A Herpes Simplex Virus Type 1 Latency-Associated Transcript Mutant with Increased Virulence and Reduced Spontaneous Reactivation

Abstract

The herpes simplex virus type 1 (HSV-1) latency-associated transcript (LAT) gene is essential for efficient spontaneous reactivation of HSV-1 from latency. We previously reported that insertion of the LAT promoter and just the first 1.5 kb of the 8.3-kb LAT gene into an ectopic location in the virus restored wild-type spontaneous reactivation to a LAT null mutant. This mutant, LAT3.3A (previously designated LAT1.5a), thus showed that the expression 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, deletion of LAT nucleotides 76 to 447 (LAT mutant dLAT371) had no effect on spontaneous reactivation or virulence. We report here on a LAT mutant designated LAT2.9A. This mutant is similar to LAT3.3A, except that the ectopic LAT insert contains the same 371nucleotide deletion found in dLAT371. We found that LAT2.9A had a significantly reduced rate of spontaneous reactivation compared to marker-rescued and wild-type viruses. This was unexpected, since 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 infection of rabbits. This was unexpected, since LAT null mutants and LAT3.3A have wild-type virulence. These results suggest for the first time (i) that regions past the first 1.5 kb of LAT can compensate for deletions in the first 1.5kb of LAT and may therefore play a role in LAT dependent spontaneous reactivation and (ii) that regions of LAT affect viral virulence.

Herpes simplex virus type 1 mutants containing the KOS strain ICP34.5 gene in place of the McKrae ICP34.5 gene have McKrae-like spontaneous reactivation but non-McKrae-like virulence

“…Following ocular infection of rabbits with 2×10 5 p.f.u. per eye of the McKrae strain of HSV-1, approximately 50% of the animals die (Perng et al ., 1994 , 1995 , 1999 ). In contrast, ocular infection with the KOS(M) strain of HSV-1 does not result in the death of any rabbits (Hill et al ., 1987 ; S. L. Wechsler, unpublished results).…”

Bovine Herpes Virus 1 (BHV-1) and Herpes Simplex Virus Type 1 (HSV-1) Promote Survival of Latently Infected Sensory Neurons, in Part by Inhibiting Apoptosis

“…The McKrae strain of wild-type HSV-1, however not a LAT null mutant, is consistently detected in tears of infected rabbits, due to spontaneous reactivation. 139,142–145 These same LAT null mutants grow with wild-type efficiency in cultured cells and in acutely infected rabbits. When just the first 1.5 kb of LAT coding sequences (Fig.…”

Multiple Applications For Replication‐Defective Herpes Simplex Virus Vectors

“…The functions of the LATs remain unknown, although several putative roles have been suggested. These include: efficient establishment of latency [44, 45]; effective reactivation from latency [46‐52]; antisense regulation of IE gene transcripts [53‐55]; prevention of apoptosis in infected neurons [56]; expression of proteins [57] that may compensate for the absence of IE gene expression during latency [58], and functions relating to RNA‐mediated catalysis [59]. However, it is clear that the LAT genes are not an absolute requirement for establishment, maintenance, or reactivation from latency [60‐63].…”