Si P t JSimon Petersen‐Jones
What is a DNA‐based Test Uses sample of DNA
Bl d Blood Cheek SwabId ifi h ( h ) f h i l Identifies the genotype (not phenotype) of the animal
There are Two Categories of Test:
Mutation DetectionDirect detection of presence/absence of disease causing mutation
Many potential ways of doing thisLinked marker testsLinked marker tests Genotypes animal for polymorphic marker, one version of which is known to be linked one version of which is known to be linked to the disease allele
How to create a DNA‐based test1. First identify the genetic mutation that causes
disease OR very closely map the disease on the disease OR very closely map the disease on the genome
2 Be 100% sure that the mutation causes the disease 2. Be 100% sure that the mutation causes the disease –or the mapping information is accurate
3 If the suspect mutation is proven to cause the disease 3. If the suspect mutation is proven to cause the disease then it is very easy to create a rapid and accurate test for the identification of the mutation BUT using a for the identification of the mutation BUT using a linked marker before the mutation is identified is more problematic.more problematic.
How to prove a DNA change is theHow to prove a DNA change is the cause of the disease Genetic information
Statistically significant association between the DNA change Statistically significant association between the DNA change and the disease status
BUT can be some problems:d h ll b d h h d Region surrounding the mutation will be associated with the disease
Is disease fully penetrant?
What is mutation predicted to do?What is mutation predicted to do? Null mutation Missense mutation
Any physiological supporting evidence? Functional? Gene expression?
Mutation detection tests
Di l id if h b f h l Directly identify the presence or absence of the causal mutationA i l i d i l Assuming no sample mix up and appropriate controls these are
E l Extremely accurate Extremely specific
O l d h i Only detects that gene mutation
Mutation Detection Tests Size of PCR product for insertion/deletionsS i f PCR d Sequencing of PCR product
Allele‐specific PCR PCR – restriction enzyme digestion test
Mismatch PCR – restriction enzyme digestion test TaqMan Assay
DNA‐based test for RPE65DNA based test for RPE65 Mutation
PCR a 135bp fragment spanning mutation site Resolve on acrylamide or high percentage agarose gel ‐ Resolve on acrylamide or high percentage agarose gel see 4bp difference in size
N N N A C CN N N A C C
Allele-specific PCR teste e spec c C test
Amplify inAmplify in PCR that is designed to normal affected carrier
amplify only the normal
gene + - +TESTDNA
gene
Amplify inAmplify in PCR that is designed to - + +
amplify only the mutant
gene
PCR Diagnostic Test for PRA in Cardigan Welsh Corgis1a 1b 2a 2b 3a 3b 4a 4b 5a 5b 6a 6b M
a = PCR forfor normal
b = PCR for mutant
7a 7b 8a 8b Na Nb Ca Cb Aa Ab -ve a -ve b M
PCR‐Restriction Enzyme
B
Tests
B/B A/BU A/A
BA
B/B A/BU A/A
Restriction enzymeenzyme
digestionA
d1 PRA i I i h Srcd1‐PRA in Irish SettersMutation in cGMP phosphodiesterase beta subunit
N l tid 2420 G ANucleotide 2420 G A
Normal TGG = tryptophanMutant TAG = stop codon
Exon 21
Mismatch PCR Test for rcd1‐PRA inMismatch PCR Test for rcd1 PRA in Irish Setters
Mutant allele Normal allele
PCR
Restriction Bfa1 Bsr1Bfa1 Bsr1enzyme digest
Bfa1
Mismatch PCR Test for rcd1 in Irish SettersMismatch PCR Test for rcd1 in Irish SettersM U
A C N 1 2 3
Cl t t l (1993) C E R 12 861 866Clements et al (1993) Curr Eye Res. 12:861-866
Petersen-Jones et al (1995) J Small Anim Pract. 36:310-314
Codon 616 1bp Deletion in PDE6A inCodon 616 1bp Deletion in PDE6A in Cardigan Welsh Corgis
Normal1921 TACCAGATGAAGTCCCAGAACCCACTGGCCAAGCTCCATGGGTCCTCCATCTTGGAAAGA610 Y Q M K S Q N P L A K L H G S S I L E R610 Y Q M K S Q N P L A K L H G S S I L E R
CACCACTTGGAGTTCGGCAAAACGTTGCTGCGAGATGAGAGCCTGAATATCTTTH H L E F G K T L L R D E S L N I F
Mutant1921 TACCAGATGAAGTCCCAGACCCACTGGCCAAGCTCCATGGGTCCTCCATCTTGGAAAGAC610 Y Q M K S Q T H W P S S M G P P S W K D
ACCACTTGGAGTTCGGCAAAACGTTGCTGCGAGATGAGAGCCTGAT T W S S A K R C C E M R A *
28 abnormal amino acids starting at position 616 before a premature stop codon. Normal protein is 861 amino acids.
GTCCCAGAACCCACTGGCCAAGCTCCATGNormal Allele
….GTCCCAGAACCCACTGGCCAAGCTCCATG…..| |||||||||||||||||||TAGGTGACCGGTTCGAGGTACTAGGTGACCGGTTCGAGGTAC
PCRHinfI PCRf
….GTCCCAGAATCCACTGGCCAAGCTCCATG|||||||||||||||||||||||||||||
….CAGGGTCTTAGGTGACCGGTTCGAGGTAC
….GTCCCAG|||||||CAGGGTCTTA….CAGGGTCTTA
Mutant Allele
….GTCCCAGACCCACTGGCCAAGCTCCATG…..| |||||||||||||||||||TAGGTGACCGGTTCGAGGTAC
PCRHinfI PCRHinfI
….GTCCCAGATCCACTGGCCAAGCTCCATG||||||||||||||||||||||||||||||||||||||||||||||||||||||||
….CAGGGTCTAGGTGACCGGTTCGAGGTAC
a) Normal allele
PCR
HinfI HinfIHinfI53bp 75bp
HinfI
20bp
b) Mutant allele
PCR
HinfIHinfI53bp 94bp
Results - Mismatch PCR- Restriction Enzyme Digest Test
M U1 2 3 4 5 6 A N C
Real Time PCR
TaqMan Assayq y
TaqMan AssaTaqMan Assay
TaqMan AssaTaqMan Assay
TaqMan Assay – For Diagnostic TestAllele Discrimination
UnaffectedUnaffected
Carrier
AffectedAffected
Linked marker test Identifies presence or absence of a DNA marker known to be closely linked to the disease locusto be closely linked to the disease locus
An indication of animals likely to be affected or unaffectedunaffected
PRCD Mutation
1 2 1 1 2 2
prcd *1 ?
Diff b d f d 1 21 1 1 11 1**Different breeds of dog 1 21 1
*1 1
2 21 2*** *
Marker/Linkage Test for PRCDMarker/Linkage Test for PRCD
P A2 2
Pattern A = normal
Pattern B1 2*
1 2Pattern B
most = carriers
1 1**
1 1*
1 1Pattern C
most = affected** *
Linked Marker Test for prcdp Uses DNA markers very close to the defective gene
Divides dogs into 3 groups Group A – all dogs in this group are homozygous normal
Group B – many dogs in this group are carriers but some may be normal
ll ff d d h Group C – all affected dogs are in this group, BUT also contains some carriers and some normals
www.optigen.com
Laboratories for PRA Testingg Commercial labsU i i l b University labs
Is lab reputable?
Advantages of DNA‐based Testsg Mutation detection tests allow genotype of animal to be ascertainedbe ascertained for recessive
affected carrier homozygous normal
i f b di d PRA ff d can use carriers for breeding and prevent PRA‐affected dogs from being born
Can be performed at any age Can be performed at any age Mutation detection tests are theoretically 100% accurate & 100% specific (barring human error)accurate & 100% specific (barring human error)
Collection of Samples for DNA‐Collection of Samples for DNA‐based Genetic Tests
Follow testing Lab’s instructions Follow testing Labs instructions Identification of animal C f l l b li f l Careful labeling of samples Require a source of cells with nuclei
Blood sample Cheek swab
Blood Samples for DNA Extraction
Follow testing Lab’s instructions Follow testing Labs instructions Citrate or EDTA tubs DO not allow to clot DO not allow to clot Label clearly, accurately and immediatelyK l d i h i k Keep cool – send with ice pack
Cheek Swabs for DNA Extraction Follow testing Lab’s instructionsN i hi f h f f di d Not within a few hours of feeding dog
Cytology brush to brush gently inside cheek Aiming to collect cells
Label sample clearly, accurately & immediately
Limitations of DNA‐based Tests Only applicable for a specific gene mutationO l f li bl f l i l b d Only a few tests are applicable for multiple breeds
Some conditions are very genetically heterogeneous e.g. need many different tests for PRA
Advantages Offered by DNA‐based TestsAdvantages Offered by DNA based Tests Detect affected animals prior to onset of clinical signs Mutation detection tests for recessive disease identify: Mutation detection tests for recessive disease identify:
Homozygous normal Heterozygous Heterozygous Homozygous affected
Enable retention of heterozygotes in breeding poolEnable retention of heterozygotes in breeding pool
U i C i f R i Di fUsing Carriers of Recessive Disease for Breedingg
If have a mutation detection DNA‐based test Aim is to avoid producing affected animals If carrier animal has other outstanding features it can b d f b dibe used for breeding Cross carrier with genetically normal animal Test offspring Test offspring
Use carriers if disease has high incidence Avoiding carriers will restrict gene poolAvoiding carriers will restrict gene pool
Risk of incidence of other hereditary disorder Risk of losing desirable features
Future for Eye Screening in the DNA Era –Future for Eye Screening in the DNA Era Can we hang up our ophthalmoscopes?
Tests very specific –h h l ifiophthalmoscopy not specific
Possibility of new forms of hereditary eye disease Previous low incidence – emerge via popular sire effect
Occurrence of new hereditary di i eye disease‐causing gene
mutations
We must continue frequent eye screenings