EMBOSS: cpgplot

| cpgplot

## Function

Identify and plot CpG islands in nucleotide sequence(s)

## Description

cpgplot identifies CpG islands in one or more nucleotide sequences. The ratio of observered to expected number of GC dinucleotides patterns is calculated over a window (sequence region) which is moved along the sequence. The calculated ratios are plotted graphically, together with the regions which match this program's definition of a "CpG island" (a CG dinucleotide rich area). A report file is written giving the input sequence name, CpG island parameters and data on any CpG islands that are found.

The ratio of observered to expected number of GC dinucleotides patterns is calculated over a window of user-specified size (`-window` parameter). The window is slid along the sequence and the ratio recalculated until the end of the sequence is reached.

By default, cpgplot defines a CpG island as a region where, over an average of 10 windows and not less than 200 bases, the calculated (%G + %C) content is over 50% and the calculated Observed/Expected ratio is over 0.6. These conditions can be modified by setting the values of the appropriate parameters.

The Observed number of CpG patterns in a window is simply the number of times a 'C' is found followed immediately by a 'G'.

The Expected number of CpG patterns is calculated for each window as the number of CpG dinucleotides you would expect to see in that window based on the frequency of C's and G's in that window. Thus, the Expected frequency of CpG's in a window is calculated as the number of 'C's in the window multiplied by the number of 'G's in the window, divided by the window length.

    Expected = (number of C's  * number of G's) / window length

## Usage

Here is a sample session with cpgplot

    % cpgplot tembl:u68037 -graph cps
    Identify and plot CpG islands in nucleotide sequence(s)
    Window size [100]:
    Minimum length of an island [200]:
    Minimum observed/expected [0.6]:
    Minimum percentage [50.]:
    Output file [u68037.cpgplot]:
    Features output [u68037.gff]:

    Created cpgplot.ps

Go to the input files for this example
Go to the output files for this example

## Command line arguments

       Standard (Mandatory) qualifiers (* if not always prompted):
      [-sequence]          seqall     Nucleotide sequence(s) filename and optional
                                      format, or reference (input USA)
       -window             integer    [100] The percentage CG content and the
                                      Observed frequency of CG is calculated
                                      within a window whose size is set by this
                                      parameter. The window is moved down the
                                      sequence and these statistics are calculated
                                      at each postition that the window is moved
                                      to. (Integer 1 or more)
       -minlen             integer    [200] This sets the minimum length that a
                                      CpG island has to be before it is reported.
                                      (Integer 1 or more)
       -minoe              float      [0.6] This sets the minimum average observed
                                      to expected ratio of C plus G to CpG in a
                                      set of 10 windows that are required before a
                                      CpG island is reported. (Number from 0.000
                                      to 10.000)
       -minpc              float      [50.] This sets the minimum average
                                      percentage of G plus C a set of 10 windows
                                      that are required before a CpG island is
                                      reported. (Number from 0.000 to 100.000)
      [-outfile]           outfile    [*.cpgplot] This sets the name of the file
                                      holding the report of the input sequence
                                      name, CpG island parameters and the output
                                      details of any CpG islands that are found.
    *  -graph              xygraph    [$EMBOSS_GRAPHICS value, or x11] Graph type
                                      (ps, hpgl, hp7470, hp7580, meta, cps, x11,
                                      tekt, tek, none, data, xterm, png, gif)
      [-outfeat]           featout    [unknown.gff] File for output features

       Additional (Optional) qualifiers: (none)
       Advanced (Unprompted) qualifiers:
       -[no]plot           toggle     [Y] Plot CpG island score
       -[no]obsexp         boolean    [Y] If this is set to true then the graph of
                                      the observed to expected ratio of C plus G
                                      to CpG within a window is displayed.
       -[no]cg             boolean    [Y] If this is set to true then the graph of
                                      the regions which have been determined to
                                      be CpG islands is displayed.
       -[no]pc             boolean    [Y] If this is set to true then the graph of
                                      the percentage C plus G within a window is
                                      displayed.

       Associated qualifiers:

       "-sequence" associated qualifiers
       -sbegin1            integer    Start of each sequence to be used
       -send1              integer    End of each sequence to be used
       -sreverse1          boolean    Reverse (if DNA)
       -sask1              boolean    Ask for begin/end/reverse
       -snucleotide1       boolean    Sequence is nucleotide
       -sprotein1          boolean    Sequence is protein
       -slower1            boolean    Make lower case
       -supper1            boolean    Make upper case
       -sformat1           string     Input sequence format
       -sdbname1           string     Database name
       -sid1               string     Entryname
       -ufo1               string     UFO features
       -fformat1           string     Features format
       -fopenfile1         string     Features file name

       "-outfile" associated qualifiers
       -odirectory2        string     Output directory

       "-graph" associated qualifiers
       -gprompt            boolean    Graph prompting
       -gdesc              string     Graph description
       -gtitle             string     Graph title
       -gsubtitle          string     Graph subtitle
       -gxtitle            string     Graph x axis title
       -gytitle            string     Graph y axis title
       -goutfile           string     Output file for non interactive displays
       -gdirectory         string     Output directory

       "-outfeat" associated qualifiers
       -offormat3          string     Output feature format
       -ofopenfile3        string     Features file name
       -ofextension3       string     File name extension
       -ofdirectory3       string     Output directory
       -ofname3            string     Base file name
       -ofsingle3          boolean    Separate file for each entry

       General qualifiers:
       -auto               boolean    Turn off prompts
       -stdout             boolean    Write first file to standard output
       -filter             boolean    Read first file from standard input, write
                                      first file to standard output
       -options            boolean    Prompt for standard and additional values
       -debug              boolean    Write debug output to program.dbg
       -verbose            boolean    Report some/full command line options
       -help               boolean    Report command line options. More
                                      information on associated and general
                                      qualifiers can be found with -help -verbose
       -warning            boolean    Report warnings
       -error              boolean    Report errors
       -fatal              boolean    Report fatal errors
       -die                boolean    Report dying program messages

Standard (Mandatory) qualifiers | Allowed values | Default
--- | --- | ---
[-sequence] (Parameter 1) | Nucleotide sequence(s) filename and optional format, or reference (input USA) | Readable sequence(s) | Required
-window | The percentage CG content and the Observed frequency of CG is calculated within a window whose size is set by this parameter. The window is moved down the sequence and these statistics are calculated at each postition that the window is moved to. | Integer 1 or more | 100
-minlen | This sets the minimum length that a CpG island has to be before it is reported. | Integer 1 or more | 200
-minoe | This sets the minimum average observed to expected ratio of C plus G to CpG in a set of 10 windows that are required before a CpG island is reported. | Number from 0.000 to 10.000 | 0.6
-minpc | This sets the minimum average percentage of G plus C a set of 10 windows that are required before a CpG island is reported. | Number from 0.000 to 100.000 | 50.
[-outfile] (Parameter 2) | This sets the name of the file holding the report of the input sequence name, CpG island parameters and the output details of any CpG islands that are found. | Output file | <*>.cpgplot
-graph | Graph type | EMBOSS has a list of known devices, including ps, hpgl, hp7470, hp7580, meta, cps, x11, tekt, tek, none, data, xterm, png, gif | EMBOSS_GRAPHICS value, or x11
[-outfeat] (Parameter 3) | File for output features | Writeable feature table | unknown.gff
Additional (Optional) qualifiers | Allowed values | Default
(none)
Advanced (Unprompted) qualifiers | Allowed values | Default
-[no]plot | Plot CpG island score | Toggle value Yes/No | Yes
-[no]obsexp | If this is set to true then the graph of the observed to expected ratio of C plus G to CpG within a window is displayed. | Boolean value Yes/No | Yes
-[no]cg | If this is set to true then the graph of the regions which have been determined to be CpG islands is displayed. | Boolean value Yes/No | Yes
-[no]pc | If this is set to true then the graph of the percentage C plus G within a window is displayed. | Boolean value Yes/No | Yes

## Input file format

Any DNA sequence USA.

### Input files for usage example

'tembl:u68037' is a sequence entry in the example nucleic acid database 'tembl'

### Database entry: tembl:u68037

    ID   U68037; SV 1; linear; mRNA; STD; ROD; 1218 BP.
    XX
    AC   U68037;
    XX
    DT   23-SEP-1996 (Rel. 49, Created)
    DT   04-MAR-2000 (Rel. 63, Last updated, Version 2)
    XX
    DE   Rattus norvegicus EP1 prostanoid receptor mRNA, complete cds.
    XX
    KW   .
    XX
    OS   Rattus norvegicus (Norway rat)
    OC   Eukaryota; Metazoa; Chordata; Craniata; Vertebrata; Euteleostomi; Mammalia;
    OC   Eutheria; Euarchontoglires; Glires; Rodentia; Sciurognathi; Muroidea;
    OC   Muridae; Murinae; Rattus.
    XX
    RN   [1]
    RP   1-1218
    RA   Abramovitz M., Boie Y.;
    RT   "Cloning of the rat EP1 prostanoid receptor";
    RL   Unpublished.
    XX
    RN   [2]
    RP   1-1218
    RA   Abramovitz M., Boie Y.;
    RT   ;
    RL   Submitted (26-AUG-1996) to the EMBL/GenBank/DDBJ databases.
    RL   Biochemistry & Molecular Biology, Merck Frosst Center for Therapeutic
    RL   Research, P. O. Box 1005, Pointe Claire - Dorval, Quebec H9R 4P8, Canada
    XX
    FH   Key             Location/Qualifiers
    FH
    FT   source          1..1218
    FT                   /organism="Rattus norvegicus"
    FT                   /strain="Sprague-Dawley"
    FT                   /mol_type="mRNA"
    FT                   /db_xref="taxon:10116"
    FT   CDS             1..1218
    FT                   /codon_start=1
    FT                   /product="EP1 prostanoid receptor"
    FT                   /note="family 1 G-protein coupled receptor"
    FT                   /db_xref="GOA:P70597"
    FT                   /db_xref="InterPro:IPR000276"
    FT                   /db_xref="InterPro:IPR000708"
    FT                   /db_xref="InterPro:IPR001244"
    FT                   /db_xref="InterPro:IPR008365"
    FT                   /db_xref="UniProtKB/Swiss-Prot:P70597"
    FT                   /protein_id="AAB07735.1"
    FT                   /translation="MSPYGLNLSLVDEATTCVTPRVPNTSVVLPTGGNGTSPALPIFSM
    FT                   TLGAVSNVLALALLAQVAGRLRRRRSTATFLLFVASLLAIDLAGHVIPGALVLRLYTAG
    FT                   RAPAGGACHFLGGCMVFFGLCPLLLGCGMAVERCVGVTQPLIHAARVSVARARLALALL
    FT                   AAMALAVALLPLVHVGHYELQYPGTWCFISLGPPGGWRQALLAGLFAGLGLAALLAALV
    FT                   CNTLSGLALLRARWRRRRSRRFRENAGPDDRRRWGSRGLRLASASSASSITSTTAALRS
    FT                   SRGGGSARRVHAHDVEMVGQLVGIMVVSCICWSPLLVLVVLAIGGWNSNSLQRPLFLAV
    FT                   RLASWNQILDPWVYILLRQAMLRQLLRLLPLRVSAKGGPTELSLTKSAWEASSLRSSRH
    FT                   SGFSHL"
    XX
    SQ   Sequence 1218 BP; 162 A; 397 C; 387 G; 272 T; 0 other;
         atgagcccct acgggcttaa cctgagccta gtggatgagg caacaacgtg tgtaacaccc        60
         agggtcccca atacatctgt ggtgctgcca acaggcggta acggcacatc accagcgctg       120
         cctatcttct ccatgacgct gggtgctgtg tccaacgtgc tggcgctggc gctgctggcc       180
         caggttgcag gcagactgcg gcgccgccgc tcgactgcca ccttcctgtt gttcgtcgcc       240
         agcctgcttg ccatcgacct agcaggccat gtgatcccgg gcgccttggt gcttcgcctg       300
         tatactgcag gacgtgcgcc cgctggcggg gcctgtcatt tcctgggcgg ctgtatggtc       360
         ttctttggcc tgtgcccact tttgcttggc tgtggcatgg ccgtggagcg ctgcgtgggt       420
         gtcacgcagc cgctgatcca cgcggcgcgc gtgtccgtag cccgcgcacg cctggcacta       480
         gccctgctgg ccgccatggc tttggcagtg gcgctgctgc cactagtgca cgtgggtcac       540
         tacgagctac agtaccctgg cacttggtgt ttcattagcc ttgggcctcc tggaggttgg       600
         cgccaggcgt tgcttgcggg cctcttcgcc ggccttggcc tggctgcgct ccttgccgca       660
         ctagtgtgta atacgctcag cggcctggcg ctccttcgtg cccgctggag gcggcgtcgc       720
         tctcgacgtt tccgagagaa cgcaggtccc gatgatcgcc ggcgctgggg gtcccgtgga       780
         ctccgcttgg cctccgcctc gtctgcgtca tccatcactt caaccacagc tgccctccgc       840
         agctctcggg gaggcggctc cgcgcgcagg gttcacgcac acgacgtgga aatggtgggc       900
         cagctcgtgg gcatcatggt ggtgtcgtgc atctgctgga gccccctgct ggtattggtg       960
         gtgttggcca tcgggggctg gaactctaac tccctgcagc ggccgctctt tctggctgta      1020
         cgcctcgcgt cgtggaacca gatcctggac ccatgggtgt acatcctgct gcgccaggct      1080
         atgctgcgcc aacttcttcg cctcctaccc ctgagggtta gtgccaaggg tggtccaacg      1140
         gagctgagcc taaccaagag tgcctgggag gccagttcac tgcgtagctc ccggcacagt      1200
         ggcttcagcc acttgtga                                                    1218
    //

## Output file format

### Output files for usage example

### File: u68037.cpgplot

    CPGPLOT islands of unusual CG composition
    U68037 from 1 to 1218

         Observed/Expected ratio > 0.60
         Percent C + Percent G > 50.00
         Length > 200

     Length 406 (104..509)

     Length 329 (596..924)

### File: u68037.gff

    ##gff-version 3
    ##sequence-region U68037 1 924
    #!Date 2008-07-15
    #!Type DNA
    #!Source-version EMBOSS 6.0.0
    U68037	cpgplot	located_sequence_feature	104	509	0.000	+	.	ID="U68037.1"
    U68037	cpgplot	located_sequence_feature	596	924	0.000	+	.	ID="U68037.2"

### Graphics File: cpgplot.ps

## Notes

"CpG" refers to a C nucleotide immediately followed by a G. The 'p' in 'CpG' refers to the phosphate group linking the two bases. Regions of genomic sequences rich in the CpG pattern or "CpG islands" are resistant to methylation and tend to be associated with genes which are frequently switched on. It's been estimated that about half of all mammalian genes, and, possibly all mammalian house-keeping genes, have a CpG-rich region around their 5' end. Non-mammalian vertebrates have some CpG islands that are associated with genes, but the association gets equivocal in the farther taxonomic groups. The detection of CpG island upstream of predicted exons or genes is evidence in support of a highly expressed gene.

As there is no official definition of what is a CpG island is or how to identify where they begin and end, we work with two definitions and thus two methods. These are:

1. cpgplot and newcpgreport use a sliding window within which the Observed/Expected ratio of CpG is calculated. For a sequence region to reported as a CpG island, it must satisfy the following contraints:

       Observed/Expected ratio > 0.6
       % C + % G > 50%
       Sequence Length > 200

2. newcpgseek and cpgreport use a running sum calculated from all positions in a sequence rather than a window to produce a score. If there is not a `CG` dinucleotide at a position, the score is decremented, if there is one, the score is incremented by a constant (user-defined) value. If the score for a region in the sequence is higher than a threshold (17 at the moment) then a putative island is declared. Sequence regions scoring above the threshold are searched for recursively.

This method overpredicts islands but finds the smaller ones around primary exons. newcpgseek uses the same method as cpgreport but the output is different and more readable. For most purposes you should probably use newcpgreport rather than cpgreport. It is used to produce the human cpgisland database you can find on the EBI's ftp server as well as on the EBI's SRS server.

newcpgseek and cpgreport both now display the actual CpG count, the (%C + %G) and the Observed/Expected ratio in the region where the score is above the threshold.

The geecee program measures CG content in the entire input sequence and is not to be used to detect CpG islands. It can be useful for detecting sequences that might contain an island.

## References

The original program was described in:

1. Larsen F, Gundersen G, Lopez R, Prydz H "CpG islands as gene markers in the human genome." Genomics 1992 Aug;13(4):1095-107

## Warnings

None.

## Diagnostic Error Messages

None.

## Exit status

0 if successful.

## Known bugs

None.

## See also

Program name | Description
--- | ---
cpgreport | Identify and report CpG-rich regions in nucleotide sequence(s)
geecee | Calculate fractional GC content of nucleic acid sequences
newcpgreport | Identify CpG islands in nucleotide sequence(s)
newcpgseek | Identify and report CpG-rich regions in nucleotide sequence(s)

As there is no official definition of what is a cpg island is, and worst where they begin and end, we have to live with 2 definitions and thus two methods. These are:

1. newcpgseek and cpgreport - both declare a putative island if the score is higher than a threshold (17 at the moment). They now also displaying the actual CpG count, the % CG and the observed/expected ration in the region where the score is above the threshold. This scoring method based on sum/frequencies overpredicts islands but finds the smaller ones around primary exons. newcpgseek uses the same method as cpgreport but the output is different and more readable.

2. newcpgreport and cpgplot use a sliding window within which the Obs/Exp ratio of CpG is calculated. The important thing to note in this method is that an island, in order to be reported, is defined as a region that satisfies the following contraints:

       Obs/Exp ratio > 0.6
       % C + % G > 50%
       Length > 200.

For all practical purposes you should probably use newcpgreport. It is actually used to produce the human cpgisland database you can find on the EBI's ftp server as well as on the EBI's SRS server.

geecee measures CG content in the entire input sequence and is not to be used to detect CpG islands. It can be usefull for detecting sequences that MIGHT contain an island.

## Author(s)

Alan Bleasby (ajb © ebi.ac.uk)
European Bioinformatics Institute, Wellcome Trust Genome Campus, Hinxton, Cambridge CB10 1SD, UK

## History

Completed 23rd March 1999.

## Target users

This program is intended to be used by everyone and everything, from naive users to embedded scripts.

## Comments

None