Lecture 4: Recombination

Introduction: Genome Maintenance and Recombination 🧬

The maintenance of a genome involves a critical balance between ensuring genetic stability (through processes like DNA repair and high-fidelity DNA replication, which aim to prevent mutations) and facilitating genetic diversity (which drives evolution and is prominent in processes like meiosis, often involving recombination).

Recombination is a fundamental process where DNA molecules are broken, and the resulting fragments are re-joined in new combinations. This can lead to various outcomes, including gene conversion (a non-reciprocal transfer of genetic information, often without an exchange of flanking DNA, also known as a non-crossover event) or a crossover (a reciprocal exchange of DNA segments between chromosomes).

Defects in recombination can have severe consequences. For example, individuals with Bloom Syndrome exhibit a dramatically increased rate of exchange between homologous chromosomes compared to normal individuals, leading to genomic instability and a predisposition to cancer.

Comparison of chromosome preparations from (A) a normal individual and (B) a Bloom Syndrome patient. The harlequin staining (darker chromatids are newly synthesized) reveals a significantly higher frequency of sister chromatid exchanges in the Bloom Syndrome patient, indicative of genomic instability.

Types of Recombination 🔄

There are four main types of DNA recombination:

• Homologous Recombination (HR)

• Non-Homologous End Joining (NHEJ)

• Transposition

• Site-Specific Recombination

Mechanism 1: Homologous Recombination (HR)

Homologous Recombination (HR) is a genetic exchange process that occurs between a pair of DNA sequences that are identical or very similar (homologous). These sequences can be located on homologous chromosomes or, following DNA replication, between sister chromatids.

Key Roles of HR:

• Meiosis: HR, particularly through crossover events between non-sister chromatids of homologous chromosomes, generates new combinations of alleles, thus creating genetic diversity in offspring.

• DNA Repair: HR is a crucial pathway for maintaining genetic stability by accurately repairing DNA damage, especially DNA double-strand breaks (DSBs). It typically uses an undamaged homologous chromosome or, more commonly after replication, the identical sister chromatid as a template for repair.

The fundamental mechanisms of HR are remarkably conserved across all kingdoms of life, from bacteria to humans. Our understanding of HR has been built from research on bacterial recombination (e.g., during phage infection), meiotic recombination in yeast, and DNA repair processes in various organisms.

Basic Steps in Homologous Recombination:

1. Double-Strand Break (DSB) Formation: The process usually initiates with the creation of a DSB in one of the DNA molecules. DSBs can be caused by:

   • Exogenous DNA damaging agents like ionizing radiation.

   • Endogenous cellular processes, such as the collapse of a stalled DNA replication fork.

   • Intentional enzymatic cleavage, for instance, by the Spo11 endonuclease during meiosis to initiate recombination, or by the HO endonuclease during mating-type switching in budding yeast.

2. End-Resection: The broken DNA ends are processed by nucleases to generate long 3' single-stranded DNA (ssDNA) overhangs. In Saccharomyces cerevisiae (budding yeast), this is carried out by the MRX trimeric nuclease complex (Mre11-Rad50-Xrs2). In E. coli, the RecBCD trimeric nuclease complex (RecB-RecC-RecD) performs this role.

3. Coating of ssDNA (Nucleoprotein Filament Formation): The 3' ssDNA overhangs are coated by specialized ssDNA-binding proteins, forming a nucleoprotein filament. The key protein involved is Rad51 (a monomer) in eukaryotes like yeast, and its homolog RecA (a monomer) in bacteria like E. coli. This filament is essential for the subsequent homology search and strand invasion steps.

4. Strand Invasion and D-Loop Formation: The RecA/Rad51 nucleoprotein filament facilitates the search for a homologous DNA duplex and promotes the invasion of this duplex by one of the 3' ssDNA overhangs. The invading strand base-pairs with its complementary sequence on one strand of the homologous duplex, displacing the other strand of that duplex to form a structure called a Displacement loop (D-loop). The region where strands from the two different DNA molecules are paired is known as heteroduplex DNA.

5. DNA Synthesis and Branch Migration: The invading 3' end is then used as a primer by DNA polymerase, which extends it by synthesizing new DNA using the homologous DNA molecule as a template. As synthesis proceeds, the D-loop may enlarge, and the point where the DNA strands cross over can move along the DNA in a process called branch migration, due to the complementarity of the DNA sequences. This results in interconnected DNA molecules that may contain single-strand nicks.

6. Resolution of HR Intermediates: The interconnected DNA structures formed can be processed through several pathways:

   • Synthesis-Dependent Strand Annealing (SDSA) Pathway: The newly synthesized strand (magenta in the diagram) that invaded the homologous duplex is displaced and then re-anneals with the original complementary strand on the other side of the initial DSB. Any remaining gaps are filled by DNA synthesis, and DNA ligase seals the nicks. SDSA is a highly conservative form of HR and results in non-crossover products, meaning the DNA sequences flanking the original DSB are not exchanged between the chromosomes.

   • Double Holliday Junction (dHJ) Formation and Processing: If both 3' ends of the DSB invade the homologous duplex, are extended by DNA synthesis, and the newly synthesized strands are captured and ligated, two Holliday Junctions (HJs) can form. An HJ is a four-way DNA structure where two homologous DNA molecules are covalently linked. These dHJs can be processed in two main ways:

     • dHJ Dissolution: This pathway is carried out by a complex of proteins such as Sgs1-Top3-Rmi1 in budding yeast (or BLM-TOP3-RMI1 in humans). Sgs1 (or BLM) is a DNA helicase that promotes branch migration of the HJs towards each other until they form a hemicatenane structure. The Top3 topoisomerase then decatenates (separates) the intertwined strands. Dissolution always results in non-crossover products.

     • dHJ Resolution: Alternatively, HJs can be cleaved by specific endonucleases known as Holliday Junction Resolvases. Examples include the RuvABC complex in bacteria, and GEN1 or YEN1, and the SMX complex (Slx1-Slx4, Mus81-Eme1, XPF-XRCC4) in eukaryotes.

       • If both HJs are cut (resolved) in the same orientation, the outcome is a non-crossover product.

       • If the two HJs are cut in opposite orientations, the outcome is a crossover product, where the DNA sequences flanking the original DSB are reciprocally exchanged between the two homologous DNA molecules. This is a critical mechanism for generating genetic diversity during meiosis.

Overview of different homologous recombination (HR) pathways for the repair of DNA double-strand breaks (DSBs). Pathways include Synthesis-Dependent Strand Annealing (SDSA), and the formation of double Holliday Junctions (dHJs) which can be either dissolved or resolved, leading to non-crossover or crossover products.

HR plays a dual role: in meiosis, it primarily aims to create genetic diversity through crossovers, while in DNA repair (especially of DSBs outside meiosis), it aims to maintain genetic stability, often favoring non-crossover outcomes.

Mechanism 2: Non-Homologous End Joining (NHEJ) 🛠

Non-Homologous End Joining (NHEJ) is the predominant pathway for repairing DSBs in human cells, particularly active during the G0 and G1 phases of the cell cycle when a sister chromatid is typically unavailable to serve as a template for HR.

• Key Characteristics:

  • Does not require any extensive sequence homology or a repair template.

  • Generally error-prone, often leading to small insertions or deletions (indels) at the break site, which can result in point mutations.

• Mechanism:

  1. End Binding: The Ku70/Ku80 heterodimer recognizes and binds to the broken DNA ends. Ku then recruits the catalytic subunit of the DNA-dependent protein kinase (DNA-PKcs).

  2. End Processing: The DNA ends may undergo minimal processing. This can involve trimming of overhangs by nucleases (such as Artemis, potentially with the MRN complex) or filling in of gaps by specialized DNA polymerases. Some NHEJ sub-pathways, like microhomology-mediated end joining (MMEJ), utilize short stretches of microhomology (a few base pairs) to help align the ends before joining.

  3. Ligation: The processed DNA ends are then joined together by the DNA Ligase IV complex, which includes XRCC4 and XLF (or PAXX).

• Importance in V(D)J Recombination: NHEJ plays a vital role in the adaptive immune system. It is essential for V(D)J recombination, the process that assembles functional immunoglobulin (antibody) genes in B cells and T-cell receptor genes in T cells from separate Variable (V), Diversity (D), and Joining (J) gene segments. The inherent imprecision and error-prone nature of NHEJ during the joining of these segments contributes significantly to the diversity of the antibody and T-cell receptor repertoire, allowing the immune system to recognize a vast array of antigens.

Mechanism 3: Transposition ("Jumping Genes") 🐒

Transposition is a recombination process involving the movement of specific DNA sequences, known as transposons or transposable elements (TEs) (also called "jumping genes"), from one genomic location to another. This phenomenon was first discovered by Barbara McClintock.

• Key Features:

  • Transposition generally does not require sequence homology between the transposon and its new insertion site; TEs can often insert into many different, sometimes random, locations in the genome.

  • The process is catalyzed by a specific enzyme called transposase, which is usually encoded by the transposon itself. The transposase recognizes specific DNA sequences at the ends of the transposon and mediates its excision (for some types) and insertion into a new target DNA site.

Major Classes of Transposable Elements:

Table adapted from Alberts et al., Molecular Biology of The Cell, depicting three major classes of transposable elements. These elements range in length from 1000 to about 12,000 nucleotide pairs.

• DNA-only Transposons:

  • Found predominantly in bacteria and are largely responsible for the spread of antibiotic resistance genes among bacterial strains.

  • Typically use a "cut and paste" transposition strategy, where the transposase excises the transposon from the donor DNA and integrates it into a target site. This process often results in the duplication of a short target DNA sequence at the site of insertion, flanking the integrated transposon.

• Nonretroviral Retrotransposons:

  • These are particularly abundant in vertebrate genomes, including humans. For instance, L1 elements (a type of LINE – Long Interspersed Nuclear Element) and SINEs (Short Interspersed Nuclear Elements) make up approximately 30% of the human genome.

  • Movement involves transcription of the element into RNA, followed by reverse transcription (often primed by a nick in the target DNA made by an endonuclease encoded by the element) directly into the new genomic site.

  • While most nonretroviral retrotransposons in the human genome are immobile due to accumulated mutations, a few active L1 elements can still "jump." Such events can cause genetic diseases if they insert into critical genes; for example, L1 insertion into the gene encoding blood clotting protein Factor VIII has been shown to result in hemophilia.

Mechanism 4: Site-Specific Recombination 🎯

Site-specific recombination is a type of DNA rearrangement that occurs between two specific, short DNA recognition sequences (recombination sites). Unlike HR, it does not require extensive homology between the recombining molecules.

• Mechanism: The process is carried out by specialized enzymes called site-specific recombinases. These enzymes recognize the specific DNA sites on both the donor and recipient DNA molecules and catalyze the breakage and rejoining of the DNA strands at these sites.

• Outcomes: The outcome of site-specific recombination depends on the relative orientation of the two recombination sites.

  • Integration: If the sites are on two different DNA molecules (e.g., a circular DNA integrating into a linear chromosome), the result is the insertion of one DNA molecule into the other.

  • Excision (Deletion): If two sites on the same DNA molecule are oriented as direct repeats, recombination between them will result in the deletion of the DNA segment located between the sites.

  • Inversion: If two sites on the same DNA molecule are oriented as inverted repeats, recombination will result in the inversion (flipping the orientation) of the DNA segment between them.

• Examples of Site-Specific Recombination Systems:

  • Bacteriophage P1: Cre recombinase enzyme acts on 34 bp DNA sequences called LoxP sites.

  • Saccharomyces cerevisiae 2µ plasmid: Flp recombinase enzyme acts on FRT sites.

  • Bacteriophage λ: Lambda integrase mediates the integration and excision of the phage genome into and out of the E. coli chromosome at specific att sites.

• Application: Conditional Gene Knockouts (Cre-LoxP System):

  • The Cre-LoxP system is a widely used and powerful tool in genetic engineering, particularly for creating conditional knockouts in model organisms like mice. This allows researchers to delete a specific gene (Gene of Interest - GOI) in a particular tissue or at a specific developmental stage.

  • Strategy:

    1. One line of mice is genetically engineered so that the GOI is "floxed," meaning it is flanked by two LoxP sites oriented in the same direction.

    2. A second line of mice is engineered to express Cre recombinase under the control of a tissue-specific promoter (e.g., a liver-specific promoter ensures Cre is only produced in liver cells) or an inducible promoter.

    3. When these two lines are crossed, offspring inheriting both the floxed GOI and the Cre transgene will express Cre recombinase only in the targeted tissue(s) or at the induced time.

    4. In these specific cells, Cre recombinase recognizes the LoxP sites and catalyzes recombination between them, leading to the excision (deletion) of the DNA segment (the GOI) located between the LoxP sites. The GOI is then lost as the cells divide. In other tissues where Cre is not expressed, the GOI remains intact and functional.

Diagram illustrating tissue-specific gene deletion using the Cre-LoxP system. In a specific tissue (e.g., liver), a tissue-specific promoter activates the Cre recombinase gene. Cre recombinase protein is made and then excises the gene of interest, which is flanked by LoxP sites. In other tissues, the Cre gene is off, and the gene of interest functions normally.

Summary and Further Reading 📚

This lecture has covered four main types of DNA recombination:

• Homologous Recombination (HR)

• Non-Homologous End Joining (NHEJ)

• Transposition

• Site-Specific Recombination

Each plays distinct roles in genome dynamics, from generating genetic diversity and repairing DNA damage to shaping genome architecture and providing powerful tools for genetic manipulation.