表面抗原可以不依赖cccDNA单独合成?
[Extracted from Serological Studies into the Natural History of Chronic Hepatitis B
A thesis submitted in fulfilment of the requirements for The degree of Doctor of Medicine
By Tin Quang Nguyen
t Department of Gastroenterology, St Vincent’s Hospital, Melbourne
And
Victorian Infectious Diseases Reference Laboratory
The University of Melbourne
February 2011]
Interest in quantitative HBsAg serology as a clinical biomarker has been based
upon studies which showed a positive association with intrahepatic HBV cccDNA
levels[103, 162, 167] and serum HBV DNA[103, 168]. Currently, HBVDNA quantification is
the gold standard in selecting patients who are potential candidates for therapy,
monitoring response to therapy, and detecting the emergence ofdrug resistance.
Compared to HBV DNA, the assays for HBsAg quantification are far less expensive,
and are fully automated with a high throughput capacity. However,the utility of
HBsAg titres as a reliable surrogate for both HBV cccDNA and HBVDNA remains
unclear, as other studies have also shown a poor correlation with HBV cccDNA[169],
and only a positive correlation with HBV DNA in HBeAg positive CHB in our
collaborative group[164].
An understanding of HBsAg titre changes throughout HBV infection may
provide some potentially useful insights into hepatitis B pathogenesis and viral life
cycle. The mechanisms linking HBsAg and viral replication during different phases of
CHB are currently unclear. This study observed a modest correlation of serum HBsAg
with HBV DNA in the IC phase of CHB (r = 0.77, p=0.0001). No correlation was
observed in the IT, LR or ENH phases. Furthermore, the ratio of HBsAg to HBV
DNA was significantly higher in the low replicative phase compared to all other
phases (1.09 vs 0.55, 0.55, 0.69. p<0.0001), a finding which is in accordance with
previous studies[162]. The apparent “disconnect” between HBsAg and HBV DNA at
different phases may possibly be due to the expression of HBsAg from integrated
viral envelope sequences, instead of HBsAg production off mRNA derived from the
HBV cccDNA template. A second possible explanation is a difference in the immune
regulation of viral replication during different phases of infection, resulting in altered
ratios of HBV virion to sub-viral HBsAg particles[31].
HBsAg synthesis during the HBV viral life cycle is complex, and typically
occurs at the endoplasmic reticulum (ER) (Figure1.3 and 3.11). The envelope open
reading frame (ORF) contains three in-frame “start” codons which sub-divide it into
preS1, preS2, and S domains. Envelope proteins are generated from two HBV mRNA
transcripts, with subsequent translation resulting in production of the small (S),
medium (Pre-S2+ S) and large surface envelope proteins(Pre-S1+Pre-S2+S); these
are also known as S, M and L surface proteins, respectively.
Figure 3.11. The two separate pathways of HBsAg and HBVDNA production.. RC-DNA, relaxed
circular DNA; DSL DNA, double stranded linear DNA; cccDNA, covalently closed circular DNA;
mRNA, messenger RNA; HBeAg, hepatitis B e antigen; HBsAg, hepatitis B surface antigen.
Newly synthesized envelope protein interacts with mature HBV nucleocapsids
at the ER prior to secretion from the hepatocyte. However, HBsAg production far
exceeds that required for virion assembly, and excess surface envelope proteins are
secreted as non infectious filamentous or spherical subviral particles[31]. These subviral
particles may play a role in evading or subverting the host immune response[84], and
may also co-exist with anti-HBs as part of circulating immune complexes (see
Chapter 5 of thesis)[170]. It is important to appreciate that whilst HBsAg quantification
detects all three forms of systemic HBsAg (part of HBV virion, spherical,
filamentous), differentiation between the relative proportions is currently technically
demanding, and not routinely performed, requiring either density gradient
[attach]249624[/attach]
Uncoating脱壳
ER内质网
Mature Nucleocapsid成熟核衣壳
Immature Nucleocapsid未成熟的核衣壳
Nuclear Transport核运输
RC-DNA TranscriptionRC - DNA转录
viral RNA
HBV DNA
Pathway途径
GOLGI高尔基体(Golgi apparatus)
Translation翻译
DSL- DNA
HBsAg乙肝表面抗原
Pre-S truncation前S截断
Viral Integration病毒集成
Spherical & Filamentous HBsAg球面及丝状乙肝表面抗原
Mature HBV virion成熟的乙肝病毒病毒粒子
Viral Integration Pathway病毒的整合途径
DSL-DNA
Reverse Transcription转录
HBsAg Pathway乙肝表面抗原
centrifugation or non-denaturing gel electrophoresis for separation and immunoblot
for initial detection and subsequent clarification.
HBsAg may also be produced from HBV DNA integrated into the host
genome. Although viral integration is an essential component of the life cycle of
retroviruses such as HIV, it is not required for normal productive hepadnaviral
infection. Rather, integration of HBV DNA occurs illegitimately through
recombination mechanisms using host enzymes such as topoisomerases acting on the
double-stranded linear (DSL) HBV DNA (Figure1.3 and 3.11)[32, 33]. In HBV
infection, viral integration does seem to occur early in infection. Whilst HBV
integration is believed to be a random event, a high preference for integration occurs
at the direct repeat 1 (DR1) and DR2 sequences on the HBV genome[34]. Such
integrated sequences cannot provide a template for productive viral replication as a
complete genome is typically not present[36]. However, given that sequences of the S
genes of the Enh I elements are often present in integrated segments, HBsAg may be
produced[36].
The phase of CHB is currently determined by three main factors;HBeAg/anti-
HBe status, HBV DNA titre and serum ALT level. This study demonstrated that
HBsAg titres change during the natural history of CHB, and suggests that there may
be HBsAg titre “set-points” within each phase. Further evaluation of baseline HBsAg
titres in other cohorts of patients with CHB are required to confirm the findings of this
study, and may help refine the current definition of the different phases of CHB.
The status of a patient’s HBeAg/anti-HBe, HBV DNA and serum ALT are
also the parameters which are currently used to assess the response to antiviral
therapy. Sustained suppression of HBV replication as assessed by HBV DNA
measurement currently represents the cornerstone of evaluation of antiviral efficacy.
In the absence of HBsAg loss, long-term therapy with potent oral NAs is required to
maintain effective suppression of HBV DNA. Thus, there is now a paradigm shift
towards striving to achieve HBsAg loss and/or seroconversion. HBsAg loss is
believed to be associated with both successful immunological control of HBV and
durable suppression of viral replication, and consequently may represent an indication
to cease oral NA therapy. Evaluation of HBsAg titres may allow determination of
baseline levels which may be more predictive of HBsAg loss.Furthermore,
assessment of on-treatment changes in HBsAg titres may facilitate new algorithms
and future trials which are aimed at achieving this important endpoint.
StephenW 发表于 2011-11-2 07:09
表面抗原可以不依赖cccDNA单独合成?[Extracted from Serological Studies into the Natural History of Chr ...
欢迎光临 肝胆相照论坛 (http://hbvhbv.info/forum/) | Powered by Discuz! X1.5 |