Adenovirus (Molecular Biology)

The adenoviruses constitute a large group of DNA viruses, the Adenoviridae family. Genera include Mastadenovirus (human, simian, bovine, equine, porcine, ovine, canine, and opossum) and Aviadenovirus (birds). Adenoviruses are nonenveloped icosahedral particles of 70 to 100 nm in diameter, with 252 capsomeres, of which 240 are hexons and 12 are penton bases. A projecting fiber attaches to each penton base to form a penton.

Human adenoviruses (Ads) include 47 serotypes, which can be classified into six subgroups, A to F, based on their ability for red-blood-cell agglutination and oncogenicity in rodents, plus their DNA homology. Ads cause respiratory tract infections, conjunctivitis, hemorrhagic cystitis, and gastroenteritis. Highly oncogenic group A (eg, Ad12 and Ad18), weakly oncogenic group B (eg, Ad3 and Ad7), and Ad9 of group D can induce tumors in rodents, but no conclusive evidence has been reported linking adenoviruses with malignant diseases in the human. All Ads that have been tested can transform cultured rodent cells. No infectious virus is present, but viral transforming genes (E1A and E1B) are present and expressed in tumors and in transformed cells induced by adenoviruses.

The viral genome consists of a single linear molecule of double-stranded DNA [MW ~23 x 106, size ~36 kilobase pairs (kbp)], varying somewhat with the type. Human adenovirus type 2 (Ad2), the first to be sequenced completely, has a total of 35,937 bp. The Ad genome has inverted terminal repeats of 103 bp to 163 bp, depending on the type, and two identical replication origins, one in each terminal repeat. A 55-kDa terminal protein (TP) covalently bound to each 5′ end of the viral DNA molecule serves as a primer for protein-primed viral DNA replication. The viral genome carries five early transcription units (E1A, E1B, E2, E3, E4), two delayed early transcription units (IX and IVa2), and one late transcription unit (major late), all of which are transcribed by RNA polymerase II. The viral genome also carries one or two (depending on the type) VA genes transcribed by RNA polymerase III. The E1A, E1B, IX, major late, VA, and E3 are on the rightward reading DNA strands (r strand), and others are on the leftward reading DNA strand (l strand).

The E1A is the first transcription unit to be expressed shortly after infection, using cellular transcription factors, encoding two major mRNAs of 12S and 13S. The encoded E1A proteins are required for productive viral replication and play a key role in cell transformation. E1A proteins transactivate early and late viral genes and cellular transcription units, and they bind cellular proteins, including p300, a family member of CBP (CREB binding protein)/p300, plus the RB family members, RB (retinoblastoma susceptibility gene product, tumor suppressor), p107, and p130. Binding to these cellular proteins is required for cell transformation. Binding of E1A to RB disrupts the RB • E2F complex to activate a cellular transcription factor, E2F, resulting in the activation of E2F-dependent cell-cycle-related genes.

Two major proteins of 19 kDa and 55 kDa, encoded by 13S and 22S messenger RNAs, respectively, are generated from the E1B region. The E1B proteins are required for efficient viral growth and cooperate with E1A products to transform rodent cells, preventing E1A-induced apoptosis. The E1B 19-kDa protein has a functional similarity to the cellular anti-apoptotic Bcl-2, and the E1B 55-kDa protein interacts with tumor suppressor and apoptosis-related p53.

Region E2 encodes proteins involved in viral replication, including the viral terminal protein precursor, pTP (coded by E2B), viral DNA polymerase (coded by E2B), and the DNA-binding protein (coded by E2A). None of the E3 proteins are required for productive infections of adenoviruses in cultured cells, but are important for in vivo infections in humans, suppressing host defense mechanisms by cytotoxic T lymphocytes or tumor necrosis factor-a. The E4 proteins are involved in transcriptional regulation, preferential transport of viral mRNA, and efficient viral DNA replication. Other functions of the E4 gene, such as transformation with E1A, tumor-suppressor p53 binding, and apoptosis-inhibiting activities, have been recently reported. The Ad9 E4 gene can oncogenically transform rat cells in the absence of the E1A and E1B genes and is required for mammary tumorigenesis.

The transcription of adenovirus late genes is triggered by the onset of viral DNA replication, yielding at least 18 distinct mRNAs by differential poly(A) site utilization and alternative splicing. Viral DNA replication activates a major late promoter (MLP) located at 16.4 map units of the viral genome, and one large primary transcript is generated, terminating at 99 map units at the right end of the genome. This transcript is processed to five families of late mRNAs, L1 to L5, based on the use of common poly(A) addition sites. The late mRNAs encode structural polypeptides of the viral particle and proteins involved in polypeptide processing, capsomere assembly, and packaging of the viral genomic DNA.

Adenoviruses are being used as vectors for delivery of therapeutic genes to target organs in vivo. Recombinant adenoviruses can be constructed by replacing the E1A and E1B genes with foreign ones. The E3 region also can be deleted without significant changes in virus growth. Recombinant adenoviruses can propagate efficiently in 293 cells, which complement the defect. The advantages are that the growth of recombinant viruses does not require host cell division, and a high titer of recombinant viruses can be easily obtained.

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