Introduction to Mobile DNA Elements
Mobile DNA elements are found in all forms of life, ranging from bacteria to humans. Research has increasingly shown that the insertion of mobile DNA into genes can disrupt gene expression and is linked to various diseases, including cancer, Duchenne muscular dystrophy, β-thalassemia, and factor IX hemophilia.
The Discovery of Mobile DNA
For many years, it was widely believed that genes were organized linearly along chromosomes. This perspective changed with the discovery of mobile DNA, also known as transposons or jumping genes, by Barbara McClintock. She noted that a specific DNA segment in maize would frequently detach from a chromosome and insert itself at a new location on the same chromosome, which impacted the pigmentation of the kernels. Since then, mobile DNA has been identified in numerous organisms, with estimates suggesting that over half of the human genome originates from these mobile DNA segments.
Current Understanding of Mobile DNA
While most mobile DNA elements are no longer capable of transposition, some retain this ability and can influence gene expression, playing significant roles in various diseases. A recent review published in The New England Journal of Medicine delves into the impact of mobile DNA elements on disease development. Below are key findings from this review, particularly regarding the association of transposons with different health conditions.
Types of Transposons and Their Impact
Long Interspersed Element 1 (LINE-1)
One significant class of transposons is the long interspersed element 1 (LINE-1). For example, the insertion of LINE-1 elements into the gene for coagulation factor VIII on the X chromosome has been linked to hemophilia. Additionally, disruptions caused by LINE-1 insertions are known to lead to Duchenne muscular dystrophy, β-thalassemia, and various cancers.
Gene Disruptions and Deletions
Transposition not only disrupts gene expression through insertion but can also result in the deletion of large DNA segments at target sites. For instance, LINE-1 insertion can cause deletions in the PDHX gene, leading to a deficiency in the pyruvate dehydrogenase enzyme complex. Similarly, deletions in the NF1 gene that occur alongside SVA transposon insertions are associated with neurofibromatosis.
LINE-1 Transpositions in Health and Disease
LINE-1 transpositions have been detected in the brain and gastrointestinal tract, although their significance in the brain remains uncertain. These transpositions are also noted in various cancers, including esophageal and gastric cancers. It remains unclear whether LINE-1 insertions in tumor tissues are responsible for initiating cancer or if they arise from the high mutation rates typical of cancerous tissues. However, there are documented cases where LINE-1 insertions into tumor suppressor genes like APC and PTEN have disrupted these genes and potentially contributed to tumor progression.
Alu Elements and Genetic Diversity
In addition to causing disease-related mutations, DNA recombination events mediated by Alu elements can lead to variations in the number of copies of certain DNA segments, known as copy number variations. These variations can differ among individuals and populations, contributing to the expansion of gene families and playing a potential role in evolution. Such genetic diversity is crucial for the long-term survival of species.
Conclusion
Significant advancements have been made in understanding the roles of transposable elements in genetic diversity and disease over the past three decades, as outlined in the review. However, further research is essential to address ongoing questions regarding their functions and their involvement in cancer development, brain development, and psychiatric disorders.
References
1. McClintock B. The origin and behavior of mutable loci in maize. Proc Natl Acad Sci U S A. 1950 Jun;36(6):344-55.
2. McClintock, B. The discovery and characterization of transposable elements. The collected papers of Barbara McClintock. (Garland, New York; 1987).
3. Smit AF. Interspersed repeats and other remnants of transposable elements in mammalian genomes. Curr Opin Genet Dev. 1999 Dec;9(6):657-63.
4. Kazazian HH Jr, Moran JV. Mobile DNA in Health and Disease. N Engl J Med. 2017 Jul 27;377(4):361-370. doi: 10.1056/NEJMra1510092.