Dr. Aiyar received his Bachelor’s degree in Life Sciences & Biochemistry from St. Xavier’s College (Bombay, India) in 1987.  He completed his Ph.D. in Biochemistry (1994) at Case Western Reserve University working on the replication of oncogenic retroviruses in the laboratory of Jonathan Leis.  He did his post-doctoral fellowship with Bill Sugden at the McArdle Laboratory for Cancer Research working on the replication of Epstein-Barr virus.  He joined the faculty of the Feinberg School of Medicine in 1999.

Research Description

Latent infection by Epstein-Barr virus (EBV) is associated with several human diseases ranging from infectious mononucleosis to several lymphomas and carcinomas.  In all these latently infected cells, EBV genomes share two characteristics with cellular chromosomes – they are replicated once per cell-cycle, and are partitioned equally into both daughter cells.  Our laboratory works on the mechanism by which Epstein-Barr viral genomes are replicated and partitioned during latent infection.

Research Abstract

EBV is a human herpesvirus that efficiently immortalizes the cells that it infects.  Within these proliferating immortalized cells, EBV is latent, so that no infectious virus is released, and very few viral genes are expressed.  During latency, EBV’s genome is resident as a large circular plasmid in the nucleus of infected cells.  This plasmid is replicated once per cell-cycle in synchrony with cellular chromosomes, and is partitioned equally to daughter cells at mitosis.  Both these processes require a single viral cis-element, termed oriP, and a single viral protein, the Epstein-Barr nuclear antigen 1 (EBNA1).  The major focus of our work is to understand the mechanism by which EBNA1 functions, and its contributions to the immortalization of primary cells by EBV.

OriP contains two clusters of binding sites for EBNA1, termed the dyad symmetry element (DS) and the family of repeats (FR) (see Figure 1).  EBNA1 binds each site within DS and FR as a dimer.  DS functions as an origin of replication within oriP.  Work from the laboratories of Schepers, Yates and Dutta has demonstrated that EBNA1 bound to DS recruits the human origin recognition complex to DS, although the mechanism by which this occurs remains to be elucidated.  EBNA1 bound to FR maintains replicated genomes within nuclei, and it partitions them into daughter cells during mitosis.

This domain, that resides in the C-terminal one-third of EBNA1, is called its DNA binding domain (DBD), and the structure of this domain was solved by the group of Lori Frappier.  The amino-terminal two-third of EBNA1 associates with cellular chromosomes (see figure 3). 

Thus for some time now, EBNA1 has been proposed to act as a molecule that tethers EBV genomes to chromosomes. Partitioning was explained by proposing that the tethers were equally likely to each of the sister chromatids.  Thus when they are separated during metaphase, the tethered EBV genomes are partitioned.

AT-hook proteins are cellular proteins that bind the minor groove of AT-rich sequences on cellular chromosomes. Cellular AT-hook proteins include HMG-I/Y (now renamed HMGA1a), and several other transcription factors.  AT-rich sequences on cellular chromosomes clustered in AT-rich satellites or in scaffold attached regions (SARs).  Thus when proteins like HMGA1a are immuno-stained on cellular chromosomes, a punctate staining like that of EBNA1 is observed (figure 5).

Publications

Sears J, Ujihara M, Wong S, Ott C, Middeldorp J, Aiyar A.  The amino terminus of Epstein-Barr nuclear antigen 1 (EBNA1) contains AT-hooks that facilitate the replication and partitioning of latent EBV genomes by tethering them to cellular chromosomes. J Virol. 2003 (in press).