| Large-scale cancer genome sequencing studies have indicated that the accumulation of somatic mutations in human tissues may be affected by environmental exposure,anatomical location of the tissue,and the type of tissue.Accumulating evdence supports the hypothesis that most of cancer-related mutations may also exist in normal tissues of healthy individuals.While sevral previous studies in healthy indivaduals have reported the mutation rates and clonal expansions of mutated cells in normal tissues or organs,the samples they analyzed usually obtained from different donors having distinct genetic backgrounds and living enviroments,thus making a cross-organ comparison unrealiable.In order to eliminated the individual differences and better explain the relationship between mutation and carcinogenesis,somatic mutations and clonal expansions should be investigated in different organs or tissues collected from the same individual.With written informed consent,we collected the bronchia,esophagus,cardia,stomach,duodenum,liver,pancreas,colon and rectum tissues and paired peripheral blood from 5 body donors aged over 85 years.After histopathological diagnosis as normal,laser-capture microdissection was used to obtain 50 samples of target cells(-600 cells)in each organ or tisseue.These mini-bulk were subjected to exome sequencing.We totally performed low-depth whole-genome sequencing and 56x whole-exome sequencing in 1,764 samples.The sequencing data and clonal expansion were analyzed using the commonly used piplines.We identify somatic mutations in various organs or tissues of 5 healthy individuals with remarkable heterogeneity(medain mutation numbers=17-43.5 per exome).Among all organs,the pancreas parenchyma has the lowest mutation burden(10 mutations/exome)while the liver has the highest mutation burden(69 mutations/exome).Analysis of somatic copy number shows that copy number alterations(CNAs)exist substantial organ preference.We observe diploid genome in most cells(1,608/1,764,91.2%)obtained from the colon,rectum and bronchia;however,cells from the esophagus and cardia had CNAs at chromosomes 2,3,5,7,8,13 and 20,which are mainly amplification.De novo single-base-substitution(SBS)mutational signature extraction analysis reveals two age-related endogenous mutational signatures(SBS1 and SBS5)and two exogenous mutational signatures(SBS4 and SBS22),which is related to smoking or aristolochic acid exposure.Surprisingly,we find that in addition to liver,SBS22 was also detected in the stomach and esophagus in two donors.We identify 32 potential cancer driver mutations in morphologically normal tissues of these 5 healty individuals,including NOTCH1,TP53,ARID1A and ERBB2,which have been commonly reported in esophageal carcinoma and other types of cancer.The proportion of samples with the cancer-driver mutations vary dramatically across the organs and among the donors and the mutations show potential organ preference.Based on somatic mutations we reconstruct the spatial somatic clonal architectures with sub-millimeter resolution.In the esophageal and cardiac epithelial tissues,we observe macroscopic mutated clones expanded to several millimeters,whereas in tissues from colon,rectum and duodenum,the mutated clones are all microscopic size and evolved independently.In summary,based on laser captured microdissection of specific cells and mini-bulk genomic sequencing,we have for the first time depicted a body map of somatic mutations and clonal expansions in multiorgans from the same healthy individuals.We have shown that the cancer-driver mutations and the degree of mutated clonal expansion are highly organ specific.These results suggest that mutations and mutated clonal expansions are not the only factors enough to drive carcinogenesis.Our genomic analyses with high spatial resolution provide new insights into the organ-specific pattern of somatic mutation and mutational clonal expansions,laying a foundation for further research on diseases such as cancer and aging. |
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