Genetic Lesion Segregation Found in Chemically Induced Tumors in Mice


Genetic instability has lengthy been acknowledged as a trademark of oncogenesis and tumor development.  The phenomenon was first recognized cytogenetically, most famously by the Philadelphia chromosome in continual myelogenous leukemia (see Wapner, The Philadelphia Chromosome: A Genetic Mystery, a Lethal Cancer, and the Improbable Invention of a Lifesaving Treatment), later appreciated as the results of a translocation between a human protooncogene (c-abl) with a heterologous sequence that resulted in growth of the illness.  But it was later acknowledged by histological analysts that most cancers cells might be characterised by a number of aneuploidies, together with chromosome loss and duplication in addition to adjustments in high-quality construction options of the genome.

The introduction of fast and (comparatively) cheap complete genome sequencing (WGS) methodologies has resulted in much more delicate assessments of adjustments in human genomic DNA related to most cancers development.  This week, the scientific journal Nature printed a paper* entitled “Pervasive lesion segregation shapes cancer genome evolution” that confirmed for the primary time a heritable sample of DNA strand-specific adjustments brought on by contact with chemical mutagenic brokers and the implications for most cancers growth.  The experiments have been carried out by treating inbred male C3H/HeOuJ mice (in addition to “divergent” CAST/EiJ mice) with one dose of dimethynitrosamine (DEN), a identified cancer-causing agent.  Treatment with this mutagenic agent was discovered consequence in predominantly (76%) mutations in A:T basepairs (T→N or A→N, the place N is any of the opposite bases), which the paper notice is “consistent with the long-lived thymine adduct O4-ethyl-deoxythymidine being the principal mutagenic lesion.”  Whole genome sequencing (WGS) was carried out on a complete of 371 independently-arising tumors from 104 C3H mice and these researchers reported discovering about 60,000 level mutations in every tumor (about 13 adjustments per megabasepair, Mb).  Perhaps unsurprisingly (in view of what’s identified about how this chemical produces mutations), insertion and deletion mutations have been not often detected.

Upon evaluation of those mutations, the authors discovered “multimegabase genomic segments with pronounced Watson-versus-Crick-strand asymmetry of mutations, frequently encompassing entire chromosomes” (i.e., “an excess of T→N over A→N mutations when called on the forward strand of the reference genome, and Crick-strand bias as the converse of this”), with a median span of those lesions being 55Mb lengthy.  The researchers state that in this end result “DEN-induced lesions remaining unrepaired before genome replication.”  The mechanistic consequence is proven in Figure 2i:

the place “(1) A mutagen generates lesions (red triangles) on both DNA strands.  (2) If not removed, lesions will segregate into sister chromatids: one carrying only Watson-strand lesions (blue) and the second carrying only Crick-strand lesions (gold).  Postmitotic daughter cells will have independent lesions and resulting replication errors (3), resolved into full mutations in later replication (4).  (5) Only lineages containing driver changes (* in (1)) will expand into substantial populations.”

As the authors clarify, “[a]symmetric regions show a 23-fold excess (median) of their preferred mutation over its reverse complement, thus more than 95% of lesions that generate a mutation segregate for at least one mitotic division,” a phenomenon they time period “lesion segregation.”  The authors clarify situations the place  these asymmetries change in a chromosome as being the results of sister chromatid exchanges ensuing from homologous recombination-mediated DNA restore occasions.  While these asymmetries are reported to be equally distributed on the Watson and Crick strands in the genome, this isn’t noticed at loci encoding genes related to tumorigenesis (what the researchers time period “driver genes”: Braf, Hras, and Egfr).  These websites present “oncogenic selection,” outlined as a bias for retaining mutations related to their position in oncogenesis.  The researchers additionally discovered transcription-coupled nucleotide-excision restore of such lesions preferentially in parts of the chromosome encoding the template (mRNA) strand of genes in the chromosomes, whereby “mutations in highly expressed genes were reduced by 79.8 ± 1.0% (mean ± s.d.) if the tumour had template-strand lesions.”

Discovery of lesion segregation supplies a mechanism for the genetic heterogeneity discovered in many tumors.  As the authors clarify, “[a] segregating lesion may act as template for multiple rounds of replication in successive cell cycles (as shown in the Figure).  Each replication could incorporate different incorrectly or correctly paired nucleotides opposite a persistent lesion, resulting in multiple alleles at the same position.  Consistent with this notion, multiallelic mutations have been reported in human cancers . . . .”  Here, 8% of mutated websites confirmed multiallelic variants (amounting to 1.Eight million websites in C3H tumors induced by DEN), whereas solely 0098% of websites between tumors confirmed nucleotide adjustments.  The consequence:  “[t]he generation of multiallelic variation produces combinatorial generic diversity that would not be expected under purely clonal expansion.”  The researchers clarify the consequence of those outcomes:

Tumours with excessive charges of genetic range have constantly excessive charges of multiallelism all through their genome[].  They are prone to have expanded from a first-generation daughter of the unique DEN-mutagenized cell, in which all DNA is a duplex of a lesion-containing and non-lesion-containing strand.  Therefore, replication utilizing lesion-containing strands because the template in subsequent generations produces multiallelic variation uniformly throughout the genome.  Tumours with decrease whole ranges of genetic range exhibit discrete genomic segments of excessive and low multiallelism[].  These tumours in all probability developed from a cell some generations after DEN remedy.  Each mitosis following DEN publicity is anticipated to dilute the variety of lesion-containing strands in every daughter cell by roughly 50%.  Only lesion-retaining fractions of the genome generate multiallelic and combinatorial genetic range in the daughter lineages; in step with this, the multiallelic segments mirror the mutational asymmetry segmentation sample.

The penalties of DEN mutagenesis reported are hanging:  “[i]n 67% of C3H tumours and 21% of CAST tumours, the initial burst of mutations was instantly transformative.”

Lesion segregation may also be noticed, as reported in this paper (if to search for it) in chemically mutagenized human induced pluripotent stem cells (iPSCs), in addition to in tumors induced by daylight (UV irradiation), tobacco smoke (benzo[a]pyrene diol-epoxide) and sure chemotherapeutic brokers.  The authors notice, nevertheless, that in contrast to experimentally induced tumors most naturally occurring tumors are the results of a collection of longitudinal genetic insults reasonably than a single harm.  Accordingly, they count on that lesion segregation could also be masked as a consequence of those a number of insults.  Nevertheless, in addition they report discovering lesion segregation in renal, biliary, and hepatic tumors from a human most cancers genome compendium (n=18,850 tumors from 22 major tumor websites).

The paper summarizes the importance of their findings thusly:

Once recognized, lesion segregation is a deeply intuitive idea.  Its sensible functions present new vistas for the exploration of genome upkeep and elementary molecular biology.  The discovery of pervasive lesion segregation profoundly revises our understanding of how the structure of DNA restore and clonal proliferation can conspire to form the most cancers genome.

* Sarah J. Aitken, Craig J. Anderson, Frances Connor, Oriol Pich, Vasavi Sundaram, Christine Feig, Tim F. Rayner, Margus Lukk, Stuart Aitken, Juliet Luft, Elissavet Kentepozidou, Claudia Arnedo-Pac, Sjoerd V. Beentjes, Susan E. Davies, Ruben M. Drews, Ailith Ewing, Vera B. Kaiser, Ava Khamseh, Erika López-Arribillaga, Aisling M. Redmond, Javier Santoyo-Lopez, Inés Sentís, Lana Talmane, Andrew D. Yates, Liver Cancer Evolution Consortium, Colin A. Semple, Núria López-Bigas, Paul Flicek, Duncan T. Odom & Martin S. Taylor.

Cancer Research UK Cambridge Institute, Department of Pathology, University of Cambridge, UK; Department of Histopathology, Cambridge University Hospitals NHS Foundation Trust, UK; MRC Human Genetics Unit, MRC Institute of Genetics and Molecular Medicine, School of Mathematics and Maxwell Institute, Higgs Centre for Theoretical Physics, University of Edinburgh, UK;  Institute for Research in Biomedicine (IRB Barcelona), The Barcelona Institute of Science and Technology, Barcelona, Spain; European Molecular Biology Laboratory, European Bioinformatics Institute, Hinxton, UK;  Edinburgh Genomics (Clinical), The University of Edinburgh, Edinburgh, UK; Universitat Pompeu Fabra (UPF), Barcelona, Spain; Institució Catalana de Recerca i Estudis Avançats (ICREA), Barcelona, Spain; German Cancer Research Center (DKFZ), Division of Regulatory Genomics and Cancer Evolution, Heidelberg, Germany.