About a third to two-thirds down the shaft of a chromosome is a constricted site called the centromere. When a chromosome replicates, the old and new pair (called chromatids) are held together at this centromere. During cell division, centromeres bind to microtubules that pull the old and new pairs toward opposite ends of the dividing cell, ensuring each daughter cell gets a full set of the parent’s genome. Defects in centromeres lead to an abnormal distribution of DNA in daughter cells that can in turn result in cell death or cancer.
In a study on the transparent, microscopic worm, Caenorhabiditis elegans, scientists have found that the transmission of the correct location of centromeres from parent to the offspring is not mediated by genes, but by an epigenetic memory.
The study conducted by two teams from the University of Geneva (UNIGE) and published in the PLOS Biology article “Transgenerational inheritance of centromere identity requires the CENP-A N-terminal tail in the C. elegans maternal germ line” was financially supported by the Swiss National Science Foundation and the Republic and Canton of Geneva.
From the worms to humans, all living organisms inherit physical and behavioral traits from their parents. Most of these biological traits are inherited through DNA, but there are exceptions to this rule. Some characteristics can be transmitted from one generation to the next, independent of genes through epigenetic means.
Florian Steiner, PhD, professor in the department of molecular biology at the UNIGE and senior author on this study, says, “Transgenerational transfer of epigenetic information not only exists, but can be molecularly understood (something that is not yet so clear in humans). This epigenetic transfer from one generation to the next can differ from cell-to-cell inheritance. It is therefore important to study these processes in organisms in addition to cultured cells.”
Reinier Prosée, PhD, researcher in the department of molecular biology at the UNIGE and first author on the study explains, “Studying these processes is greatly facilitated in C. elegans, since this small worm is transparent and allows live observation of cell divisions and the fate of chromosomes from one generation to the next.”
The location of the centromere on the chromosome is defined by a histone protein called centromere protein A (CENP-A) that is the research focus of the groups of both Steiner and Monica Gotta, PhD, professor at the UNIGE Faculty of Medicine. Together Steiner and Gotta’s teams discovered CENP-A finds its location on the chromosome to define the centromere with the help of a particular DNA region that serves as a guide. The researchers then mutated this region of DNA that guides CENP-A.
“The prediction was that this mutant would not be viable, since the position of the centromere could not be defined in the absence of the guide part of the protein. This, we expected, would lead to the incorrect distribution of chromosomes,” says Steiner.
“However,” says Prosée, “we found that even in the absence of this ‘guide’ region, the truncated protein positions itself correctly and is functional. The worms are therefore perfectly viable!”
Once the centromeric sites are defined in the mother, this information is transmitted to the next generation even in the absence of the DNA that codes for the “guide” region of the protein, the authors infer. In contrast, the offspring of the mutant worms cannot divide their cells properly and therefore do not survive because they have not inherited the epigenetic information about the correct position of the centromeric sites from their mutant mother. This epigenetic memory only lasts for one generation and is not transmitted to the next.
“We combined traditional worm genetics and fluorescence microscopy with more recently developed techniques, such as the auxin-inducible degron (AID) system to degrade proteins in a specific tissue at a specific developmental time. This was important to distinguish the roles of CENP-A and KNL-2 [a protein that binds to CENP-A] in the germ line from those during development,” says Steiner. “We needed to generate multiple precise deletions in the endogenous CENP-A gene to remove specific parts of the protein, and add tags to be able to detect these mutant proteins. This would not have been possible with the techniques that existed before the development of the CRISPR/Cas9 genome editing technique.”
In their next experiments, Steiner’s team will attempt to explain the epigenetic mechanism by which this memory is made and endures through the stages of development.
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