Human chromosomes often undergo breakage due to agents that damage the DNA. It is critical to repair such breaks, to maintain genome integrity and to prevent mutations that can give rise to cancer. All organisms have the capacity to repair such breaks by a process called homologous recombination, which restores the continuity of the genome without introducing mutations. A non-homologous recombination process, called end-joining, often introduces mutations and thus is only used by a cell when homologous recombination is not possible. Maria Jasin pioneered genetic and physical assays for recombination in human cells and she was the first scientist to directly demonstrate the importance of both homologous recombination and non-homologous end-joining for repair of chromosomal breaks. Her discovery has important implications for both normal cellular function, embryonic development, fertility, and for the etiology of diseases such as cancer. In the course of this work, Dr Jasin demonstrated that breaks in chromosomes greatly increase the frequency of recombination at the site of the break. This important discovery laid the groundwork for efficient modification of mammalian genomes by site-specific nucleases, an approach that is currently being widely exploited for gene therapy and basic research.
 
Dr Jasin developed her interest in gene modification as a graduate student in Paul Schimmel’s lab at MIT, working in bacterial systems. She began pursuing ideas about double-strand break (DSB)-provoked recombination in mammalian cells as a postdoctoral fellow with Walter Schaffner in Zurich. From her work there, she published a paper on recombination between endogenous and incoming copies of SV40 viral genomes.  However, in order to focus more specifically on genome modification and recombination, Dr Jasin moved to Paul Berg’s lab at Stanford where she showed that recombination was greatly stimulated by a DSB on the transfected DNA within a region of homology to the chromosomal target. Like her work with Schaffner, this study used targeting of SV40 sequences in monkey kidney cells. She then followed this up by targeting an endogenous human gene, the CD4 locus.
 
On the basis of this important work, Jasin then started her own lab at Memorial Sloan Kettering Cancer Center in New York in 1990. In Jasin’s groundbreaking 1994 work, her laboratory devised an ingenious method to create a DSB in the mouse genome to provoke recombination. To do this, she used a specialized nuclease enzyme from yeast that had a well-characterized, 18 nucleotide long DNA recognition sequence so as to break a unique site in even complex genomes. The gene encoding the yeast enzyme was introduced into mouse cells after the companion recognition sequence, not normally present in any mouse chromosome, was genetically engineered into a gene that could be scored for its function in the cells. When the recognition sequence is cut by the yeast enzyme, Jasin found how the mouse cells patch it up by the normal cellular process of repair.
 
Using this strategy, Jasin performed the first gene editing. Importantly, she showed that introduction of a site-specific DSB into the genome of mammalian cells produced a 1000-fold increase in the targeting of a homologous fragment of DNA to that site. This groundbreaking work laid the foundation for all subsequent gene-editing studies, because now it was clear that a DSB in the genome is the critical step.
 
Of course, none of us work in a vacuum. The intellectual backdrop of the time included the then-recent pioneering work of Terry Orr-Weaver, Jack Szostak, and Rodney Rothstein on gene targeting in yeast, leading to their influential model (with Frank Stahl) for repair of plasmid DSBs by homologous recombination. Other important foundational work in yeast was the characterization of mating locus type switching triggered by a mating-specific endonuclease (HO)-mediated DSB (work of Jim Haber, Jeff Strathern, and others), and the engineering of the yeast endonuclease that Jasin used (I-SceI) by Bernard Dujon’s laboratory.  And in work contemporaneous to Jasin’s, Dujon also analyzed effects of a chromosomal DSB on recombination with other chromosomal templates, not as Jasin did using a transfected donor template for the recombination reaction. Importantly, Jasin’s work was the first to demonstrate that a chromosomal DSB could recombine with a transfected piece of homologous DNA, or alternatively, undergo non-homologous repair. This particular arrangement in her 1994 papers presages the precision genome editing of the modern era.
 
Jasin’s discovery forms the basis for subsequent work on highly specific but flexible nucleases – zinc fingers nucleases, TALENs, and CRISPR – that are currently being used for genome modification. All of these methods describe new and increasingly refined ways to introduce enzymes and DSBs into DNA. Nonetheless, they all rely fundamentally on Jasin’s discovery of the stimulation of recombination by a double strand DNA break and the strategy to introduce a DNA cleaving enzyme to make the precise break. In her visionary 1994 paper, Jasin modestly concluded: “This could facilitate the creation of subtle genetic alterations at targeted loci”.
 
Using the methods developed in her lab and now applied worldwide, Dr. Jasin also discovered that the two major familial breast/ovarian tumor suppressor genes, BRCA1 and BRCA2, are required for homologous recombination, a finding that explained how the loss of either of these two genes increases the frequency of potentially carcinogenic genetic alterations (note the 2018 Shaw Prize in Life Science and Medicine to Dr Mary-Claire King for the discovery of the BRCA1 and 2 genes in breast cancer). The importance of these results cannot be overstated, and they are being exploited in novel therapies for the treatment of breast, ovarian, and other cancers with BRCA1 and BRCA2 mutations, and potentially cancers with mutations in other homologous recombination genes.
 
Maria Jasin’s research has contributed to the textbook view of how cells survive breaks in their chromosomes, which is critical for the life of all cells. Equally important, her insights paved the way for today’s current revolution in genome editing.