Genotyping Issues. Chapter 15

Transcription

1 Genotyping Issues Chapter 15

2 Turns peaks into profiles Overall goal is to determine a DNA profile: 1. Extract and isolate DNA from sample 2. PCR amplify DNA With fluorescent dye labels 3. Separate PCR products 4. Detect PCR products 5. Analyze peaks and determine the genotypes

3 Data Collection Peak Identification Matrix file Internal sizing standard (e.g., GS500-ROX) Allelic ladder sample Color Separation Peak Sizing Comparison to Allelic Ladder Genotype Assignment to Alleles GeneScan or FMBIO Analysis software Genotyper or StaR Call software GeneMapperID software Data Review by Analyst/Examiner Confirmation of Results by Second Analyst/Examiner Expert Systems under Development (e.g., True Allele) Figure 15.1, J.M. Butler (2005) Forensic DNA Typing, 2 nd Edition 2005 Elsevier Academic Press

4 Data Collection Techniques: Common Electrophoresis Machines ABI Capillary machine Currently most commonly used 96 capillaries currently FMBIO Polyacrylamide gel Then scan gel to read bands

5 Color Separation Matrixes files: Assign the correct color spectra to the correct fluorescent dye Subtract overlap between dyes Run each dye separately To determine the true spectra in given conditions Then run dyes together To determine amount of overlap

6 (A) ABI 310 Matrix Samples Blue (5FAM) (B) ABI 3100 Matrix (Spectral Calibration) Sample Green (JOE) Red (ROX) Yellow (NED) Green (JOE) Blue (5FAM) Yellow (NED) Red (ROX) Separate samples run for each dye color Each sample contains multiple peaks All peaks labeled with the same dye color Single sample run containing all dye colors Only one peak per dye color Injected into each capillary of the array A separate spectral calibration file is created for each capillary Figure 14.5, J.M. Butler (2005) Forensic DNA Typing, 2 nd Edition 2005 Elsevier Academic Press

7 Overlapping Spectra Although each dye emits its maximum fluorescence at a different wavelength they do overlap quite a bit Where laser reads emission Fluorescence Wavelength (nm)

8 Peak Sizing Internal size standards are run within each lane/capillary Size standard is a pre-determined ladder Every peak is a known sized product Before you can determine the size of the sample PCR products Must go through and label every peak in the size standard Then computer uses size standard to determine the size of sample peaks

9 (a) (b) DNA Size bp bp DNA fragment peaks are sized based on the sizing curve produced from the points on the internal size standard 100 DNA fragment peaks in sample Data Point Figure 15.2, J.M. Butler (2005) Forensic DNA Typing, 2 nd Edition 2005 Elsevier Academic Press

10 Size Algorithms Local Southern Method: Take two size standard peaks above the unknown sample Two size standard peaks below Therefore can only accurately call unknown peaks that exist within the 50 bp to 490 bp range Any peaks outside this range cannot be called with standard size standard mix

11 Size Algorithms Global Southern Method: Fitting all the size standard peaks onto a best fit line Then using one line to determine all unknown peaks Seems to be more accurate method Still need to keep within 50 to 490 bp range

12 Comparison to Allelic Ladder First all peaks are sized In terms of base pairs Then determine the actual allele of each peak Allele = the number of repeat units of the STR marker Need to be able to compare alleles between different labs with different technologies

13 Allelic Ladder Allelic ladder contains every single allele for the marker within one lane Determine the size of all peaks within the allelic ladder Then compare sample peaks to the allelic ladder determine which alleles present Sample peaks need to be exactly the same as the allelic ladder to be called with confidence

14 D8S1179 (12 alleles) D21S11 (24 alleles) D7S820 (10 alleles) CSF1PO (10 alleles) Blue panel D3S1358 (8 alleles) TH01 (10 alleles) D13S317 (8 alleles) D16S539 (9 alleles) D2S1338 (14 alleles) Green panel D19S433 (15 alleles) VWA (14 alleles) TPOX (8 alleles) D18S51 (23 alleles) Yellow panel AMEL (2 alleles) D5S818 (10 alleles) FGA low (19 alleles) FGA high (9 alleles) Red panel 100 bp 139bp 150 bp 160 bp 200 bp 250 bp* Orange panel 300 bp 340 bp 350 bp LIZ-labeled GS500 DNA sizing standard Figure 5.6, J.M. Butler (2005) Forensic DNA Typing, 2 nd Edition 2005 Elsevier Academic Press

15 Off-ladder Alleles Accepted range of any allele Within 0.5 bp of allelic ladder Any peaks that are outside this range are considered Off-ladder Two types of off ladder peaks: Peaks that are bigger or smaller than ladder s complete range Peaks that exist between the ladder s peaks

16 Off-ladder Alleles If allele is smaller than smallest allele in ladder genotype is written: CSF1PO < 6 If allele is larger than largest allele: CSF1PO > 15 Alleles between alleles written: TH (8 repeat units plus 3 bases) Or TH01 8.x

17 Off-ladder Alleles An expert may call a technically off-ladder allele as the closest allele For example if the peak is only 0.7 bps off Or if they know that certain colors always run a certain amount high or low Experts should compare any off ladder alleles to databases of known microvariants More than 220 microvariants are known for 13 core STR markers

18 Genotyping (Calling Alleles) If only one allele is present for a given STR marker = homozygous If two alleles are present = heterozygote Genotype is usually reported as number of repeats present Number of time the repeat unit is present Based on the size of PCR product All STR markers genotypes are put together forming a DNA profile

19 Genotyping Software Currently software exists that can: Size all peaks automatically given the size standard s known sizes Then compare unknown peaks to allelic ladders and determine the alleles automatically Convert the alleles into a genotype Collect all alleles into a DNA profile

20 COfiler STR data GeneScan view Genotyper view Allele call (repeat number) determined by comparison of peak size (bp) to allelic ladder allele peak sizes run under the same electrophoretic conditions Peak height in relative fluorescence units (RFUs)

21 Data Review All computer analyzed alleles must be evaluated by a real life expert Computer algorithms will call alleles based on the information you feed into it: The size standard s expected sizes The alleles present in the allelic ladder The acceptable bin for each unknown peak Natural variations require that a human actually examine the data and verify the alleles make common sense

22 Allele Binning Before computer can determine any alleles Give computer a base pair size range Known as a bin Based on allelic ladder Standard recommendation is 0.5 bps below and above each peak in the ladder However, the ladder s alleles may show variation Not always at exact same position!

23 Allele Binning Say the allelic ladder s peaks vary by 0.5 bps to either side Then you set your bin for that allele to be another 0.5 bp to each side of that range Therefore your bin will be 2 bps wide Usually fine for tetranucleotide repeats May cause problems with microvariants Bin

24 Partial DNA Profiles In some conditions only a partial Profile may be obtained Usually due to degradation of the DNA or the presence of PCR inhibitors Often the smaller PCR products may continue to show up when the larger products do not For this reason may decide to go back and re-genotype with mini-strs

25 Partial DNA Profiles Only some of the genotypes can be obtained Significance of a match will go down Because there are less markers Therefore the overall profile is less informative and specific

26 Mixture Interpretation While determining genotypes must also determine whether unknown sample is a mixture Better chance of identifying that the sample is a mixture when examining: More markers; all highly polymorphic Because more chance that both samples will be heterozygous see 3 or 4 peaks

27 Extra Peaks Sometimes extra peaks may not be caused by a mixture Possible reasons for extra peaks: Stutter bands Incomplete Adenylation of products True rare anomalies causing extra chromosomes or chromosomal regions Mixtures

28 Extra Peaks Sometimes extra peaks caused by technology related artifacts Possible reasons for extra peaks: Strong alleles bleeding through Dye blobs Air bubbles Urea crystals Sample contaminants that fluoresce

29 Deciphering Artifacts from the True Alleles Biological (PCR) artifacts Stutter products STR alleles 6.0% 7.8% Dye blob stutter spike D3S1358 Blue channel Incomplete adenylation +A -A -A +A Pull-up (bleed-through) Green channel Yellow channel D8S1179 Red channel Figure 15.4, J.M. Butler (2005) Forensic DNA Typing, 2 nd Edition 2005 Elsevier Academic Press

30 Contamination Possibilities Certain contaminants fluoresce When DNA is extracted along with any of the following: Dyes in fabrics Chlorophyll from plants People with certain pathological conditions Lead poisoning, Blood Porphyrins People who took tetracycline antibiotics

31 Confirmation of Genotypes DNA profile is determined by: Computer software Expert review Second expert Only when a genotype is confirmed by two experts is it accepted Both individuals need to be unaware of each other s allele calls Blind allele calls

32 Interpretation Strategy Each laboratory must develop a strategy for calling alleles consistently: Conduct validation studies of lab equipment and personnel Gain experience examining peaks Learn when to trust the computer and when changing allele calls is appropriate Reference literature and databases often to keep up to date with CODIS markers

33 A Match? Three possible outcomes of comparing two DNA profiles: 1. Inclusion if the DNA profiles match Probability of seeing this match at random is calculated 2. Exclusion non-match The profiles are too different to possibly be the same individual 3. Inconclusive - unknown There is not enough data to determine

34 A Match! Three possible explanations for a match: 1. Suspect left DNA at crime scene Trial needs to determine whether that information proves suspect committed crime 2. Suspect s profile matches by chance This is why statistics of seeing this DNA profile at random are calculated 3. Match is a false positive result This is avoided at all times by validating technology, running controls and duplicates

35 Any Questions? Read Chapter 16

36 Capillaries Electrodes for Injection Figure 14.4, J.M. Butler (2005) Forensic DNA Typing, 2 nd Edition 2005 Elsevier Academic Press

37 Replace capillary Refill syringe with polymer solution Performed only once per batch of ~96 samples Fill buffer vials Prepare samples (denature, cool, and mix with size standard) Prepare sample sheet and injection list Allelic ladder every tenth injection Automated Automated Sample Sample Injection, Injection, Electrophoresis Electrophoresis and and Data Data Collection Collection Size DNA Fragments Genotype STR alleles Perform Data Analysis GeneScan Software Genotyper Software Manually inspect the data ELECTROPHORESIS and DETECTION steps are simultaneous Figure 14.2, J.M. Butler (2005) Forensic DNA Typing, 2 nd Edition 2005 Elsevier Academic Press

38 FMBIO III Gel Imager System Penta E FGA Penta D CSF1PO D18S51 D16S539 TPOX D7S820 D8S1179 D21S11 TH01 D13S317 VWA D5S818 D3S1358 Amelogenin PowerPlex 16 BIO

39 Pour Gel Prepare samples: denature, cool, and mix with loading dye Load Samples (allelic ladder every third lane) Electrophoresis Pour Gel Prepare samples: denature, cool, and mix with loading dye Load Samples (allelic ladder every third lane) Electrophoresis Post-Electrophoresis FMBIO II Fluorescence Imaging System SCAN SCAN GEL GEL ReadImage Software Size DNA Fragments FMBIO Analysis Software Genotype STR alleles Perform Data Analysis STaR Call Genotyping Software Manually inspect the data ELECTROPHORESIS and DETECTION steps are separate Figure 14.7, J.M. Butler (2005) Forensic DNA Typing, 2 nd Edition 2005 Elsevier Academic Press

40 Blue Green Yellow Figure 15.3, J.M. Butler (2005) Forensic DNA Typing, 2 nd Edition 2005 Elsevier Academic Press