a protective humoral immune response [2-6]. These epitopes are localized in the region known as the "a" determinant, between amino acid residues 99 and 169 [7-10], and are involved in the binding of antibodies against HBsAg. Amino acids changes in this region render mutant strains able to escape immune responses induced by vaccines [5,6,11,12]. Immune-escape mutants occur naturally [13-15], and in strains from patients with a weak or negative HBsAg reactivity in detection assays [10,15].
HBV strains are classified into 8 main genomic groups or genotypes, designated A through H [16-18], arbitrarily defined by an intergroup divergence of more than 8% based on the complete genomes [16,19-21]. The relationship between subtypes and genotypes has been reported, and several studies have described the geographic distribution for the different HBV subtypes/genotypes [16,17,20,22-25]. The study of HBV genotype has recently been focused on the clinical outcome of the infection, since studies suggest a possible association with the natural history and severity of the infection [26].
In Mexico, frequency of HBV chronic carriers is low, and the prevalence of HBsAg in blood donors ranges from 0.15 to 1.4% [27,28]; the frequency of hepatocellular carcinoma is also low [29]. Scarce studies have addressed genetic diversity and amino acids changes in HBsAg of HBV strains from Mexico [30]. The aim of the present work was to study genetic diversity in the S gene of HBV strains from asymptomatic and symptomatic carriers in Mexico. Genotype and mutations in the "a" determinant of the S gene leading to amino acids changes were compared between these two groups.
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