Fusion Genes and Chromosomal Translocations

Chromosomal translocations join segments of two different chromosomes, and when a break falls within or near genes they can create a fusion gene - a hybrid that produces a chimeric protein or places one gene under the control of another. Such fusions are among the most distinctive oncogenic alterations in cancer, defining the biology of many leukemias, lymphomas, sarcomas, and a subset of carcinomas.

Definition

A chromosomal translocation is the relocation of a chromosomal segment to a non-homologous chromosome; a fusion gene is a hybrid gene formed when such a rearrangement (or other structural change) joins parts of two previously separate genes, often yielding a chimeric transcript and protein with oncogenic activity.

Scope

This entry covers how translocations generate fusion genes, the ways fusions drive cancer, their value as diagnostic markers, and the methods used to detect them. It treats fusions as a topic within tumor molecular profiling and describes biology and methodology rather than offering testing or treatment recommendations.

Core questions

• How do chromosomal translocations create fusion genes?
• By what mechanisms do fusion genes drive cancer - chimeric protein versus promoter swap?
• Why are particular fusions diagnostic hallmarks of specific tumor types?
• How are fusions detected, and what are the strengths of cytogenetic, FISH, and sequencing approaches?

Key concepts

• Reciprocal translocation
• Fusion gene and chimeric protein
• Promoter or enhancer hijacking
• Constitutive kinase activation
• Diagnostic fusion markers
• Breakpoint and fusion partner
• Detection by FISH, RT-PCR, and RNA sequencing

Mechanisms

When a double-strand break on one chromosome is mis-repaired by joining to a break on another, the resulting translocation can fuse the coding sequences of two genes or move a gene next to a strong regulatory element. Two broad oncogenic mechanisms follow. In the first, a chimeric protein with new or deregulated activity is produced - for example, fusions that join a kinase domain to a partner causing constitutive, ligand-independent signaling, as in the EML4-ALK fusion of lung cancer. In the second, a translocation places an otherwise normal gene under the control of an active promoter or enhancer, driving its overexpression. Because the same fusion recurs in a given tumor type, it serves both as a driver and as a highly specific diagnostic marker, detectable by karyotyping, fluorescence in situ hybridization, reverse-transcription PCR, or RNA sequencing.

Clinical relevance

Fusion genes are among the clearest examples of molecularly defined cancers and figure prominently in both diagnosis and the rationale for targeted therapy, including agents directed at kinase fusions across tumor types. This entry explains the biology and detection of fusions; it characterizes mechanisms and evidence and is not a basis for selecting tests or treatments for an individual.

Epidemiology

Recurrent fusions define a substantial share of hematologic malignancies and soft-tissue sarcomas and occur as drivers in subsets of common carcinomas such as lung adenocarcinoma. Certain fusions are present across diverse tumor types, supporting tissue-agnostic, fusion-defined groupings, while large genomic studies continue to catalogue the prevalence of fusions across cancers.

History

The link between chromosomal translocation and cancer was established with the recognition of a characteristic rearrangement in chronic myeloid leukemia and its BCR-ABL fusion, the first molecularly defined oncogenic fusion. Subsequent decades identified recurrent fusions across leukemias, lymphomas, and sarcomas, and the discovery of the EML4-ALK fusion in lung cancer in 2007 extended the paradigm to common solid tumors. Studies of acquired resistance to targeted therapy, as in BCR-ABL, further illuminated how fusion-driven cancers evolve.

Key figures

• Charles Sawyers
• Hiroyuki Mano

Seminal works

• soda-2007
• gorre-2001
• drilon-2018

Frequently asked questions

How does a chromosomal translocation cause cancer?

A translocation can create a fusion gene encoding a chimeric protein with abnormal activity, such as a constitutively active kinase, or it can move a gene next to a strong promoter or enhancer that drives its overexpression; both can give the cell a growth advantage.

Why are fusion genes useful as diagnostic markers?

Because particular fusions recur in specific tumor types and are rarely found elsewhere, detecting a characteristic fusion can help define the tumor's identity with high specificity, using methods such as FISH, RT-PCR, or RNA sequencing.