Kataegis is a localized hypermutation occurring when a region is enriched in somatic SNVs (Nik-Zainal S. et al 2012). Kataegis can result from multiple cytosine deaminations catalyzed by the AID/APOBEC family of proteins (Lada AG et al 2012). A first step to understand kataegis requires the ability to reproducibly and reliability identify it. Although a formal, quantifiable definition of kataegis has not been reached, we have provided the first operational definition in the form of SeqKat, a R package that predicts kataegis from paired tumour normal human whole genome samples. This package contains functions to detect kataegis from SNVs in BED format.
SeqKat uses a sliding window (of fixed width) approach to test deviation of observed SNV trinucleotide content and inter-mutational distance from expected by chance alone. Additionally, an exact binomial test is performed to test that the proportion of each of the 32 tri-nucleotides within each window is higher than expected. The resulting p-values are then adjusted for multiple hypothesis testing using FDR. Hypermutation and kataegic scores are calculated for each window as follows
hypermutation score = $-log_{10}$(binomial $p_{adj}$) * $\frac{N observed Mutations}{N expected Mutations}$ [Equation 1]
kataegis score = hypermutation score * $\frac{N TCX bases}{N expected TCX bases}$ [Equation 2]
SeqKat reports both hypermutation score and an APOBEC mediated kataegic score along with the start and end position of each detected event. A reference paper will be added upon publication in an upcoming version of this package.
SeqKat accepts a SNV BED file per patient with the following columns:
seqkat(sigcutoff = 5, mutdistance = 3.2, segnum = 4, ref.dir = NULL, bed.file = "./", output.dir = "./", chromosome = "all", chromosome.length.file = NULL, trinucleotide.count.file = NULL )
sigcutoff: The minimum hypermutation score used to classify the windows in the sliding binomial test as significant windows. The score is calculated following [Equation 1]. Recommended value: 5
mutdistance: The maximum intermutational distance allowed for SNVs to be grouped in the same kataegic event. Recommended value: 3.2
segnum: Minimum mutation count. The minimum number of mutations required within a cluster to be identified as kataegic. Recommended value: 4
ref.dir: Path to a directory containing the reference genome. Each chromosome should have its own .fa file and chromosomes X and Y are named as chr23 and chr24. The fasta files should contain no header.
bed.file: Path to the SNV BED file. The BED file should contain the following information: Chromosome, Position, Reference allele, Alternate allele. The file should be named {SAMPLE_NAME}_snvs.bed. Below is an example of a BED file:
chr4 17185 G A chr4 38640 T C chr4 52598 C T chr4 53102 C G chr4 71989 G A chr4 91099 C G chr4 91139 G C chr4 192852 G C chr4 201573 G C chr4 212498 C G
output.dir: Path to a directory where output will be created
chromosome: The chromosome to be analysed. This can be (1, 2, ..., 23, 24) or "all" to run sequentially on all chromosomes
chromosome.length.file (provided): A tab separated file containing the lengths of all chromosomes in the reference genome. Below is an example of a chromosome.length.file for hg19:
"num" "length" "1" 249250621 "2" 243199373 "3" 198022430 "4" 191154276 "5" 180915260 "6" 171115067 "7" 159138663 "8" 146364022 "9" 141213431 "10" 135534747 "11" 135006516 "12" 133851895 "13" 115169878 "14" 107349540 "15" 102531392 "16" 90354753 "17" 81195210 "18" 78077248 "19" 59128983 "20" 63025520 "21" 48129895 "22" 51304566 "23" 155270560 "24" 59373566 "sum.f" 3036303846 "sum.m" 3095677412
trinucleotide count ACA 118307548 ACC 67377361 ACG 15031779 ACT 94371148 ATA 120046083 ATC 78231773 ATG 107040211 ATT 145343907 CCA 107257513 CCC 75979793 CCG 16160851 CCT 103230644 CTA 75285140 CTC 98529730 CTG 118268845 CTT 117139678 GCA 84247974 GCC 68786498 GCG 13995012 GCT 81425256 GTA 65911575 GTC 54961008 GTG 88082699 GTT 86132552 TCA 114943318 TCC 90485129 TCG 13117247 TCT 130196437 TTA 120231763 TTC 117019750 TTG 111364601 TTT 225211336
note: sigcutoff, multidistance and segnum default parameters are optimized using Alexandrov et al's "Signatures of mutational processes in human cancer" dataset.
note: trinucleotide.count.file and chromosome.length.file have been provided for GRCh38 reference as well
If Kataegic events are detected, SeqKat generates a tab delimited file that includes details about the detected events. Each line represents one detected hypermutation or kataegic event. The file includes the following columns:
note: if no event is detected then no file is generated
A subset BED file from the publically available breast cancer sample PD4120a is provided in the package. This BED contains 2804 SNVs in the first 2,000,000 bases of chromosome 4. A subset FASTA file and chromosome length file have also been provided for testing purposes only.
example.bed.file <- paste0( path.package("SeqKat"), "/extdata/test/PD4120a-chr4-1-2000000_test_snvs.bed" ); example.ref.dir <- paste0( path.package("SeqKat"), "/extdata/test/ref/" ); example.chromosome.length.file <- paste0( path.package("SeqKat"), "/extdata/test/length_hg19_chr_test.txt" ); seqkat( 5, 3.2, 2, bed.file = example.bed.file, output.dir = ".", chromosome = "4", ref.dir = example.ref.dir, chromosome.length.file = example.chromosome.length.file );
To view the detected events, you can check the file PD4120a-chr4-1-2000000_chr4_cutoff5_mutdist3.2_segnum2.txt
sample chr start end variants score.hm score.kat PD4120a-chr4-1-2000000 4 1009070 1009541 4 119920.029973896 0
In this example, SeqKat detected one hypermutation window on chromosome 4 between 1009070 and 1009541, containing 4 SNVs with a hypermutation score of 119920.03. The kataegic score is 0, indicating that it is not an APOBEC mediated event.
Nik-Zainal S, Alexandrov LB, Wedge DC, Van Loo P, Greenman CD, Raine K, Jones D, Hinton J, Marshall J, Stebbings LA, Menzies A, Martin S, Leung K, Chen L, Leroy C, Ramakrishna M, Rance R, Lau KW, Mudie LJ, Varela I, McBride DJ, Bignell GR, Cooke SL, Shlien A, Gamble J, Whitmore I, Maddison M, Tarpey PS, Davies HR, Papaemmanuil E, Stephens PJ, McLaren S, Butler AP, Teague JW, Jönsson G, Garber JE, Silver D, Miron P, Fatima A, Boyault S, Langerød A, Tutt A, Martens JW, Aparicio SA, Borg Å, Salomon AV, Thomas G, Børresen-Dale AL, Richardson AL, Neuberger MS, Futreal PA, Campbell PJ, Stratton MR; Breast Cancer Working Group of the International Cancer Genome Consortium.. Mutational processes molding the genomes of 21 breast cancers. Cell. 2012 May 25;149(5):979-93. doi: 10.1016/j.cell.2012.04.024. Epub 2012 May 17. PubMed PMID: 22608084; PubMed Central PMCID: PMC3414841.
Lada AG, Dhar A, Boissy RJ, Hirano M, Rubel AA, Rogozin IB, Pavlov YI. AID/APOBEC cytosine deaminase induces genome-wide kataegis. Biol Direct. 2012 Dec 18;7:47; discussion 47. doi: 10.1186/1745-6150-7-47. PubMed PMID: 23249472; PubMed Central PMCID: PMC3542020
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