Molecularvirology /certified fixed orthodontic courses by Indian dental academy

Overview of Molecular Epidemiological
Methods for the Subtyping and
Comparison of Viruses
INDIAN DENTAL ACADEMY
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Uses of molecular
epidemiological methods
 Subtyping - in some viruses, different subtypes are
associated with different clinical manifestations e.g.
enteroviruses, adenoviruses, and human papillomaviruses.
 General Epidemiology - by identifying the viral subtypes at
different times and geographical locations, one can detect
major changes in the epidemiological patterns of infection
e.g. HIV and HCV.
 Investigation of Outbreaks - to support or disprove a link
between the donor and recipient viruses e.g. HIV, HBV,
HCV, Norwalk virus.

Methods Used - Complete or
Partial genome?
 For greatest degree of accuracy, the complete genome
should be used for the purpose of comparison.
 However, since viral genomes ranges from 3500 bp to over
200,000 bp, it would be highly impractical to sequence the
whole genome.
 Certain simple methods are still used for the comparison of
complete genomes e.g. RFLP for CMV, HSV, and
Adenoviruses.
 Nowadays in practice, a small part of the genome is
amplified first by PCR and the product investigated by
sequencing or other methods.

Strategies for identification of the PCR
Product (Commonly used methods)
 Sequencing of the PCR product
 the gold standard but expensive and not widely available.
 PCR product may be sequenced directly or cloned before sequencing.
 However, it is the test of choice in outbreak situations where there are
serious public health and/or medical-legal implications.
 Sequencing can be used to confirm results of other molecular
epidemiological assays. As a matter of fact, all other assays can be
considered as simpler screening assays.
 Restriction Fragment Length Polymorphism (RFLP) - very simple,
rapid and economical technique but the result may be difficult to read.
 Hybridization with a specific oligonucleotide probe - A wide variety of
formats is available e.g. dot-blot, Southern blot, reverse hybridization, DNA
enzyme immunoassay etc.

Principles behind Restriction Enzyme
Analysis and Hybridization Probes
EcoRI (GAATTC)
0 10032
32
68
GAATTC
Target
Target
Hybridization
Probes
REA

PCR-RFLP (PRA)
 The gene target must be present in all viral strains.
 It is amplified with primers directed against conserved areas in the
target gene so that all subtypes can be amplified.
 The PCR product is then digested with one or more restriction
enzymes and on an agarose or polyacrylamide gel.
 The species or genotype is identified from the restriction patterns seen.
 Therefore PRA can be considered as probably the simplest DNA
fingerprinting technique.
 The principle of PRA is similar to that of RFLP of whole viral
genomes and pulse field gel electrophoresis.
 It is quick, simple and cheap and this is why it is preferred by many
molecular biologists.
 Examples include HCV genotyping and identification of
mycobacteria. www.indiandentalacademy.com

Nature of Restriction Enzymes
 4-cutter Enzymes (frequency of cutting = 1/256)
taq 1 T↓CGA
Hae III G↓GCC
Sau 96I G↓GNCC
Eco RI G↓AATTC
Hind III A↓AGCTT
Not I GC↓GGCCGC

Specific Oligonucleotide
Probe
 Simple to carry out, particularly suitable for large scale testing
 Results are usually easier to read than REA and requires less skill to
interpret
 Preferred strategy by commercial companies e.g. INNO-LIPA HCV,
Sorin DEIA, Roche Amplicor and Taqman.
 Can be made into a highly automated closed system e.g. Roche-
Amplicor.
 Therefore more attractive than PRA for the routine laboratory but the
costs could be prohibitive.
 Specific nucleic acid probe assays are available where the specimen is
tested directly without amplification. However the sensitivity is much
lower.

Choice of Genomic Region
 The choice of genomic region to use for analysis is critical and
could affect the outcome of results.
 Too conserved – will not be able to demonstrate any differences
between subtypes.
 Too variable – may not be able to demonstrate a link between
source and recipient viruses in outbreak studies because of the
high mutation rate.
 In general, RFLP is not suitable to highly conserved regions
while nucleic acid probes are not suitable for highly variable
regions.
 It is often advisable to use more than one gene region, especially
where there are serious medical-legal implications.

Summary
 A wide variety of molecular epidemiological methods are available, of which
DNA sequencing is the gold standard.
 It is now usual to analyze a small part of the genome rather than the complete
genome. The target fragment is first amplfied by PCR before analysis.
 The most widely used screening methods involve either restriction enzyme
analysis or hybridization with specific nucleic acid probes, or a combination
of the two.
 Other screening methods such as SSCP, dHPLC and other heteroduplex
analysis techniques are rarely used outside a research setting because they
often suffer from poor inter-laboratory reproducibility.
 The choice of the genomic region to use is critical: it is often advisable to use
more than one genomic region.
 It is important to remember that all molecular epidemiological methods
available for viruses can be applied to bacteria but not vice-versa.

Points to Consider
 Molecular epidemiology techniques are can be used to good effect to
disprove a link between donor and recipient strains of a particular
infectious agent but they cannot prove a definitive link.
 Therefore a negative result is much greater predictive value than the
positive result.
 The probability of a link depends on many factors including the
prevalence of that particular genotype and the methods used.
 Where the outbreak carries huge medical-legal implications e.g. HIV
transmitted through blood factors, the case would have to be argued on
an individual basis in court, preferably with the help of a statistician.
 It is important to remember that molecular epidemiological
investigation does not replace a good basic epidemiological
investigation

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Molecularvirology /certified fixed orthodontic courses by Indian dental academy

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