Adenoviruses

Isolated in 1953 in latent form in tonsils and adenoids removed from healthy children

Capsid and Genome Structure

Adenoviruses are nonenveloped – have a protein capsid instead of a lipid membrane

icosohedral arrangement (20 equilateral triangular faces, 12 vertices)
each face has 12 identical hexons and each vertex has 1 penton; each penton is attached a fiber

12 x 20 = 240 hexons; also 12 penton-fiber proteins, so total of 252 proteins in the capsid (capsomers)

Adenovirus is a double-stranded DNA virus; its genome is 36 kb total and is linear

linear DNA forms a loop due to terminal protein (covalently linked to each 5’ end) and terminal base pairing

transcription occurs from both strands and is spiced (splicing was discovered in adenovirus)

mRNAs are polycistronic (many genes from few mRNAs) and many genes have the same 3’ stop codon

not all RNAs are translated – viral associated RNAs (VA-RNAs) control rate of transcription of late mRNAs (capsid proteins), block IFN-a

Classification

There are more than 40 recognized serotypes of human adenovirus, loosely grouped into six subgroups (labeled A-F)

classification originally was based upon ability to agglutinate erythrocytes (hemagglutinin groups)
all subgroups can form tumors in tissue culture, only subgroup A (and B less so) can produce tumors in animals

Life Cycle

adenovirus capsid proteins bind specific cell surface receptors

fiber binds CAR (coccaxi/adenovirus receptor), penton binds integrins (normally interacts with ECM)

internalization occurs by receptor-mediated endocytosis – possibly activates MAPK signal transduction cascade

endosome acidification causes penetration into cytoplasm – blocked by bafilomycin (prevents acidification)

capsid proteins disassemble and uncoated DNA enters nucleus where it is transcribed by cell enzymes

mRNAs are translated in cytoplasm like normal cellular proteins (have 5’ methylated caps, 3’ poly A tails)
viral proteins are translocated into the nucleus, and assembly of new adenovirus capsid occurs here
DNA is added at the end (often incomplete integration, resulting in "defective" particle)

new viral particles lyse the cell; occasionally lysogeny (integration into host chromosome) occurs

Functions of Adenovirus Genes

E = early (before replication, <8 hours); L = late (after replication, >12 hours)

Gene	Function
E1A	regulates transcription of host and viral genes inactivates Rb
E2	viral DNA replication, inactivates p53
E3	counteracts host defense mechanisms
E4, E1B	progression to late phase
L1-L5	capsid proteins

Disease and Pathogenesis

only 45% of infections result in disease, and only fatal to immunocompromised individuals

types 1, 2, and 5 are common endemic forms (usually early in life, respiratory or fecal-oral transmission)
types 3 and 7 produce acute febrile illness in young children
types 4 and 7 produce acute respiratory disease in military recruits
after infection, serotype-specific immunity is long-lasting

pathogenesis does NOT result from viral proteins (as was originally assumed) but from early gene products

human adenovirus do not replicate in mice but cause pneumonia

E3-deficient adenoviruses cause more severe disease (no counteraction of immune response)

probable pathogenic mechanism: immune response (cytotoxic T-cells, TNF-a ) to early genes and endocytosis

virus blocks host defense – mostly by E3

E3-19kDa binds MHC class I antigens in ER, E3-13.7 downregulates EGF receptor, another E3 blocks TNF-a

Adenovirus Vectors for Gene Therapy

advantageous vector for gene therapy (wide range of tissues infected, including non-proliferating cells)

delete E1 to prevent viral replication – add temperature-sensitive E2A (grow only in culture) – delete E4 for extra insurance

however, DNA is not integrated so effect is temporary; also size of foreign gene limited because it must fit into virus

first generation of vectors were pathogenic since they deleted the E3 gene (produced severe inflammatory response)