A human genome sequencing project conducted in 2001 revealed that more than 45% of our genome came from sequences known as transposons. Also called "jumping" genes, these structures can move within the genome and generate new copies of themselves through cut-and-paste or copy-and-paste molecular mechanisms.
Because of these characteristics, transposons are dubbed as "selfish genes," which are involved only in the replication process. A recent study, however, has revealed that they also have crucial and unexpected functions.
Selfish Jumpers
Transposons refer to segments of DNA that can move from one genetic location to another. They are useful to pathogenic organisms in helping them fight antimicrobial agents. In humans, they were thought to have no biological functions and were even considered a nuisance.
There are two basic types of transposons: Class I and Class II. Class I transposons, also known as retrotransposons, move through an RNA intermediate and are responsible for almost half of the human genome. Meanwhile, Class II transposons are also called DNA transposons, which move directly from one location to another within the genome.
Most of the copies generated by transposons are currently inactive, but in the human genome and that of other mammals, there are about a hundred copies of transposons that belong to the Long Interspersed Nuclear Elements (LINE) family, which are still potentially active. This means that they can be transcribed, producing mRNA, which, when translated into proteins, can contribute to copying and pasting genetic sequences.
This function of LINE transposons poses a risk to genome integrity since randomly inserting a new gene copy can interfere with important genetic functions. Over the course of evolution, animals learn to protect themselves from this danger by developing cellular defense mechanisms that can block or limit transposons' activity, thus helping preserve genome stability.
Unexpected Functions of Transposons
A collaboration between Scuola Internazionale Superiore di Studi Avanzati (SISSA) and Istituto di Tecnologia (IIT) sought to answer the fundamental question of why the human genome allows and finely regulates the activity of transposons. Led by Professors Stefano Gustincich and Remo Sanges, the studies led to the publication of three articles that shed new light on the contribution of LINE elements to molecular processes in living organisms.
The first article reveals that the RNA produced by LINE transposons is essential for developing the cerebral cortex in mice. Without this RNA, the proportion of the different cell types that make up the cerebral cortex is compromised.
The second article examines the data on the early stages of embryonic development when a two-cell embryo produces its RNA. It was discovered that LINE elements are recognized by protein complexes, which trigger transcription processes important for proper embryonic development.
In the final article, the researchers examined the behavior of LINE transposons in the brains of people with autism spectrum disorders. They found out that the transcription regulation of these sequences is different only in a small group of individuals with autism.
The experts believe that these studies hold the key to new and innovative lines of research to identify and understand the regulatory signals contained in transposon sequences. They also offer new insights in identifying novel treatments for neurodevelopmental and aging-related disorders.
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