New Light Shed on the “Dark Side” of the Genome
In human beings – and more generally in higher organisms (eukaryotes) the DNA interacts with specific proteins to form a compact nucleoproteic structure called chromatin that contains genetic information and contributes to regulating its expression. Chromatin has two states: euchromatin, which is less condensed and associated with intense gene expression, and constitutive heterochromatin that is structurally more compact and located in “peripheral” regions, such as centromeres and telomeres.
Although constitutive heterochromatin represents a quantitatively significant part of the genome (30% of the human genome), it has traditionally been considered devoid of functional genes and without any transcriptional activity. This is why constitutive heterochromatin is frequently regarded as a sort of genomic dump of “junk DNA” or, in the best case, a “silent” part of the genome.
Now, a new perspective on constitutive heterochromatin has been presented and discussed by a group of researchers, including Patrizio Dimitri from the Sapienza “Charles Darwin” Department of Biology and Biotechnology, in an Italian review published on Trends in Genetics.
The authors, who are experts in this sector, have reviewed the literature, shedding further light on aspects that the international scientific community tends to neglect. What emerged by pioneering studies of genetics and more recently studied of genomics is that in the model organism of the Drosophila melanogaster, constitutive heterochromatin contains a surprising and heterogeneous variety of functional genes. Some of them show a very large size and are essential for cellular differentiation and development. Functional genes in heterochromatin have been also found in other organisms, including humans.
“In Drosophila, “giant” heterochromatic genes,” points out Patrizio Dimitri, “include the Y-chromosome fertility genes that have been extensively studied since the 80s by Maurizio Gatti and Sergio Pimpinelli, and Myosin 81F, which has more recently been identified as codifying a new protein of the myosin family. These genes – with more than 2 million base pairs of DNA – are comparable in size to an entire bacterial genome.”
“Our review,” explains Prof. Dimitri, “is aimed to promote a new functional “portrait” of constitutive heterochromatin both in didactic and scientific perspective. In fact, the study and understanding of epigenetic mechanisms that allow the expression of genes in constitutive heterochromatin of Drosophila may contribute to better understand the relationships between disfunctions of the above-mentioned mechanisms and the onset of tumours and other human pathologies caused by alterations of chromatin states.”
A New Portrait of Constitutive Heterochromatin: Lessons from Drosophila melanogaster - Marsano R. M., Giordano E., Messina G., Dimitri P. - Trends in Genetics Volume 35, Issue 9, September 2019, Pages 615-631; DOI https://doi.org/10.1016/j.tig.2019.06.002
“Charles Darwin” Department of Biology and Biotechnology, Sapienza University of Rome