| Abstract |
Hepatitis B virus infection in childhood remains a consirable health problem worldwide,
even after the introduction of passive and active immunoprophylaxis for children born to
HBV positive mothers. The great majority (90%) of newborns that will acquire HBV
during their perinatal period and a significant percentage (30%) of children that will acquire
HBV infection during their first 6 years of life will develop chronic infection, compared to
a lesser degree (10%) of adults. The risk of HBV transmission from an HBsAg positive
mother to her infant is associated with different factors including the timing of exposure,
the maternal HBeAg carriage associated with significant infectivity and the viral load at
birth.
The contribution of genetic factors to the outcome of ΗΒV infection is shown in recent
genetic association studies. Mannose binding lectin (MBL) is an innate immunity factor
known to activate the complement after binding to HBV surface glycoproteins and to elicit
viral opsonisation. Variants in mbl2 gene, including the single point mutations in exon 1
(codons 54 and 57) and at position 221 of the promoter reduce plasma MBL levels. MBL
deficiency has been related to increased susceptibility to infections in infants and poor
outcome of hepatitis B and C in adults.
Tumor necrosis factor alpha (TNF-α) is also an efficient factor of HBV clearance, able to
disrupt viral particles, transcriptional templates and episomal covalently closed circular
DNA. Plasma TNF-α level is modified by a variant at position −308 of its gene. Vitamin D
affects viral presentation by suppressing antigen presentation and effector T cells activation
in a vitamin D receptor (VDR) transcription-dependent way. Low level of viral antigens,
expressed by antigen presenting cells, are unable to induce the CD4 T cells but sufficient to
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activate cytotoxic T cells, which mediate liver injury without clearing HBV. Two variants
in the VDR gene, ApaI A/a at intron 8 and TaqI T/t at exon 9, have already been related to
the outcome of hepatitis B in adults.
The effect of the mbl2 (Cdn54 GGC to GAC, Cdn57 GGA to GAA, -221 G to C), TNF-α (-
308 A to G) and VDR gene (ApaI G toT, TaqI T to C) variants on the acquisition of HBV
infection and the outcome of infection (progress to chronicity or clearance) was studied on
102 children and adolescents (53 male and 49 female), aged 0.5 to 18.5 years (mean age
9.17 years), who were born to HBV positive mothers and had not received
immunoprophylaxis. The children were assessed for HBV serology and those found to be
HBsAg positive were retested later for potential HBsAg seroconversion. The group of
chronic HBV infection (presence of HbsAg for more than 6 months) consisted of 33
HBsAg positive children, 14 male and 19 female, aged 3.0 to 18.5 (mean 11.4) years. The
group of spontaneous HBV clearance (disappearance of HbsAg and the development of
anti-Hbs antibodies) consisted of 36 children, 17 male and 19 female, aged 0.5 to 18 (mean
9.47) years and the group of naïve subjects consisted of 33 children, 22 male and 11
female, aged 1.0 to 14 (mean 6.58) years. Initially genomic DNA was isolated from whole
blood, followed by polymerase chain reaction (PCR), restriction fragment length
polymorphism (RFLP) methods and electrophoresis in agarose gels. MBL serum levels
were determined in 28, 32, and 16 children with HBV chronic infection, clearance and
naivety, respectively by indirect enzyme-linked immunosorbent assay (ELISA). MBL
levels were defined as low when΄&λτ500 and normal when΄&γτ500 ng/ml.
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The allele, genotype, haplotype frequencies and MBL levels were compared among the
groups with Fisher’s exact probability test and risk ratios (RR) and 95% confidence
intervals (CI) were calculated. The following differences were observed:
Females were at higher risk of acquiring HBV infection than males (p=0.032), but not at a
different risk of progressing to chronicity (p=0.44).
The prevalence of mutant alleles in codons 54 and 57 and at position -221 and the
distribution of mbl2 haplotypes did not differ among the study groups. Low serum MBL
values were found in 13.2% of the whole cohort, in 10.7% of children with chronic
infection, 15.6% of children with HBV clearance and 12.5% of HBV naïve subjects. MBL
deficiency as defined by mbl2 genotypes or MBL serum levels was not shown to have
significant impact in early HBV infection.
TNF-alpha-308 G allele was found more frequently in children with chronic infection than
in HBV naïve children (p=0.050) and in infected than non-infected children, though not at a
significant level (p 0.089).
VDR ApaI A allele (as compared to a allele) was more frequent, though at a marginal level
of significance, in children with chronic than in children with resolved infection (p=0.071).
On the other hand, VDR ApaI α allele in VDR ApaI and TaqI joint haplotype αΤ (as
compared to At and AT haplotypes) was more frequent in children with resolved than in
children with chronic infection (p=0.049). TaqI and ApaI markers were found to be in
linkage disequilibrium, consistently with the findings of previous studies. Three joint
genotypes (AAtt, AaTt, aaTT) were more common exhibiting frequencies of 22.5%, 39.2%
and 17.6% compared to the less common genotypes AATt, AATT, AaTT (9.8%, 2.9% and
7.8%, respectively).
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Our findings suggest that TNF-alpha and possibly vitamin D pathways may be involved in
the progress of early HBV infection. The present study as all genetic association studies
provide a better understanding of HBV infection pathogenesis early in life. Future global
multicohort studies that will include multiple genetic variations, haplotypes and clinical
data are required for the clarification of HBV immunogenetics. Polymorphisms may serve
as genetic markers and facilitate the estimation of long-term outcome. Genetic studies raise
hopes of the emergence of automated, rapid and affordable tools in the diagnostics and
management of hepatitis B.
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Εγγενής ανοσία και κλινική έκβαση της λοίμωξης από τον ιό της ηπατίτιδας Β στα παιδιά
