Study yields secrets of chromosome movement

Investigators at St. Jude Children's Research Hospital have used the lowly yeast to gain insights into how a dividing human cell ensures that an identical set of chromosomes gets passed on to each new daughter cell. Errors in this critical part of cell division can cause one daughter cell to get extra copies of some chromosomes that should have moved into the other daughter cell, or no copies of other chromosomes—a problem that is prevalent in cancer and can cause miscarriages or disease, such as Down syndrome.

St. Jude researchers made their discovery by tracking the activity of a small army of molecules with exotic names like argonaute (Ago1) and dicer; these molecules help maintain a specialized, tightly packaged form of DNA called heterochromatin at the part of the chromosome called the centromere.

The investigators also showed the order in which certain critical events occur in setting up and maintaining this heterochromatin. The work is important because it gives scientists insight into how each daughter cell receives the normal number of chromosomes; and it offers important clues to understanding the genetic cause of certain catastrophic diseases. A report on this work appears in the May 25 issue of Molecular Cell.

All of the cell’s DNA is wrapped around a series of structures, called histone octamers, to generate chromatin—much like thread wound around a spool. This chromatin is then further compacted to form the characteristic, thick structures commonly recognized in illustrations and photographs as chromosomes. At the centromere, DNA is packaged into an even more compact and specialized form of chromatin called centromeric heterochromatin.

The centromere is the last point at which the two identical chromosomes are joined before the cell divides. Centromeric heterochromatin helps to yoke together the “sister chromatids” of each chromosome pair as they line up in the center of the dividing cell before separating and moving into their respective daughter cells. When the cell has ensured that it is safe to continue dividing, each sister chromatid moves in opposite directions toward the two new daughter cells that are forming.

“The cell must establish and then maintain centromeric heterochromatin to ensure that each chromosome pair is stable and securely linked together until it’s time to separate,” said Janet Partridge, Ph.D., assistant member of the St. Jude Department of Biochemistry. “Otherwise, the chromosome pairs would drift apart and leave daughter cells with too many or too few chromosomes.” Partridge is the report’s senior author.

The St. Jude team studied combinations of molecules in yeast called the RITS and RDRC complexes, which together with an enzyme called Clr4 (Suv39 in humans), establish and maintain centromeric heterochromatin in the yeast cell during a carefully choreographed series of steps.

RITS is composed of the proteins Ago1, Tas3 and Chp1 and works closely with RDRC. RDRC produces a type of genetic material called double-stranded RNA, which an enzyme, called dicer, then chops into smaller pieces called small interfering RNA (siRNA). siRNA is bound by RITS, and in turn, helps RITS to reinforce the centromeric heterochromatin and keep it stable.

In addition, the Clr4 enzyme puts chemical tags onto the histone “spool” in a process called methylation. Methylation attracts a protein called Swi6 (HP1 in humans) to the chromosome to reinforce heterochromatin.

Previously, scientists—including Partridge’s team—showed that cells lacking any component of RITS, RDRC or the Clr4 complex fail to assemble intact centromeric heterochromatin and suffer loss of chromosomes. However, researchers did not know whether the same components of these complexes are needed to support both establishment and the maintenance of centromeric heterochromatin. Therefore, Partridge’s team developed specially modified yeast cells that allowed them to study these events individually.

By separating the Ago1 component of the RITS complex away from the Chp1-Tas3 components, rather than completely removing Ago1 from the cell, the researchers were able to generate yeast that could still maintain heterochromatin it had already established. These yeast cells had normal chromosome movement during cell division.

The investigators then showed that the same yeast cells also require the continuous presence of Clr4 to initially assemble normal centromeric heterochromatin. Specifically, the researchers used these yeast cells to show that the establishment of centromeric heterochromatin requires Clr4 to methylate histones at the centromere. The methylated centromere then attracts the RITS complex to this site. The St. Jude researchers’ work also suggests that the siRNA is not so important for this first step, but does play an important role in propagating and maintaining centromeric heterochromatin.

“Until we did this study, it was virtually impossible to figure out which molecular events were specifically required for the two different processes of establishing and maintaining centromeric heterochromatin,” Partridge said. “Now we have the tools to ask what is required for the cell to perform each task. This has important implications not just for understanding how centromeric heterochromatin assembles, but also for learning how heterochromatin forms elsewhere on the chromosome, a process that is often disturbed in cancer.”

Source: St. Jude Children's Research Hospital

Related stories:

Exploring the Dark Matter of the Genome
Not so long ago, the difficult-to-sequence, highly repetitive, gene-poor DNA found in regions of chromosomes known as heterochromatin was called "junk." Like dark matter in the universe, the true nature of heterochromatin was unknown.
Centromeres cross over, a lot
Recombination at centromeres is higher than anywhere else on the chromosome, even though methyltransferases do their best to prevent it, say Jaco et al., as published in the June 16 issue of the Journal of Cell Biology.
Research shines spotlight on a key player in the dance of chromosomes
Cell division is essential to life, but the mechanism by which emerging daughter cells organize and divvy up their genetic endowments is little understood. In a new study, researchers at the University of Illinois and Columbia University report on how a key motor protein orchestrates chromosome movements at a critical stage of cell division. The study appeared in the Proceedings of the National Academy of Sciences.
Scientists clarify a mechanism of epigenetic inheritance
Although letters representing the three billion pairs of molecules that form the “rungs” of the helical DNA “ladder” are routinely called the human “genetic code,” the DNA they comprise transmits traits across generations in a variety of ways, not all of which depend on the sequence of letters in the code.
Cancer stem cell marker also drives transcription in normal cells
New research links the recently discovered function of a multi-faceted transcriptional complex to control of gene expression in both normal cells and cancer stem cells. Two separate studies, published by Cell Press in the January 18th issue of Molecular Cell, provide insight into novel subunits associated with an evolutionarily conserved transcriptional regulatory complex and reveal a previously undescribed chromatin function that is required for full activity of nuclear receptors in normal cells and for the MYC oncoprotein in tumor cells.
Coming together: tDNAs, RNA pol III and chromatid cohesion
In the September 1st issue of G&D, Drs. Rudra Dubey and Marc Gartenberg (UMDNJ) reveal a surprising new role for tDNAs and RNA polymerase III-associated proteins in sister chromatid cohesion.
Where broken DNA is repaired
Ionizing radiation, toxic chemicals, and other agents continually damage the body's DNA, threatening life and health: unrepaired DNA can lead to mutations, which in turn can lead to diseases like cancer. Intricate DNA repair mechanisms in the cells' nuclei are constantly working to fix what's broken, but whether the repair work happens "on the road" — right where the damage occurs — or "in the shop" — at specific regions of the nucleus — is an unanswered question.
Regulating the nuclear architecture of the cell
An organelle called the nucleolus resides deep within the cell nucleus and performs one of the cell's most critical functions: it manufactures ribosomes, the molecular machines that convert the genetic information carried by messenger RNA into proteins that do the work of life.

News discussion:

Medicine & Health news