A herpes simplex virus type 1 latency-associated transcript mutant with increased virulence and reduced spontaneous reactivation

The herpes simplex virus type 1 (HSV-1) latency-associated transcript (LAT) gene is essential for efficient spontaneous reactivation of HSV-1 from lat ency. We previously reported that insertion of the LAT promoter and just th e first 1.5 kb of the 8.3-kb EAT gene into an ectopic location in the virus restored wild-type spontaneous reactivation to a LAT null mutant. This mut ant, LAT3.3A (previously designated LAT1.5a), thus showed that the expressi on of just the first 1.5 kb of LAT is sufficient for wild-type spontaneous reactivation. We also showed that in the context of the entire LAT gene, de letion of EAT nucleotides 76 to 447 (LAT mutant dLAT371) had no effect on s pontaneous reactivation or virulence. We report here on a LAT mutant design ated LAT2.9A. This mutant is similar to LAT3.3A, except that the ectopic LA T insert contains the same 371-nucleotide deletion found in dLAT371, We fou nd that LAT2.9A had a significantly reduced rate of spontaneous reactivatio n compared to marker-rescued and wild-type viruses. This was unexpected, si nce the combined results of dLAT371 and LAT3.3A predicted that spontaneous reactivation of LAT2.9A would be wild type. We also found that LAT2.9A was more virulent than wild-type or marker-rescued viruses after ocular infecti on of rabbits. This was unexpected, since LAT null mutants and LAT3.3A have wild-type virulence. These results suggest fur the first time (i) that reg ions past the first 1.5 kb of LAT can compensate for deletions in the first 1.5kb of EAT and may therefore play a role in LAT dependent spontaneous re activation and (ii) that regions of LAT affect viral virulence.