Defective HBsAg secretion was also reported to be associated with substitutions in the MHR of OBIs (5, 13, 17, 26, 27). Some of these substitutions were observed in the OBI sequences selleckchem Tipifarnib studied here, but their negative effect if any on HBsAg phenotype remained unclear. For example, serine at position 126, previously associated with a moderate decrease of HBsAg secretion (5), was present in OBI pattern 1 TW5004-cl3, with no evidence of a secretion defect. Similarly, Q129R was found in four OBI clones irrespective of their HBsAg excretion pattern (pattern 1 clone TW4576-cl3, pattern 2 clone TW0498-cl3, and pattern 3 clones TW6639-cl1 and TW8964-cl1). Moreover, the correction R129Q did not restore efficient HBsAg excretion in TW0498-cl3 (Table 2), and the M88-cl4 excretion pattern was not modified by Q129R (Table 3).

The substitution G145A was reported to impair HBsAg secretion in a genotype A OBI strain, but it showed no obvious effect in an Asian strain in another study (5, 13). This substitution was present in genotype B OBI TW0498-cl3 clone with pattern 2. Similarly, D144E, previously reported to impair HBsAg detection/secretion as mentioned above, was also observed in pattern 1 HK6921-cl3. Together, these data suggest that alteration of HBsAg secretion can be due not only to a single substitution but also more frequently to a combination of amino acid substitutions in different regions of the protein. This is supported by reports showing both positive and negative transcomplementation effect of S mutations on HBsAg secretion inhibition (13, 16, 17).

It might be difficult to evaluate the exact importance of mutations altering HBsAg secretion in the genesis of OBI, since such mutations appear to be rare and strain specific in most cases. In addition, the HBsAg phenotype associated with these mutations was generally examined from individual clones that may not be representative of the variants constituting the quasispecies in infected individuals, as observed for M75T carriage in HK01556 sample (data not shown). In contrast, the presence of P178R in every closely related variant within HK3110 and HK3475 quasispecies strongly supports an association between HBsAg excretion defect and OBI phenotype in these donors. A more complex situation was observed in HK6794 diverse quasispecies reflected by diversity in the patterns of HBsAg phenotype, suggesting that the HBsAg phenotype of only few clones may not truly represent the overall strain phenotype.

At present, the possible pathogenic role of intracellularly retained HBsAg in the development of liver disease Cilengitide in individuals with OBI can be only speculated on. However, impaired secretion of mutated misfolded/unfolded proteins is known to induce ER stress that can affect host cell physiology by activating intracellular transduction pathways (28).