*Chapter 32* – Molecular Diagnosis in the Clinical Laboratory

Three main areas of hemetopathologic molecular testing:

a) Detection of chromosomal translocation in hematologic malignancies and inherited hematologic disorders

b) Identification of hematologically important infectious diseases

c) Monitoring of minimal residual disease after cancer treatment

Structure and Function of DNA

The central dogma: DNA to RNA to Protein The central dogma in genetics is that

information is stored in the DNA is replicated to daughter DNA, transcribed to messenger ribonucleic acid (mRNA), and translated into a functional protein

DNA is long-term storage. It is stable, packaged, and inert.

RNA is short-term storage. It is unstable and lacks secondary structure. Some RNA has enzymatic activity.

Proteins are the 'programs' of the cells. They are the physical manifestations of the abstract information recorded in the genome.

DNA at the molecular level DNA is the genetic material of all living

cells and of many viruses. DNA is: an alpha double helix of two

polynucleotide strands. The genetic code is the sequence of

bases on one of the strands. A gene is a specific sequence of bases,

which has the information for a particular protein.

DNA is self-replicating - it can make an identical copy of itself.

Replication allows the genetic information to pass faithfully to the next generation.

Replication occurs during the ‘S’ (= synthesis) stage of interphase just before nuclear division.

The chromosomes contain 90% of the cell’s DNA.

10% is present in mitochondria and chloroplasts.

Adenine (A) and Guanine (G) are purine bases

Thymine (T) and Cytosine (C) are pyrimidine bases

Hydrogen bonds link the complementary base pairs:

a) Two between A and T (A=T) b) Three between G and C (G≡C)

A single unit in the chain is a nucleotide. This consists of a phosphate

group A pentose sugar (D = DNA; R =

RNA) and An organic base (ATGC = DNA;

AUGC = RNA)

Transcription and translation

RNA Synthesis: Transcription RNA is an important type of nucleic acid

that plays several roles in the production of protein

RNA is necessary to carry the instructions of the DNA out of the nucleus and to the ribosomes

The genome of any organism contains all the information for making that organism. The information is encoded in various types of genes that are transcribed into 4 types of RNA:

a) mRNA - Messenger RNA: Encodes amino acid sequence of a polypeptide

b) tRNA - Transfer RNA: Brings amino acids to ribosomes during translation

c) rRNA - Ribosomal RNA: With ribosomal proteins, makes up the ribosomes, the organelles that translate the mRNA

d) snRNA - Small nuclear RNA: With proteins, forms complexes that are used in RNA processing in eukaryotes

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Messenger RNA carries the actual code that specifies the amino acid sequence in a polypeptide (protein)

Making mRNA starts with a protein encoding gene on a template strand of DNA

Step 1: Initiation RNA Polymerase binds to a promoter

which is a region of bases that signals the beginning of a gene

RNA Polymerase is bound to the TATA box of the promoter by transcription factors

The double helix unwinds and is ready to be transcribed

Step 2: Elongation RNA Polymerase moves along the

protein encoding gene adding new RNA nucleotides in the 5’ to 3’ direction and complimentary to the DNA template

Works at up to 60 nucleotides/second

Step 3: Termination RNA Polymerase reaches the terminator

region of the protein encoding gene All the enzymes and factors are released The product of these 3 steps is called

immature or pre-mRNA

RNA Processing Most eukaryotic protein encoding genes

contain non-coding segments called introns, which break up the amino acid coding sequence into segments called exons

RNA Processing includes modification and splicing

Modification At the 5' end, a cap is added

consisting of a modified GTP (guanosine triphosphate). This occurs at the beginning of transcription. The 5' cap is used as a recognition signal for ribosomes to bind to the mRNA

At the 3' end, a poly(A) tail of 150 or more adenine nucleotides is added. The tail plays a role in the stability of the mRNA

Splicing (Intron Removal) The intron loops out as snRNPs

(small nuclear ribonucleoprotein particles) bind to form the spliceosome

The intron is excised, and the exons are then spliced together

Results in mature mRNA

Protein Synthesis: Translation The language of nucleic acids in

translated into the language of proteins Nucleic acids have a 4 letter language Proteins have a 20 letter language Things needed in translation:

a) Messenger RNA (mRNA) Synthesized in Transcription Composed of Codons Codons are 3-base sequences of

mRNA

b) Ribosomes Made of rRNA and protein 2 subunits (large and small) form a

3D groove 2 major sites:

P site ---holds the growing polypeptide A site ---new amino acids enter here

c) Transfer RNA (tRNA) Carries amino acids to the ribosome During tRNA charging each tRNA

picks up an amino acid from the INP

d) Amino Acids There are 20 amino acids, each with

a basic structure Amino acids are held together by

peptide bonds

3 Steps in translation:1) Initiation2) Elongation3) Termination

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Step 1: Initiation 5’ G-cap of mRNA binds to ribosome Start codon AUG and anticodon with

Methionine bind a P site A site is open and ready to receive new

tRNAs

Step 2: Elongation (Adding New Amino Acids) Codon recognition Peptide bond formation Translocation : ribosome moves along

mRNA, aminoacyl tRNA shifts from A site to P site

Step 3: Termination A stop codon is reached UAA UAG UGA All parts release

Translation, Polypeptides, and Mutations Normally, the genetic code is translated

and the correct protein is formed from a long chain of amino acids.

Translation of codons is dependent on the reading frame, or a grouping of codons in a gene transcript.

Mutations: Any change in the nucleotide sequence

of DNA Mutations can involve large sections of

chromosomes or single base pairs Mutations can change the reading frame

of a gene transcript

Deletion or insertion mutations are most disruptive because they change the reading frame, causing a frame shift

Substitution mutations have varied impact on amino acid sequences.

Substitutions of 1st or 2nd base in codon almost always changes the amino acid

Substitution of 3rd base in codon does not always change the amino acid

What causes mutations?• Errors in DNA Replication• Errors in chromosome crossover in

meiosis• Mutagens

Mutagens are physical or chemical factors that cause mutations• UV Radiation and X-Rays• Chemicals like DDT

Many mutations are harmful and cause the organism to die or function incorrectly.

Some mutations are beneficial and help the organism to survive. (Peppered Moths)

If mutations are present in gametes, they can be passed on to offspring. This is the driving force of Natural Selection.

DNA replication and the cell cycle

DNA replication DNA Replication is semi-conservative Each newly synthesized molecule

contains 1 “parent template” strand and 1 new “daughter” strand

Step 1: Initiation Helicase unwinds DNA forming a

“replication fork” Multiple replication forks along a DNA

molecule create replication bubbles

Step 2: Elongation---Adding New Nucleotides RNA Primase adds a complimentary

RNA primer to each template strand as a starting point for replication

DNA Polymerase reads the template strand (3’ to 5’) and adds new complimentary nucleotides (5’ to 3’)

DNA synthesized in the direction of the replication fork is called the leading strand

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DNA polymerase can only add new nucleotides in the 5’ to 3’ direction

Because of the antiparallel nature of DNA, replication occurs in two directions

An RNA primer is laid down on the other strand, and new nucleotides are added 5’ to 3’ moving away from the replication fork. This is the lagging strand and the segment of DNA produced is called an Okazaki fragment

The DNA unwinds some more and the leading strand is extended by DNA polymerase adding more DNA nucleotides. Thus, the leading strand is synthesized continuously.

On the top template strand, a new RNA primer is synthesized by primase near the replication fork

DNA polymerase adds new DNA. This produces the second Okazaki fragment. Thus, the lagging strand is synthesized discontinously

Step 3: Termination A different type of DNA polymerase

removes the RNA primer and replaces it with DNA

DNA ligase joins the two Okazaki fragments with phosphodiester bonds to produce a continuous chain

Each new DNA molecule is rewound by helicase. Each molecule is identical

Summary and Other Facts: Leading Strand: 1 primer, 5’ to 3’

continuous Lagging Strand: multiple primers, 5’ to 3’

discontinuous In humans, DNA polymerase adds 50

nucleotides/second DNA polymerase can proofread its own

work and does excision repair 1 in 10,000 bases are in error After proofreading, rate of mutation is 1

in 10,000,000

Cell cycle

Gap 1-phase The first phase within interphase, from

the end of the previous M phase until the beginning of DNA synthesis.

It is also called the growth phase. During this phase the biosynthetic

activities of the cell, which had been considerably slowed down during M phase, resume at a high rate.

This phase is marked by synthesis of various enzymes that are required in S phase, mainly those needed for DNA replication.

Duration of G1 is highly variable, even among different cells of the same species.

S-phase S-phase starts when DNA synthesis

commences; When it is complete, all of

the chromosomes have been replicated, i.e., each chromosome has two (sister) chromatids.

Thus, during this phase, the amount of DNA in the cell has effectively doubled.

It is the longest phase of the cell cycle.

G2 -phase G2 phase, which lasts until the cell enters

mitosis. Again, significant biosynthesis occurs

during this phase, mainly involving the production of microtubules, which are required during the process of mitosis.

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Inhibition of protein synthesis during G2 phase prevents the cell from undergoing mitosis.

Stages of Mitosis1.Prophase

– The chromosomes gradually condense and appear as strands that become thicker and shorter; - The nuclear envelope breaks up.

2.Metaphase – The chromosomes are condensed; - A mitotic spindle is formed of microtubules; - Microtubules attach to the centromeres on chromosomes and to the centrioles at opposite poles of the cell

3.Anaphase – The chromatids separate and move to opposite poles

4.Telophase – The chromatids are at opposite poles of the cells - The nuclear envelope is formed

5.Cytokinesis – Division of the cytoplasm mediated by actin filament

Regulation of cell cycle A major cell cycle regulatory point

occurs in late G1 phase and controls progression from G1 to S.

This regulatory point was first defined by studies of budding yeast (Saccharomyces cerevisiae), where it is called as START

Once cells have passed START, they are committed to enter S phase.

In addition to serving as a decision point for monitoring extracellular signals, START is the point at which cell growth is coordinated with DNA REPLICATION and CELL DIVISION

Families of cyclins and cyclin dependent kinases

The cell cycles of higher eukaryotes are controlled not only by multiple cyclins

but also by multiple cdc2 related protein kinases-cdk’s

Active complexes o cyclins and CDKs exert their biological effects by phosphorylating proteins

During the G1 phase, a major target of cyclin/CDK complexes is the retinoblastoma protein (pRb).

pRb is a growth-suppressing protein whose activity is controlled by whether or not it is phosphorylated

pRb When pRb is in the dephosphorylated

form, during the G0 phase and early in the G1 phase, it is active

pRb exerts its growth-suppressing effects by binding to many cellular proteins, including the transcription factors of the E2F family

E2F transcription factors regulate the expression of numerous genes that are expressed during G1, or at the transition from the G1 to the S phase, to initiate DNA replication.

pRb that is bound to an E2F transcription factor inhibits the transcription factor's activity.

Following phosphorylation by cyclin/CDK complexes, pRb dissociates from E2F, allowing the transcription factor to bind DNA sequences and activate the expression of genes necessary for the cell to enter the S phase.

p53 The p53 protein senses DNA damage

and can halt progression of the cell cycle in G1 (by blocking the activity of Cdk2’

Under normal circumstances p53 levels remain very low due to its interaction with a member of the ubiquitin ligase family called MDM2.

The p53 protein is also a key player in apoptosis, forcing "bad" cells to commit suicide.

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So if the cell has only mutant versions of the protein, it can live on — perhaps developing into a cancer.

More than half of all human cancers do, in fact, harbor p53 mutations and have no functioning p53 protein.

An extreme case of this is Li Fraumeni syndrome, where a genetic a defect in p53 leads to a high frequency of cancer in affected individuals.

A genetically engineered adenovirus, called ONYX-015, can only replicate in human cells lacking p53. Thus it infects, replicates, and ultimately kills many types of cancer cells in vitro. Clinical trials are now proceeding to see if injections of ONYX-015 can shrink a variety of types of cancers in human patients.

Variations within different cells The human body contains a huge range

of cells ~ over 300 different types. The fastest cycling mammalian cells in

culture, crypt cells in the intestinal epithelium, have a cycle time as short as 9 to 10 hours.

Cells are most radiosensitive in late M and G2 phases and most resistant in late S phase

Some divide often through life, others divide only infrequently and some do not divide at all after birth.

The most actively dividing cells are found in areas of the body that receive a lot of wear and tear. (Skin, GIT)

Liver tissue is very interesting. It is usually permanent but when massive damage to the liver occurs, the cells can get out of G0 and undergo mitosis to repair the tissue

Telomere and Telomerase Telomeres, located at the ends of

chromosomes, are key genetic elements involved in the regulation of the cellular aging process.

Each time a normal cell divides, telomeres shorten

Once telomeres reach a certain short length, cell division halts and the cell enters a state known as replicative senescence or aging.

Thus, this shortening of the telomeres effectively serves as a molecular "clock" for cellular aging.

When the enzyme telomerase is introduced into normal cells, it can restore telomere length - reset the "clock" - thereby increasing the functional lifespan of the cells.

Importantly, it does this without altering the cells' biology or causing them to become cancerous.

Telomerase Human telomerase present at very low

levels, in most normal cells and tissues, but that during cancer progression, telomerase is abnormally reactivated in all major cancer types.

While telomerase does not cause cancer, the continued presence of telomerase enables cancer cells to maintain telomere length, providing them with indefinite replicative capacity.

It has been shown in various tumor models that inhibiting telomerase activity results in telomere shortening and causes aging or death of the cancer cell.

Development of anti-cancer therapies based on telomerase inhibitors and telomerase therapeutic vaccines

Molecular Diagnostic Testing Overview Generalities

Exploits the enzymes & processes of DNA replication to make copies of DNA sequence of interest

Amplicons -> copies Primers or Probes – short sequences

used to locate specific DNA or RNA sequences within a population of nucleic acids

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For mRNA or rRNA – amplification happens in a process called reverse transcriptase PCR

Most test use DNA amplification

Contamination Prevention Use UV rays and bleach to induce stand

breaks in work surfaces Use Uracil-N-Glycosylase to destroy

previously amplified DNA

Nucleic Acid Isolation (Extraction)

Isolating DNA from clinical specimen Used to test for mutation in patient DNA Used to test for microorganism DNA

(detecting infections) DNA is preferred since it is more stable

than RNA and easier to isolate Collection

Samples can include peripheral blood, bone marrow, tissue biopsy, needle aspiration, cheek swabs

Whole blood is collected in an EDTA tube to prevent clotting and inhibit enzymes that may digest DNA

White blood cells are separated by detergent and proteinase

A high salt solution removes cellular debris leaving DNA in the aqueous solution

The high salt solution neutralizes the negative charge of backbone allowing close contact of DNA with one another.

Addition of isopropanol precipitates DNA

Wash with 70% ethanol and resuspend in aqueous buffer solution

Isolated DNA sample can be stored at -80˚C

DNA can be taken from formalin fixed, paraffin embedded tissue sections

Tissue is obtained by microdissection by scraping or laser

Tissue is degraded by proteinase K to release DNA

Sample is heated to 94˚C for several minutes to inactivate the proteinase K and degrade other proteins

Fresh/Frozen tissue samples can be utilized as well by quickly thawing and mincing the frozen tissue

The minced tissue is mixed with an extraction buffer to release DNA and is purified and precipitated as described earlier

Isolating RNA from clinical specimen Much more difficult than DNA isolation The isolated RNA contains mRNA, rRNA,

tRNA. Large specimen may be needed to obtain

adequate amount of mRNA mRNA does not represent all

information stored in the DNA, only the genes being expressed.

Collection RNA released by cell lysis RNase inhibition with strong chemical

agents such as urea or guanidine isothiocyanate

Protein and DNA removal by using phenol at pH 4, chloroform and isoamyl alcohol. The chemicals separate DNA and protein in the organic layer while RNA remains in the aqueous phase. RNA resists acidic pH while DNA, carbohydrates, lipids and proteins are affected by an acidic pH

RNA precipitation by addition of salt to neutralize the charge of the phosphoester backbone and ethanol to make the nucleic acid insoluble

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Amplification of Nucleic Acid by Polymerase Chain Reaction

Polymerase chain reaction for amplifying DNA PCR is an enzyme based method for

amplifying a target sequence to allow its detection from a small volume of material

DNA is first denatured at 95˚C to separate the strands

It is then cooled to 40-60˚C to allow annealing (binding) of the primer to the target sequence

It is then warmed to 72˚C to promote the Taq polymerase. The Taq polymerase attaches to the primers and extend the strand to synthesize the new DNA

The cycle is then repeated A thermocycler is used to accurately

produce and monitor rapid temperature changes

Primer annealing accounts for PCR specificity but primers anneal to non-identical regions if annealing temperature is too low.

PCR controls include +, - and no DNA controls

- control contains DNA that lacks the sequence of interest

+ control contains DNA that has the sequence of interest

No DNA indicates if there is DNA contamination. Ex a band formed in no DNA means that sample is contaminated.

Reverse Transcription polymerase chain reaction for amplifying RNA

In RT PCR, reverse transcriptase enzyme produces complementary DNA (cDNA) from mRNA

Produce an RNA-CDNA hybrid by utilizing reverse transcriptase and a specialized primer called oligo(dT)

The primer anneals to the polyA tail on the 3’ end of the adenine nucleotides and the reverse transcriptase recognizes the hydroxyl group on the last nucleotides of the primer and reads the mRNA template strand and then adds the correct complementary deoxyribo nucleotides

Heat denaturation separated the mRNA from the cDNA strand so that the can act as a template for replication by DNA polymerase.

The cDNA is then amplified as in DNA based PCR using primers specific for the sequence of interest.

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Detection of Amplified DNA

Gel electrophoresis

Restriction endonuclease methods

Nucleic acid hybridization and the southern blotting

Hybridization level

DNA sequencing

Real Time Polymerase Chain Reaction

Minimal Residual disease

Infectious disease load

Current Developments

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