New Molecular Based Methods of Diagnosis

New Molecular Based Methods of Diagnosis
DNA based molecular methods
Why use a molecular test to diagnose an infectious disease?
Need an accurate and timely diagnosis
Important for initiating the proper treatment
Important for preventing the spread of a contagious disease
Leading uses for nucleic acid based tests
Nonculturable agents
Human papilloma virus
Hepatitis B virus
Fastidious, slow-growing agents
Mycobacterium tuberculosis
Legionella pneumophilia
Highly infectious agents that are dangerous to culture
Francisella tularensis
Brucella species
Coccidioidis immitis
Leading uses for nucleic acid based tests
In situ detection of infectious agents
Helicobacter pylori
Toxoplasma gondii
Agents present in low numbers
HIV in antibody negative patients
CMV in transplanted organs
Organisms present in small volume specimens
Intra-ocular fluid
Forensic samples
Leading uses for nucleic acid based tests
Differentiation of antigenically similar agents
May be important for detecting specific virus genotypes associated with human cancers (Papilloma viruses)
Antiviral drug susceptibility testing
May be important in helping to decide anti-viral therapy to use in HIV infections
Non-viable organisms
Organisms tied up in immune complexes
Leading uses for nucleic acid based tests
Molecular epidemiology
To identify point sources for hospital and community-based outbreaks
To predict virulence
Culture confirmation
What are the different types of nucleic acid molecular techniques that are used?
Direct probe testing – better for identification than for detection because it is not as sensitive as amplification methods
Amplification methods – used to improve the sensitivity of the nucleic acid testing technique
Target amplification
Probe amplification
Signal amplification
Combinations of the above
Review of the Structure of DNA
DNA structure
In double stranded linear DNA, 1 end of each strand has a free 5’ carbon and 1 end has a free 3’ OH group. The two strands are in the opposite orientation with respect to each other (antiparallel).
Adenines always basepair with thymines (2 hydrogen bonds) and guanines always basepair with cytosines (3 hydrogen bonds)
The Structure of DNA
Direct probe testing
Hybridization – to come together through complementary base-pairing. Can be used in identification. In colony hybridization the colony is treated to release the nucleic acid which is then denatured to single strands. Labeled single-stranded DNA (a probe) unique to the organism you are testing for is added and hybridization is allowed to occur. Unbound probe is washed away and the presence of bound probe is determined by the presence of the label.
Direct probe testing
Target amplification
Target amplification requires that the DNA to be tested for be amplified, i.e., the number of copies of the DNA is increased.
To understand this we must first review the activity of the enzyme, DNA polymerase, that is used to amplify the DNA.
Polymerase template and primer requirements
DNA polymerase cannot initiate synthesis on its own. It needs a primer to prime or start the reaction. The primer is a single stranded piece of DNA that is complementary to a unique region of the sequence to be amplified.
Polymerase template and primer requirements
DNA synthesis
Synthesis can occur only in the 5’ to 3’ direction.
DNA synthesis
Remember that DNA replication is semiconservative:
Target amplification – The PCR reaction
Polymerase chain reaction – used to amplify something found in such small amounts that without PCR it would be undetectable. Uses two primers, one that binds to one strand of a double-stranded DNA molecule, and the other which binds to the other strand of the DNA molecule, all four nucleotides and a thermostable DNA polymerase. The primers must be unique to the DNA being amplified and they flank the region of the DNA to be amplified.
PCR
The PCR reaction has three basic steps
Denature – when you denature DNA, you separate it into single strands (SS).
In the PCR reaction, this is accomplished by heating at 950 C for 15 seconds to 1 minute.
The SS DNA generated will serve as templates for DNA synthesis.
Anneal – to anneal is to come together through complementary base-pairing (hybridization). During this stage in the PCR reaction the primers base-pair with their complementary sequences on the SS template DNA generated in the denaturation step of the reaction.
PCR
The primer concentration is in excess of the template concentration.
The excess primer concentration ensures that the chances of the primers base-pairing with their complementary sequences on the template DNA are higher than that of the complementary SS DNA templates base-pairing back together.
The annealing temperature used should ensure that annealing will occur only with DNA sequences that are completely complementary. WHY?
The annealing temperature depends upon the lengths and sequences of the primers. The longer the primers and the more Gs and Cs in the sequence, the higher the annealing temperature. WHY?
The annealing time is usually 15 seconds to 1 minute.
PCR
Extension – during this stage of the PCR reaction, the DNA polymerase will use dNTPs to synthesize DNA complementary to the template DNA. To do this DNA polymerase extends the primers that annealed in the annealling step of the reaction.
The temperature used is 720 C since this is the optimum reaction temperature for the thermostable polymerase that is used in PCR. Why is a thermostable polymerase used?
The extension time is usually 15 seconds to 1 minute.
The combination of denaturation, annealing, and extension constitute 1 cycle in a PCR reaction.
PCR
Most PCR reaction use 25 to 30 of these cycles to amplify the target DNA up to a million times the starting concentration.
PCR
PCR reactions in the lab
We will be doing two different PCR reactions in the lab.
For the first PCR reaction we will be using what are called consensus sequence primers. These are primers that will bind to unique regions of the 16S ribosomal genes found in all bacteria. The sequences of these primers are not unique to a specific kind of bacteria, but they are unique to a conserved region (consensus sequence) of DNA found in the 16S ribosomal genes of all bacteria. They will be used to amplify a portion of the 16S ribosomal gene of an unknown bacteria.
PCR reactions in the lab
The sequence of the amplified DNA will be determined.
The identity of the unknown will be determined by searching the DNA sequence databases.
Note that that DNA of all bacteria should be amplified and yield a product using these consensus primers.
For the second PCR reaction we will be using primers that are unique to the genes that encode the shiga-like toxin produced by EHEC.
Only the DNA of those bacteria that carry the shiga-like toxin gene will be amplified and yield a product when using these primers.
For diagnostic purposes, only the second type of PCR, in which primers unique to a single type of organism or gene are used, is practical.
What are the advantages of using a molecular test?
High sensitivity
Can theoretically detect the presence of a single organism
High specificity
Can detect specific genotypes
Can determine drug resistance
Can predict virulence
Speed
Quicker than traditional culturing for certain organisms
What are the advantages of using a molecular test?
Simplicity
Some assays are now automated
What are the disadvantages of using a molecular test?
Expensive
So specific that must have good clinical data to support infection by that organism before testing is initiated.
Will miss new organisms unless sequencing is done as we will be doing in the lab for our molecular unknowns (not practical in a clinical setting).
May be a problem with mixed cultures – would have to assay for all organisms causing the infection.
What are the disadvantages of using a molecular test?
Too sensitive? Are the results clinically relevant?