Docstoc

Microarray Basics

Document Sample
Microarray Basics Powered By Docstoc
					  Microarray Basics
Part 1: Choosing a platform, setting
       up, data preprocessing
        Experimental design

• What type of microarray
• What overall design strategy
• How many replicates
        Type of Microarray
One colour                         Two colour


Short oligo                 cDNA                Long oligo




              Genome wide          Custom

availability, cost, represented genes, need,
perceived accuracy/reproducibility
            Experimental Design Strategies

Cy3   reference     reference                    Cy3      healthy
                                     x40                            x20
Cy5   healthy           disease                           disease
                                                 Cy5



            Cy3         healthy                 disease
                                     x10                     x10
            Cy5         disease                 healthy



                  Cy3      disease         healthy
                                                       x40
                  Cy5      healthy         disease
              How many replicates?
                             True situation


                   Not diff expressed         Diff expressed

             NS           correct             Type 2 error
Your call                                         (power)
              S        Type 1 error              correct
                        (confidence)



      Technical replicates do NOT count as different samples
      in the power calculation
Power analysis requires decisions about:
     Difference in mean that you are trying to detect
     The std dev of the population variability
     Power you are trying to achieve
     Significance level that you are trying to achieve
     Experimental design

  You have a 10,000 gene chip, and want to identify 95% of
  the genes that are 2 fold up or down regulated in samples
  following treatment. You will tolerate 1 false positive call
  out of the 10,000 genes tested. The coefficient of variability
  in your population is ~ 50%. You are doing a paired
  analysis.

      One can conclude that you will need 22 patients
           Technical replicates

• Most publications recommend at least 3 if that is
  possible
• These are considered to be replicates at the level
  of the experimental platform
• Beware of doing 2 now and hoping to add one
  more later
• In downstream analysis, generally suggested to
  use the average of technical replicates- these are
  not different samples for analysis
   RNA required to get started
• Source of both experimental and reference
  RNA
• Will need about 10-20ug of total RNA from
  each source for each experiment or chip
• This RNA needs to be of high quality
• How do you check quality?
               Common sources of RNA
Cultured animal cells: generally easy to disrupt and get large
amounts of high quality RNA
Animal tissues: some require harsh disruption treatments (such
as soft tissues like kidney or liver) and some may require
addition treatments (such as fatty tissues or fibrous tissues that may
require more stringent lysis)

Blood: may be influence by anticoagulant in collection system, and
also seems to contain enzyme inhibitors

Plant material: some metabolites make purification difficult-
extractions may also be highly viscous

Bacteria: may want to consider stabilization
       Checking RNA quality
• Conventional methods include agarose gel
  electrophoresis to look for evidence of
  degradation
• Spectrophotometric readings to give an idea
  of purity
• Bioanalyzer to provide scan- integrity and
  quantity measurements
                                  Provides an RIN
                                  Provides a [ ]




Requires 1 µl of ~50ng/µl stock
         RNA amplification
• When quantity of RNA is limited, may have
  to consider amplification
• Several strategies, but need to decide up
  front if you want sense or antisense
  amplified material
  What do you get back after an
          experiment?
• TIFF images- one image for each fluor used
  in the experiment- same chip scanned twice
  (or more times if multiple scans were done
  to compensate for intensity)
• Spreadsheet of quantitated data
              TIFF images
• Generally named as: bar code_fluor_PMT
  setting_laser setting
• These settings will not necessarily be the
  same for your two scans from the same
  chip- they are manipulated to try to produce
  scans of even intensity from the two fluors
• The final image should have only a few
  white spots over the whole array- these
  represent saturated spots
 How can you tell anything about
    the quality of your data?
• Easiest way to start is to look at your TIFF images
• Look for blank areas on the slide
• Look for areas where one fluor consistently is
  brighter than the other
• Look for gradients of intensity
• Differentiate between artifacts introduced by slide
  quality and those by RNA quality and those by
  experimental procedure
           Slide issues- printing
•   Presence of donuts
•   Smeared spots
•   Scratches on surface of slide
•   Non circular spots
•   Spots off the grid
•   No signals in areas
•   Consistent problems with the same area of each
    subarray
             RNA quality issues
• General low intensity
• Consistent problems with one sample,
  regardless of fluor used
• High level of background-
  grainy over entire slide
             Experimental issues

• One fluor consistently not giving good
  signal regardless of RNA sample labelled
• High areas of local background
  not covering entire slide
• Obvious intensity gradients
• Bubbles over surface of chip
• After looking at your images you should
  have a sense of whether or not these data
  are likely to be clean and high enough
  quality to warrant proceeding
• If not you need to try to determine where
  the problem originates
           Image processing

• Choice of methods for quantitating image
• Fixed circle
  – Good for arrays with regular sizes of spots
• Variable circle
  – Better for arrays with irregular sizes
• Histogram
  – Best for arrays with irregular sizes and shapes
           Data quantitation
• The images are quantitated, generating a
  lengthy spreadsheet
• This is done in the facility using
  QuantArray, but can be done using other
  freeware (Scanalzye) or commercial
  software
• The output can generally be opened in
  Excel for first pass manipulation of data
          QuantArray output
• QA generates a series of columns that many
  people find confusing
• In general, it provides the data in two ways
  on a single sheet- the first method is
  showing one channel as a proportion of the
  other, the second method provides absolute
  pixel counts for each channel
Information about the experiment




   Data presented as ratios




    Raw quantitated data
    Locator and identifier columns
•   A: unique number assigned to that spot
•   B: Row of subgrid
•   C: Column of subgrid
•   D: Row of spot within subgrid
•   E: Column of spot within subgrid
•   F: Gene identification
•   G: x coordinate of each spot
•   H: y coordinate of each spot
            Spot Values

• I/U:   intensity of signal in ch1/ch2
• J/V:   intensity of background in ch1/ch2
• K/W:   std dev of intensity of signal in
         ch1/ch2
• L/X:   std dev of background of signal in
         ch 1/ch2
    Quality control measurements
•   M/Y    spot diameter
•   N/Z    spot area
•   O/AA   spot footprint
•   P/AB   spot circularity
•   Q/AC   spot uniformity
•   R/AD   background uniformity
•   S/AE   signal to noise ratio
                   Data Cleaning
Are there flagged spots?
         -may see flags in last column- these are added by
         user during quantitation
Are there areas of the images that you just wouldn’t
trust?
Are there saturated spots?

Have the option of removing, recalculating, ignoring , flagging or
resetting the results of these spots so that they don’t interfere
with downstream analysis

At this stage, may also want to background subtract the raw
intensities
  On chip controls and how they
             behave
• Blank spots: generally 3XSSC (print buffer)
  – Expect no signal- can use the average or
    median intensity of these spots as the lower
    cutoff for what represents a real signal
  – However not all empty spots are the same on
    some chips
  – Possibility that there is carryover from non-
    empty spots printed with the same pin
            On chip controls
• Multiple spots of the same gene
  – In general if it is exactly the same sequence,
    can assess the variability of these spots to
    assess artifacts of geography on the chip
  – If it is not the same sequence, less
    straightforward
            On chip controls
• Housekeeping genes: if you can identify a
  set of genes that should remain at constant
  expression, can use these to standardize the
  two channels
• to correctly identify such genes is difficult
• May also have exogenous controls that can
  be added, but must identify these prior to
  hybridizing the slides
           Log transformation of data
                              Most data bunched in lower
                              left corner
                              Variability increases with
                              intensity




Data are spread more evenly
Variability is more even
        Within array normalization
In two colour arrays, are measuring two different samples,
labelled in two different reactions with two different fluors
and measured using two different lasers at two different
wavelengths

In addition, dealing with the distribution of spots across a
relatively large surface


Need to try to eliminate some of these potential sources
of variation so that the variation that is left is more likely
to be due to biological effects
                    Dye Bias
• The two dyes incorporate differently into DNA of
  different abundance
• The two dyes may have different emission
  responses to the laser at different abundances
• The two dye emissions may be measured by the
  PMT differently at different intensities
• The intensities of the dyes may vary over the
  surface of the slide, but not in synch, as the focus
  of each laser is separate
        Correcting for dye bias
• Global normalization using median or mean
• Linear regression of Cy3 against Cy5
• Linear regression of the log ratio against the
  average intensity (MA plots)
• Non linear regression of the log ratio against the
  average intensity (loess)

 **assumption that most genes are not differentially
                    expressed**
                                      Simple global normalization
                                      to try to fit the data




Slope does not equal 1 means one channel responds more at higher
intensity
Non zero intercept means one
channel is consistently brighter

Non straight line means non
linearity in intensity responses
of two channels
Linear regression of
Cy3 against Cy5
                       MA plots
Regressing one channel against the other has the disadvantage
of treating the two sets of signals separately

Also suggested that the human eye has a harder time seeing
deviations from a diagonal line than a horizontal line

            MA plots get around both these
            issues
     Basically a rotation and rescaling of the data

           X axis   A= (log2R + log2G)/2
           Y axis   M= log2R-log2G
                       Scatterplot of intensities




MA plot of same data
           Non linear normalization
Normalization that takes into account intensity effects
  Lowess or loess is the locally weighted polynomial regression



 User defines the size of
 bins used to calculate the
 best fit line

                                                Taken from Stekal (2003) Microarray Bioinformatics
                                 Adjusted values for the x
                                 axis (average intensity for
                                 each feature) calculated using
                                 the loess regression




Should now see the data
centred around 0 and
straight across the horizontal
axis
   Spatial defects over the slide
• In some cases, you may notice a spatial bias
  of the two channels
• May be a result of the slide not lying
  completely flat in the scanner
• This will not be corrected by the methods
  discussed before
     Regressions for spatial bias
• Carry out normal loess regression but treat each
  subgrid as an entire array (block by block loess)
• Corrects best for artifacts introduced by the pins,
  as opposed to artifacts of regions of the slide
   – Because each subgrid has relatively few spots, risk
     having a subgrid where a substantial proportion of spots
     are really differentially expressed- you will lose data if
     you apply a loess regression to that block
• May also perform a 2-D loess- plot log ratio for
  each feature against its x and y coordinates and
  perform regression
           Acknowledgements

•   Perseus Missirlis
•   Natasha Gallo
•   Jim Gore
•   Jennifer Kreiger

• Scott Davey

				
DOCUMENT INFO