Research

Chromosome end protection

What does one mean by chromosome end protection? Our cells are equipped with a molecular ‘rescue team’ collectively known as the DNA damage response and repair machinery, which heals and seals deleterious double-stranded (ds) breaks to restore genome integrity. Now, if one looks at a natural chromosome end, it looks very much like a double-stranded break. However, if the DNA damage-response machinery gets recruited to natural ends of chromosomes, it would lead to catastrophic inter-chromosomal end-to-end fusions. Hence, the protection of the natural ends of chromosomes from the DNA damage response machinery defines chromosome end protection.

So, how do we protect chromosomes ends? A six-protein complex known as shelterin that binds specifically to chromosome ends and protects them from end-to-end fusions performs this function in humans and other mammals. Our lab is interested in using a wide array of biochemical, crystallographic, and cell biological tools to determine the mechanisms by which end protection is established.

Chromosome end replication

What is meant by chromosome end replication? After every round of DNA synthesis by DNA polymerases in a given cell cycle, a small fraction of DNA at the extreme end of the chromosome is lost. This is called the end replication problem, which is of great significance because if chromosomes shrink beyond a certain threshold, cells would cease to divide. Although most cells that make up our body are non-dividing and hence do not have to deal with this problem, our stem cells — which are actively dividing cells responsible for replenishing and repairing tissues – require solution of the end replication problem.

So, what solves the end replication problem? Telomerase, a specialized RNA-protein (RNP) enzyme, synthesizes DNA repeats (known as telomeric DNA) at chromosome ends to compensate for the DNA lost every cell cycle. Although telomerase plays the ‘good cop’ in facilitating stem cell function, telomerase is illicitly employed by ~90% of cancers for their continued growth and division. Hence, telomerase is considered a major target for anti-cancer drug development. Our lab is interested in using a multi-disciplinary approach combining structural and functional methodologies to answer the several outstanding questions in telomerase biogenesis and action.

The role of telomeres in meiosis

Telomeres play a role in meiosis, what you do you mean? Well, it turns out that telomeres play a critical role in meiosis and the reason why this has caught less attention in the past is because of how little we knew about the basis of this function. Meiosis is a specialized form of cell division which entails one round of DNA replication followed by two rounds (meiosis I and II) of cell division to furnish haploid gametes for sexual reproduction. The biological importance of proper progression through meiosis can be gleaned from the fact that defects in meiosis are the leading cause of miscarriages. An essential step in prophase I of meiosis is homologous recombination, whereby each chromosome pairs with its homolog to exchange DNA for enhancing genetic diversity and facilitating proper segregation into gametes. However for this pairing to occur, chromosomes must first attach to the nuclear periphery so that a connection between chromosomes in the nucleus and the dynein/dynactin machinery in the cytosol is established. Telomeres, which serve to link chromosomes to the force-generating machinery during meiosis move along the nuclear membrane to allow for homologs to pair and exchange genetic information.

The mammalian proteins involved in telomere tethering and movement along the nuclear membrane were recently discovered, however the structures & dynamics at the telomeres, the nuclear membrane, and on microtubules that orchestrate homolog motion & pairing remain largely unknown. We are interested in dissecting this complex process by using biochemistry and structural biology (crystallography and EM).