What is Gene Fusion?
A gene fusion is defined as two genes that are joined so that they are transcribed and translated as a single unit protein, and this single unit protein is also called fusion protein. These fusion proteins contain complete or incomplete parts of two normal proteins originating from fused genes.
Gene fusions can occur:
In vivo: naturally and result from genetic manipulations, and
In vitro: can be constructed using recombinant DNA techniques.
In nature, gene fusions occur throughout the evolution of living organisms. For example, two genes whose products are part of a metabolic pathway are fused, producing a fusion protein that carries out both steps of the pathway.
During the evolution of living organisms, the fused chimeric genes develop amino acid substitutions that are rare or missing from the ancestral mutants. Fused genes are frequently observed in the evolutionary ranks, indicating that they did not evolve by common vertical descent.
On the other side, gene fusion may have pathological consequences. For example, it is well described that chromosomal rearrangement (translocations) and gene fusions of different genic elements cause several types of cancers.
An Example of Gene Fusion
One of the well-known examples of gene fusions concerning the onset of cancer is the example of the fusion of the ABL gene and BCR gene that gives rise to a phenomenon called the Philadelphia chromosome.
In this case, segments of chromosomes 9 and 22 break off and change places giving rise to a new fused gene. The ABL gene from chromosome 9 joins the BCR gene on chromosome 22, forming the BCR-ABL fusion gene.
This changed chromosome 22 with the BCR-ABL fusion gene on it is called the Philadelphia chromosome.
Important to mention that most patients with chronic myelogenous leukemia (CML) have the BCR-ABL fusion gene, and also the BCR-ABL fusion gene is present in some patients with acute lymphoblastic leukemia (ALL) or acute myelogenous leukemia (AML).