PCR DETECTION OF AMPLIFIED 132 BP REPEATS IN MAREKS-DISEASE VIRUS TYPE-1 (MDV-1) DNA CAN SERVE AS AN INDICATOR FOR CRITICAL GENOMIC REARRANGEMENT LEADING TO THE ATTENUATION OF VIRUS VIRULENCE

A radioactive PCR test was developed that amplified the very virulent Marek's disease virus-1 (vvMDV-1) DNA sequence containing the 132 bp r epeats. In apathogenic MDV-1 (CVI 988, Rispens), amplified DNA bands c ontaining multiple copies of 132 bp repeats were identified. In the pr esent study this PCR technique was used to monitor the passage level o f vvMDV-1 in chicken embryo fibroblasts (CEF) in which the number of t andem 132 bp repeats was increased. It was found that at passage level 32 of vvMDV-1-B isolate, the 132 bp tandem repeat was already markedl y amplified and its pattern resembled that of the MDV-1 (CVI 988, Risp ens) vaccine virus DNA. In the vvMDV-1Z strain, amplification of the 1 32 bp repeat was not detectable at a similar passage level. The PCR te st demonstrated that the apathogenic MDV-1 Md11/75c virus developed by extensive in vitro passaging has amplified 132 bp DNA repeats similar to those of the commercial vaccine virus (CVI 988, Rispense). It was also found that the pattern of viral RNA from infected cells detectabl e by Northern blot hybridization was markedly changed from a 2.4 kb RN A species in cells infected with vvMDV-1 viruses, to four RNA species (ranging from 2.2 to 4.4 kb) in cells infected with passage 32 of MDV- 1-B strain, to a very large number of undefined RNA species synthesize d in cells infected with attenuated MDV-1 viruses (CVI 988, Rispens an d Md 11/75c).