Fusion Gene

• Fusion genes are hybrid genes created when two distinct genes merge due to chromosomal changes.
• They often produce novel proteins with altered functions, which can drive disease, especially cancer.
• These genes are formed through mechanisms like translocations, inversions, and deletions in the genome.
• Specific fusion genes serve as crucial diagnostic markers and therapeutic targets in oncology.
• Examples include BCR-ABL1 in CML and EML4-ALK in non-small cell lung cancer.

What is a Fusion Gene? Definition and Formation

A Fusion Gene refers to a hybrid gene created by the joining of two previously independent genes. This genetic alteration typically arises from chromosomal rearrangements, such as translocations, inversions, or deletions, which bring two distinct genes into close proximity. The resulting fusion gene often encodes a chimeric protein, meaning a protein composed of segments from both original genes, which can possess novel or altered functions compared to the individual proteins.

How Do Fusion Genes Form?

Fusion genes primarily form through chromosomal aberrations that physically link two separate genes. The most common mechanism is a chromosomal translocation, where a segment of one chromosome breaks off and attaches to another chromosome. If this break occurs within or near two different genes, their coding sequences can become fused. Other mechanisms include inversions, where a segment of a chromosome is reversed, and deletions, where a segment is lost, both of which can lead to the juxtaposition of two genes that were originally far apart. These events result in a new, single gene sequence that is then transcribed and translated into a fusion protein.

Fusion Genes: Examples and Their Role in Cancer

The study of fusion gene definition and examples is critical in understanding their impact on human health, particularly in oncology. These genetic fusions are frequently found in various cancers, where the aberrant fusion proteins can act as oncogenes, driving tumor initiation and progression. For instance, the BCR-ABL1 fusion gene is a hallmark of chronic myeloid leukemia (CML), present in over 95% of patients, as reported by the National Cancer Institute (NCI). This fusion gene produces a constitutively active tyrosine kinase, leading to uncontrolled cell proliferation.

The role of fusion genes in cancer extends beyond CML, as they are implicated in a wide array of malignancies. The abnormal proteins produced by fusion genes can disrupt normal cellular processes, such as cell growth, differentiation, and apoptosis. This disruption often leads to uncontrolled cell division and survival, characteristic features of cancer. Consequently, fusion genes are not only important diagnostic markers but also valuable therapeutic targets. The identification of specific fusion genes allows for the development of targeted therapies that specifically inhibit the activity of the aberrant fusion protein, often leading to more effective and less toxic treatments compared to conventional chemotherapy.

Several other notable fusion genes and their associated cancers include:

• EML4-ALK: Found in a subset of non-small cell lung cancer (NSCLC), leading to the development of ALK inhibitors.
• PML-RARA: Characteristic of acute promyelocytic leukemia (APL), responsive to all-trans retinoic acid (ATRA) therapy.
• RET fusions: Identified in various cancers, including thyroid cancer and NSCLC, with specific RET inhibitors showing promise.
• NTRK fusions: Present in a broad range of tumor types, leading to the development of pan-TRK inhibitors.