Recombinant DNA Technology

Introduction

• Harper's Illustrated Biochemistry, 32nd Ed, p. 456
• Basic Medical Biochemistry: A Clinical Approach, 6th Ed, p. 542

I. Tools and Key Reagents of Recombinant DNA Technology

A. Restriction Endonucleases ("Molecular Scissors")

• Type I: Cleave DNA at sites remote from their recognition sequence; require ATP and SAM
• Type II: Cleave at or near the recognition sequence; most useful in rDNA work
• Type III: Cleave ~25 bp downstream from recognition site

• Sticky/cohesive ends - staggered cuts leaving single-stranded overhangs (e.g., EcoRI: 5'-GAATTC-3')
• Blunt ends - flush cuts with no overhangs (e.g., SmaI: 5'-CCCGGG-3')

• Basic Medical Biochemistry, 6th Ed, p. 542-543; Harper's, 32nd Ed, p. 456-457

B. DNA Ligase ("Molecular Glue")

C. Vectors (Cloning Vehicles)

• Origin of replication (ori)
• Selectable marker (e.g., antibiotic resistance gene)
• Multiple cloning site (MCS)/polylinker containing RE sites
• Promoter for expression vectors

D. Reverse Transcriptase

II. Core Techniques

A. Gene Cloning - Step-by-Step

1. Isolation of DNA - Target gene isolated from genomic DNA or synthesized as cDNA
2. Restriction digestion - Both vector and insert cut with same RE to generate compatible ends
3. Ligation - T4 DNA ligase joins insert into vector, creating recombinant plasmid
4. Transformation - Recombinant vector introduced into host (usually E. coli)
5. Selection - Colonies grown on antibiotic-containing plates; recombinant colonies identified
6. Screening - Colony hybridization or PCR used to confirm presence of correct insert

B. Polymerase Chain Reaction (PCR)

• Template DNA
• Two specific oligonucleotide primers flanking the target sequence
• Thermostable DNA polymerase (Taq polymerase from Thermus aquaticus)
• All four dNTPs (in excess)
• MgCl2 as cofactor

1. Denaturation at 94-95°C - hydrogen bonds broken, DNA strands separate
2. Annealing at 50-65°C - primers bind to complementary sequences on each strand
3. Extension at 72°C - Taq polymerase extends from 3'-OH of each primer

• Detection and quantification of infectious agents (HIV, Hepatitis B/C, Ebola, COVID-19)
• Genetic diagnosis - detection of mutations (e.g., sickle cell disease, cystic fibrosis)
• Forensic DNA analysis (from single cells, hair follicles)
• Tissue typing for transplantation
• RT-PCR (Reverse Transcriptase PCR): used for RNA/mRNA quantitation and for detecting RNA viruses
• Quantitative real-time PCR (qPCR): measures amplification in real time for gene expression studies
• Prenatal diagnosis - detection of fetal genetic disorders
• Chromatin immunoprecipitation (ChIP-PCR): mapping in vivo protein-DNA interactions genome-wide
• Harper's Illustrated Biochemistry, 32nd Ed, p. 464-465

C. DNA Hybridization and Southern/Northern Blotting

1. DNA digested with REs → fragments separated by agarose gel electrophoresis
2. DNA denatured and transferred to nitrocellulose/nylon membrane
3. Membrane incubated with labeled (radioactive/fluorescent) DNA probe
4. Probe hybridizes specifically to complementary sequences
5. Detected by autoradiography or chemiluminescence

D. DNA Libraries

• Contains fragments representing the entire genome of an organism
• Created by partial restriction digestion of total DNA
• Includes introns, exons, regulatory regions
• Stored in phage or cosmid vectors

• Constructed from mRNA (via reverse transcriptase)
• Represents only expressed genes of a specific cell type at a specific time
• Lacks introns - directly expressible in prokaryotes
• Tissue-specific (e.g., liver cDNA library is rich in albumin, coagulation factors)

E. DNA Sequencing

• Uses dideoxynucleotides (ddNTPs) that lack a 3'-OH group
• When incorporated, terminate chain elongation
• Four separate reactions with ddATP, ddTTP, ddGTP, ddCTP
• Products separated by gel electrophoresis; sequence read from bottom (smallest) to top

• Sequences millions of DNA fragments simultaneously on a single flow cell
• Each dNTP is tagged with a distinct fluorophore + chemical blocking group on 3'-OH
• Nucleotides added one at a time; fluorescence signal recorded by computer
• Computer identifies overlaps and assembles complete sequence
• Can sequence an entire human genome in <1 day
• Cost has fallen from ~$350 million (HGP, 2003) to approximately $200-600 (2024)
• Basic Medical Biochemistry, 6th Ed, p. 553-554; Harper's, 32nd Ed, p. 465

F. DNA Microarrays (DNA Chips)

• Glass chips dotted with thousands of known DNA sequences (probes)
• Sample mRNA is reverse-transcribed to cDNA, labeled with fluorescent dyes
• Hybridization with chip reveals global gene expression patterns
• Two-color arrays compare normal vs. diseased tissue simultaneously
• Applications: Cancer subtyping, drug response prediction, infectious disease diagnosis

G. Restriction Fragment Length Polymorphism (RFLP) Analysis

• Individual variation in the number or position of RE cut sites creates fragments of different lengths
• These polymorphisms are inherited in a Mendelian fashion
• Used as genetic markers (linked to disease genes)
• Detected by Southern blotting
• Applications: Prenatal diagnosis, carrier detection, forensic DNA fingerprinting, paternity testing

III. Expression of Cloned Genes - Producing Recombinant Proteins

• Prokaryotic (E. coli): High yield, rapid, inexpensive; cannot perform post-translational modifications (glycosylation, disulfide bonds); suitable for small non-glycosylated proteins (e.g., insulin, some interferons)
• Yeast (S. cerevisiae, Pichia pastoris): Can perform glycosylation; good for secreted proteins
• Baculovirus/Insect cell: Complex glycosylation; high yield
• Mammalian cell culture (CHO cells): Human-type glycosylation; required for complex proteins (e.g., erythropoietin, factor VIII, tPA, monoclonal antibodies)

IV. Clinical Applications of Recombinant DNA Technology

A. Therapeutic Protein Production

• Harper's Biochemistry, 32nd Ed, p. 465; Basic Medical Biochemistry, 6th Ed, p. 569

B. Recombinant Vaccines

• Hepatitis B vaccine: First recombinant vaccine approved for human use; HBsAg gene expressed in yeast (S. cerevisiae); replaced plasma-derived vaccines (safety concern: HIV transmission)
• HPV vaccine (Gardasil): Virus-like particles (VLPs) of L1 capsid protein; prevents cervical cancer
• COVID-19 mRNA vaccines (Pfizer-BioNTech, Moderna): Synthetic mRNA encoding spike protein encapsulated in lipid nanoparticles; a landmark application of nucleic acid technology
• COVID-19 recombinant protein vaccine (Novavax): Spike protein VLPs with adjuvant
• Attenuated vaccines can be made safer by using rDNA to delete virulence genes

C. Molecular Diagnosis

• Sickle cell disease: Point mutation in beta-globin gene (GAG→GTG); detected by Southern blot (MstII site abolished), PCR + allele-specific oligonucleotide (ASO) probes, or direct sequencing
• Thalassemias: Deletions/mutations in alpha/beta globin genes; detected by PCR-based methods
• Cystic fibrosis: Deletions in CFTR gene (DeltaF508 most common); PCR + RFLP or ASO analysis
• Duchenne muscular dystrophy: Large deletions in dystrophin gene on X chromosome
• Huntington disease: CAG trinucleotide repeat expansion; PCR-based analysis

• PCR for HIV viral load, Hepatitis B/C viral load (guides antiviral therapy)
• Detection of drug resistance mutations (e.g., HIV RT mutations causing AZT resistance)
• Rapid COVID-19 diagnosis by RT-PCR

• Philadelphia chromosome: BCR-ABL fusion gene detected by RT-PCR in CML (guides imatinib therapy)
• BRCA1/BRCA2 mutations: Sequencing for hereditary breast/ovarian cancer risk
• KRAS mutations: PCR-based sequencing guides anti-EGFR therapy in colorectal cancer
• Liquid biopsy: Cell-free circulating tumor DNA (ctDNA) detected by NGS in plasma - non-invasive cancer monitoring

D. Forensic Medicine and Paternity Testing

• STR (Short Tandem Repeat) profiling: PCR amplification of hypervariable microsatellite loci; probability of two unrelated individuals having identical profiles <1 in 10^15
• DNA fingerprinting: Used in criminal investigations, mass disaster victim identification
• DNA can be analyzed from a single hair follicle, sperm cell, or minute blood stain
• Admissible as evidence in legal proceedings worldwide

E. Gene Therapy

1. Gene replacement - add a functional copy of a defective gene
2. Gene silencing - inhibit overactive/mutant gene expression (antisense, siRNA, miRNA)
3. Gene editing - correct the mutation at its source (CRISPR-Cas9)

• Retroviral vectors: Integrate into host genome (risk of insertional mutagenesis); used for ex vivo modification of hematopoietic stem cells
• Adenoviral vectors: High transduction efficiency; episomal (non-integrating); strong immune response (as seen in Jesse Gelsinger case - ornithine transcarbamylase deficiency trial, 1999)
• Adeno-associated viral vectors (AAV): Low immunogenicity; site-specific integration; preferred for in vivo gene therapy
• Lentiviral vectors: Infect non-dividing cells; integrating; used in CAR-T cell therapy

• ADA-SCID (1990): Dr. William French Anderson - first successful gene therapy; retroviral vector carrying adenosine deaminase gene introduced into T lymphocytes of a 4-year-old girl; replaced gene remained stable into adulthood
• CFTR gene therapy: Adenoviral aerosol delivery to lung cells in cystic fibrosis; moderate improvement but no stable integration
• Beta-thalassemia and Sickle cell disease: Lentiviral vectors delivering functional beta-globin genes into hematopoietic stem cells; long-term remission in trials
• RPE65 mutation (Leber congenital amaurosis): AAV2 vector - first approved gene therapy product (Luxturna, FDA 2017) restoring vision
• Harper's Biochemistry, 32nd Ed, p. 465-466; Basic Medical Biochemistry, 6th Ed, p. 572

V. Recent Advances in Recombinant DNA Technology

A. CRISPR-Cas9 Genome Editing

• Mechanism: Guide RNA (gRNA, ~20 nt) directs Cas9 endonuclease to a specific genomic site complementary to the gRNA sequence (adjacent to a PAM sequence - NGG for SpCas9); Cas9 creates a double-strand break (DSB)
• Repair mechanisms:
  • NHEJ (Non-Homologous End Joining): Error-prone; causes insertions/deletions (indels) → gene disruption/knockout
  • HDR (Homology-Directed Repair): Precise correction using a supplied DNA template
• Applications:
  • Sickle cell disease and beta-thalassemia: CRISPR-based re-activation of fetal hemoglobin (HbF) by editing BCL11A enhancer; Casgevy (exagamglogene autotemcel) - first CRISPR therapy approved by FDA (December 2023) for sickle cell disease and beta-thalassemia
  • Transthyretin amyloidosis treatment
  • Ex vivo CAR-T cell engineering
  • Base editing and prime editing (newer, more precise variants)

B. Next-Generation Sequencing (NGS) and Whole Genome/Exome Sequencing

• NGS cost has dropped >million-fold since the Human Genome Project (completed 2003, cost ~$350 million)
• Whole Exome Sequencing (WES): Sequences all ~22,000 protein-coding genes; identifies causative mutations in rare Mendelian disorders
• Whole Genome Sequencing (WGS): Complete genome; identifies regulatory and structural variants
• Clinical use: Diagnosis of undiagnosed rare diseases, pharmacogenomics (predicting drug response), newborn screening programs
• Third-generation (long-read) sequencing: Oxford Nanopore (real-time, portable) and PacBio SMRT sequencing resolve repetitive regions, structural variants, and epigenetic marks simultaneously

C. mRNA Therapeutics

• The COVID-19 pandemic accelerated mRNA technology into mainstream medicine
• Modified mRNA (using N1-methyl-pseudouridine) reduces immunogenicity and increases stability
• mRNA encapsulated in lipid nanoparticles (LNPs) for delivery
• Applications beyond vaccines: mRNA-based protein replacement therapies, cancer immunotherapy, rare metabolic diseases
• Key advantage: No risk of genomic integration

D. Antisense Oligonucleotides and RNA Interference

• Antisense oligonucleotides (ASOs): Single-stranded DNA/RNA analogs that bind target mRNA → RNase H-mediated degradation or splicing correction
  • Nusinersen (Spinraza): ASO for spinal muscular atrophy (SMA) - corrects SMN2 splicing; FDA approved 2016
  • Inotersen: ASO for transthyretin amyloidosis
• siRNA (small interfering RNA): Double-stranded RNA ~21 bp → RISC complex → sequence-specific mRNA degradation
  • Patisiran (Onpattro): First FDA-approved siRNA drug (2018); targets TTR mRNA for transthyretin amyloidosis
  • Inclisiran: siRNA targeting PCSK9 mRNA; reduces LDL-cholesterol; given twice yearly

E. Proteomics and Systems Biology

• Two-dimensional (2D) gel electrophoresis: Separates proteins by isoelectric point (1st dimension) and molecular weight (2nd dimension)
• Mass spectrometry: Protein identification by peptide mass fingerprinting (MALDI-TOF) or tandem MS/MS
• Cancer proteomics: Comparing normal vs. tumor cell proteomes identifies upregulated/downregulated proteins
• Enables personalized oncology - druggable targets specific to each patient's tumor
• Phosphoproteomics: Maps post-translational modifications (phosphorylation) relevant to signaling and cancer

F. Pharmacogenomics

• Uses rDNA-based genetic testing to predict individual variation in drug response
• Examples:
  • CYP2D6, CYP2C9, CYP2C19 polymorphisms - affect metabolism of codeine, warfarin, clopidogrel
  • HLA-B*5701 testing before abacavir (HIV drug) to avoid hypersensitivity reaction
  • TPMT genotyping before azathioprine/6-mercaptopurine for autoimmune diseases/leukemia
  • EGFR/KRAS/ALK mutations guide targeted therapy in non-small cell lung cancer

VI. Ethical and Social Considerations

• Germ-line editing: Heritable modifications raise profound ethical concerns (He Jiankui case, 2018 - CCR5 gene editing in human embryos, widely condemned)
• Cloning: Somatic cell nuclear transfer (SCNT); therapeutic cloning for stem cells vs. reproductive cloning
• Genetic privacy: Risk of insurance/employment discrimination based on genetic data
• Gene drive technology: Potential ecological consequences of spreading genetic traits in wild populations
• Access and equity: High cost of CRISPR-based therapies (Casgevy priced at ~$2.2 million/patient) limits global access

VII. Impact on Hematology (Integration with Clinical Biochemistry)

• Molecular basis of thalassemias and sickle cell disease defined by rDNA techniques
• Recombinant erythropoietin (rEPO) available for anemia of chronic kidney disease
• Coagulation factor deficiencies (hemophilia A and B): Recombinant Factor VIII and IX replaced plasma-derived concentrates (risk of HIV/hepatitis transmission eliminated)
• BCR-ABL detection by RT-PCR monitors minimal residual disease (MRD) in CML patients on imatinib
• Leukemia: Chromosomal translocations characterized by Southern blot and FISH; monoclonal antibodies (rituximab, trastuzumab) produced by rDNA technology
• Harper's Illustrated Biochemistry, 32nd Ed, p. 544-550

Summary Table: Key Enzymes and Their Roles

Key Points for Exam

1. Restriction enzymes recognize palindromic sequences and generate either sticky or blunt ends
2. cDNA libraries are preferred for expression in prokaryotes (lack introns)
3. PCR requires two primers, Taq polymerase, and three temperature cycles; produces 2^n copies
4. Southern blot = DNA; Northern blot = RNA; Western blot = Protein
5. NGS can sequence a human genome in <1 day at ~$200-600 cost (vs. $350 million for HGP)
6. First recombinant vaccine = Hepatitis B (1986); produced in yeast
7. Recombinant human insulin (Humulin) was the first therapeutic rDNA product (FDA 1982)
8. First gene therapy = ADA-SCID (1990, Dr. William French Anderson) using retroviral vector
9. CRISPR-Cas9 uses guide RNA for precision genome editing; first approved therapy (Casgevy) for sickle cell disease/beta-thalassemia (FDA December 2023)
10. Pharmacogenomics uses rDNA techniques to personalize drug therapy

RECOMBINANT DNA TECHNOLOGY

INTRODUCTION (Definition)

Harper's Illustrated Biochemistry, 32nd Ed, p. 456

I. TOOLS AND REAGENTS

1. Restriction Endonucleases ("Molecular Scissors")

• Bacterial enzymes that recognize specific palindromic DNA sequences (4-6 bp) and cleave both strands
• Generate either:
  • Sticky/cohesive ends - staggered cuts with single-stranded overhangs → allow annealing with complementary sequences (e.g., EcoRI: 5'-G↓AATTC-3')
  • Blunt ends - flush cuts, no overhangs (e.g., SmaI: 5'-CCC↓GGG-3')
• Type II restriction enzymes are most useful in rDNA work

2. DNA Ligase

• T4 DNA ligase seals phosphodiester bonds between insert and vector
• Can join both sticky-end and blunt-end fragments

3. Vectors (Cloning Vehicles)

• Origin of replication (ori)
• Selectable marker (antibiotic resistance gene)
• Multiple Cloning Site (MCS)/polylinker
• Promoter (in expression vectors)

4. Reverse Transcriptase

• Synthesizes cDNA (complementary DNA) from mRNA
• cDNA lacks introns → directly expressible in prokaryotes

II. STEPS IN GENE CLONING

Target gene → RE digestion → Insertion into vector (ligation)
→ Transformation into E. coli → Selection → Screening → Clone

1. Isolation of target gene/fragment
2. Restriction digestion of both insert and vector with same RE
3. Ligation using T4 DNA ligase → recombinant plasmid formed
4. Transformation into competent E. coli host
5. Selection on antibiotic plates (only transformants survive)
6. Screening by colony hybridization, PCR, or blue-white selection

III. KEY TECHNIQUES

A. Polymerase Chain Reaction (PCR)

• HIV/Hepatitis B,C/COVID-19 viral load detection
• Genetic diagnosis (sickle cell, cystic fibrosis)
• Forensic DNA analysis (from a single cell)
• RT-PCR for RNA viruses and gene expression studies
• Minimal residual disease (MRD) monitoring in leukemia

B. DNA Libraries

C. Southern/Northern/Western Blotting

1. DNA digested with RE → electrophoresis on agarose gel
2. DNA denatured and transferred to nitrocellulose membrane
3. Hybridized with labeled probe → autoradiography

D. DNA Sequencing

• Uses dideoxynucleotides (ddNTPs) lacking 3'-OH → chain termination
• Four parallel reactions with ddATP, ddTTP, ddGTP, ddCTP
• Products separated by electrophoresis → sequence read gel

• Sequences millions of fragments simultaneously on a flow cell
• Each dNTP labeled with distinct fluorophore + blocking group at 3'-OH
• Computer assembles sequence from overlapping reads
• Entire human genome sequenced in <1 day at ~$200-600 (vs. $350 million for Human Genome Project)

Basic Medical Biochemistry, 6th Ed, p. 553-554

E. DNA Microarrays (Gene Chips)

• Thousands of known DNA probes spotted on a glass chip
• Sample mRNA reverse-transcribed to fluorescent cDNA → hybridized to chip
• Identifies global gene expression patterns
• Clinical use: Cancer subtyping, drug response prediction, infectious disease profiling

F. RFLP Analysis

• Variation in RE cut sites between individuals creates fragments of different sizes
• Detected by Southern blot; inherited as Mendelian markers
• Applications: Carrier detection, prenatal diagnosis, forensic DNA fingerprinting

IV. EXPRESSION OF CLONED GENES - RECOMBINANT PROTEINS

V. CLINICAL APPLICATIONS

A. Therapeutic Recombinant Proteins

B. Recombinant Vaccines

• Hepatitis B vaccine - First recombinant vaccine for humans; HBsAg expressed in yeast (S. cerevisiae)
• HPV vaccine (Gardasil) - Virus-like particles of L1 capsid protein; prevents cervical cancer
• COVID-19 vaccines - mRNA (Pfizer/Moderna) and recombinant protein (Novavax) vaccines

C. Molecular Diagnosis

• Sickle cell disease: Point mutation (GAG→GTG) in beta-globin; detected by PCR + ASO probes or Southern blot (MstII site abolished)
• Cystic fibrosis: ΔF508 deletion in CFTR; PCR-based analysis
• Thalassemias: Alpha/beta-globin mutations; multiplex PCR
• Huntington disease: CAG trinucleotide repeat expansion; PCR

• BCR-ABL fusion gene by RT-PCR in CML → guides imatinib therapy
• BRCA1/2 sequencing for hereditary breast/ovarian cancer risk
• KRAS/EGFR mutations guide targeted therapy in lung/colorectal cancer
• Liquid biopsy - Cell-free circulating tumor DNA (ctDNA) by NGS in plasma - non-invasive cancer monitoring

• STR (Short Tandem Repeat) profiling: Probability of random match <1 in 10¹⁵
• DNA from a single hair follicle, sperm cell, or blood stain can be typed

D. Gene Therapy

1. Gene replacement - Add functional copy of defective gene
2. Gene silencing - Antisense/siRNA/miRNA
3. Gene editing - Correct mutation at source (CRISPR-Cas9)

• Retroviral - Integrate; used for ex vivo HSC modification (risk: insertional mutagenesis)
• Adenoviral - High efficiency; non-integrating; immunogenic (Jesse Gelsinger, 1999 - death in OTC deficiency trial)
• AAV (Adeno-Associated Viral) - Low immunogenicity, preferred for in vivo therapy
• Lentiviral - Infects non-dividing cells; used in CAR-T cell engineering

VI. RECENT ADVANCES

A. CRISPR-Cas9 Genome Editing

• Guide RNA (gRNA, ~20 nt) directs Cas9 endonuclease to specific genomic site
• Cas9 creates a double-strand break (DSB)
• Repaired by:
  • NHEJ - Error-prone → indels → gene knockout
  • HDR - Precise correction using a DNA template
• Casgevy (FDA December 2023) - reactivates fetal hemoglobin (HbF) by editing BCL11A enhancer in HSCs; approved for sickle cell disease and beta-thalassemia
• Newer variants: Base editing (no DSB, single base changes) and Prime editing ("find-and-replace" at the genome level)

Cetin B et al., Expert Rev Mol Med, 2025 (PMID: 40160040)

B. mRNA Therapeutics

• Modified mRNA (N1-methyl-pseudouridine substitution) reduces immunogenicity
• Delivered via lipid nanoparticles (LNPs)
• COVID-19 pandemic established mRNA platform; now being extended to:
  • Rare metabolic diseases (e.g., methylmalonic acidemia, PA)
  • Personalized cancer neo-antigen vaccines
  • Protein replacement therapy

C. RNA Interference and Antisense Therapies (Approved Examples)

D. Pharmacogenomics

• CYP2D6, CYP2C9, CYP2C19 polymorphisms - metabolism of codeine, warfarin, clopidogrel
• HLA-B*5701 testing before abacavir (prevents severe hypersensitivity)
• TPMT genotyping before azathioprine/6-MP
• EGFR/ALK/ROS1 mutation testing for targeted lung cancer therapy

E. Third-Generation (Long-Read) Sequencing

• Oxford Nanopore Technology (ONT): Portable, real-time, single-molecule sequencing
• PacBio SMRT: Long reads resolve repetitive regions and structural variants
• Can detect DNA methylation (epigenetic marks) simultaneously with sequence

VII. IMPACT ON HEMATOLOGY

• Molecular basis of thalassemias and hemoglobinopathies established by gene cloning/sequencing
• Recombinant EPO transformed management of anemia of CKD
• Recombinant Factor VIII and IX replaced plasma-derived concentrates (eliminated HIV/hepatitis B/C transmission risk)
• BCR-ABL RT-PCR for MRD monitoring in CML patients on imatinib
• First gene therapy disease: ADA-SCID (lymphocytes are highly sensitive to adenosine deaminase deficiency)

Harper's Illustrated Biochemistry, 32nd Ed, p. 544-550

SUMMARY / KEY POINTS TO REMEMBER