EXPANSION OF A UNIQUE REGION IN THE MAREK'S DISEASE VIRUS GENOME OCCURS CONCOMITANTLY WITH ATTENUATION BUT IS NOT SUFFICIENT TO CAUSE ATTENUATION

Authors: Silva, Robert; REDDY, SANJAY - TEXAS A&M UNIVERSITY; LUPIANI, BLANCA - TEXAS A&M UNIVERSITY

Submitted to: Journal of Virology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/23/2003
Publication Date: 1/1/2004
Citation: Silva, R.F., Reddy, S., Lupiani, B. 2004. Expansion of a unique region in the Marek's disease virus genome occurs concomitantly with attenuation but is not sufficient to cause attenuation. Journal of Virology. 78(2):733-740.

Interpretive Summary: Marek's disease (MD) is a virus induced cancer-like disease of chickens that continues to be a major problem for the poultry industry. The causative agent is the Marek's disease virus (MDV). We have known for many years that we can generate a non-disease causing MDV by growing the virus in the laboratory. At the same time as the virus is being grown in the laboratory, portions of the MDV changes (mutates). The prevailing hypothesis was that these changes in the MDV are responsible for causing the MDV to become non-disease inducing. If this hypothesis is true, then we should be able to artificially manipulate MDVs, and thereby turn ordinary MDVs into non-disease producing viruses that could possibly be used as vaccines. To test this hypothesis, we used molecular techniques to generate the same changes that we see when MDV is grown in the laboratory. When inoculated into chickens, these altered MDVs still induced disease. Thus, the changes that we see when MDV is grown in the laboratory, is not sufficient in of itself to produce non-disease causing MDVs. The information obtained from this research is of great importance to scientists in industry and academia as it now lays to rest a long held hypothesis concerning how non-disease causing MDVs are created.

Technical Abstract: Pathogenic Marek's Disease viruses (MDVs) have two head-to-tail copies of a 132 base pair (bp) repeat. As MDV is serially passed in cell culture, the virus becomes attenuated and the number of copies of the 132 bp repeat increases from two to more than 20 copies. To determine the role of the repeats in attenuation, we used five overlapping cosmid clones that spanned the MDV genome to reconstitute infectious virus (rMd5). By mutating the appropriate cosmids, we generated clones of infectious MDVs that contained zero copies of the 132 bp repeats, rMd5(del 132), 9 copies of the 132 bp repeats, rMd5(9-132), and 9 copies of the 132 bp repeats inserted in the reverse orientation, rMd5(rev 9-132). After two passages in cell culture, wild-type Md5, rMd5, and rMd5(del 132) were stable. However, rMd5(9-132) and rMd5(rev 9-132) contained a population of viruses that contained from 3 to over 20 copies of the repeats. A major 1.8 kb mRNA, containing two copies of the 132 bp repeat, was present in wild-type Md5 and rMd5, but was not present in rMd5(del 132), rMd5(9-132), rMd5(rev 9-132) or an attenuated MDV. Instead, the RNAs transcribed from the 132 bp repeat region in rMd5(9-132), and rMd5(rev 9-132) closely resembled the pattern of RNAs transcribed in attenuated MDVs. When inoculated into susceptible day-old chicks, all viruses produced various lesions. Thus, expansion of the number of copies of 132 bp repeats, that accompanies attenuation, is not sufficient in of itself to attenuate pathogenic MDVs.