Use of a Cosmid Recombination System in Mutational Analysis of Herpes Simplex Virus Type 1 Genes UL14 to UL17

Harris, Wendy J (1996) Use of a Cosmid Recombination System in Mutational Analysis of Herpes Simplex Virus Type 1 Genes UL14 to UL17. PhD thesis, University of Glasgow.

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The HSV-1 genome is 152 kbp in length and contains at least 74 distinct genes. About half of the genes are individually not essential for viral growth in cell culture but most appear to be required for efficient viral replication and pathogenesis in experimental animal systems. The availability of the complete DNA sequence of HSV-1 since 1988 has made it possible to target any gene for mutation, including those that had been previously inaccessible to conventional genetic analysis. A cosmid-based recombination system for producing HSV-1 mutants has been developed recently. The inserts from a set of five cosmids, each about 40 kbp in size, which together contain the entire HSV-1 genome, are co-transfected into BHK C13 cells to yield wt virus via recombination processes. One cosmid containing a gene of interest can be mutated, and replacement of the wt cosmid in the set by the mutated cosmid results in mutant progeny after co-transfection. This approach has a number of advantages over traditional methods of mutagenesis, particularly in allowing minimal alterations to be introduced and in permitting the generation of mutants in the absence of a wt background. The aim of this project was to expand the use the cosmid-based system of mutagenesis to analyse HSV-1 genes UL14, UL15, UL16 and UL17 and to investigate the phenotypes of mutants obtained. The technique used takes advantage of the observation that digestion of covalently closed DNA molecules in the presence of ethidium bromide by restriction enzymes with multiple recognition sites preferentially yields full length linear molecules after a single site is cleaved. An appropriate cosmid (cos24) was linearised with the restriction enzyme Asp718, which cleaves at the sequence G'GTACC, leaving a 5' overhang. Treatment with T4 DNA polymerase in the presence of the four deoxynucleotide triphosphates, followed by religation, resulted in a 4 bp insertion, thus altering the reading frame of the gene from that point onwards. Asp718 has seven sites in cos24, and mutant cosmids were made with a 4 bp insertion in UL14, ULl5 and UL17, in addition to UL6 and UL7. Since these genes may be essential in cell culture, a number of candidate complementing cell lines were produced by three different strategies. These were generated using either cos24 (containing genes ULl to UL21), a plasmid containing genes UL14 to ULl8, or plasmids containing UL14, UL15 or UL17 individually. Overall, 122 candidate cell lines were produced, and 40 were tested to see if they could complement the mutation by transfection with the appropriate mutant cosmid sets. BHK Cl3 cells were also transfected. Following transfection, all of the progeny generated from these cell lines and BHK C13 cells were wt revertants, presumably owing to loss of the 4 bp insertion by intramolecular recombination. This indicates that the cell lines tested are not able to complement the defects and that UL14, UL15, and UL17 are essential for viral replication in cell culture. Proteins whose sequences are present in databases may be identified by newly developed techniques involving mass spectrometry. Proteins are separated by SDS-PAGE and electroblotted onto a membrane, then proteins of interest are excised and treated with trypsin. The peptides produced are subjected to laser desorption mass spectrometry, and the resulting masses from the spectrum are compared to the predicted tryptic products from proteins in a database. Using this method, evidence was obtained that the protein products of UL17 and UL14 may be present in small amounts in HSV-1 virions. Two independent UL16' viruses were available in the laboratory at the commencement of this work, and a further four independent UL16' mutants were also produced. These six mutants were characterised in cell culture and the effects of the lesions on neurovirulence and latency were investigated using a mouse model. The mutants produce smaller plaques on BHK C13 cells and yield less virus than wt and a revertant. The small plaque size and low yield were dependent on the particular site of the mutation within UL16. The mutants were able to adsorb as efficiently as wt, but were slightly reduced in their ability to bind and penetrate cell, they were not significantly temperature sensitive or cell type-specific. Particle counts by electron microscopy revealed that they have a high particle to plaque forming unit (pfu) ratio (i.e. they produce the same amount of particles but fewer are infectious). Analysis of the mutants was carried out using a mouse model. After inoculation of virus into the cranium, wt virus produced from the parental cosmid set was fully neurovirulent but the UL16' viruses were less virulent than wt (i.e. they killed fewer animals at each dose). Footpad inoculations were carried out to investigate the ability of the mutants to replicate at the periphery and to establish and reactivate from latency. The mutants were able to establish and reactivate from latent infection, but less efficiently than wt. In conclusion, although UL16 mutants are viable in cell culture they have a high particle to pfu ratio, yield less virus than wt and produce smaller plaques. The results from experiments involving infection of mice indicate that the UL16 protein has a role in neurovirulence and latency.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: Andrew J Davison
Keywords: Virology, Genetics
Date of Award: 1996
Depositing User: Enlighten Team
Unique ID: glathesis:1996-75591
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 19 Nov 2019 19:21
Last Modified: 19 Nov 2019 19:21

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