APOBEC Mutagenesis in Cancer Development and Susceptibility.

APOBEC cytosine deaminases are prominent mutators in cancer, mediating mutations in over 50% of cancers. APOBEC mutagenesis has been linked to tumor heterogeneity, persistent cell evolution, and therapy responses. While emerging evidence supports the impact of APOBEC mutagenesis on cancer progression, the understanding of its contribution to cancer susceptibility and malignant transformation is limited. We examine the existing evidence for the role of APOBEC mutagenesis in carcinogenesis on the basis of the reported associations between germline polymorphisms in genes encoding APOBEC enzymes and cancer risk, insights into APOBEC activities from sequencing efforts of both malignant and non-malignant human tissues, and in vivo studies. We discuss key knowledge gaps and highlight possible ways to gain a deeper understanding of the contribution of APOBEC mutagenesis to cancer development.

Addressing the benefits of inhibiting APOBEC3-dependent mutagenesis in cancer.

Mutational signatures associated with apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like (APOBEC)3 cytosine deaminase activity have been found in over half of cancer types, including some therapy-resistant and metastatic tumors. Driver mutations can occur in APOBEC3-favored sequence contexts, suggesting that mutagenesis by APOBEC3 enzymes may drive cancer evolution. The APOBEC3-mediated signatures are often detected in subclonal branches of tumor phylogenies and are acquired in cancer cell lines over long periods of time, indicating that APOBEC3 mutagenesis can be ongoing in cancer. Collectively, these and other observations have led to the proposal that APOBEC3 mutagenesis represents a disease-modifying process that could be inhibited to limit tumor heterogeneity, metastasis and drug resistance. However, critical aspects of APOBEC3 biology in cancer and in healthy tissues have not been clearly defined, limiting well-grounded predictions regarding the benefits of inhibiting APOBEC3 mutagenesis in different settings in cancer. We discuss the relevant mechanistic gaps and strategies to address them to investigate whether inhibiting APOBEC3 mutagenesis may confer clinical benefits in cancer.

Mechanisms of APOBEC3 mutagenesis in human cancer cells.

The APOBEC3 family of cytosine deaminases has been implicated in some of the most prevalent mutational signatures in cancer1-3. However, a causal link between endogenous APOBEC3 enzymes and mutational signatures in human cancer genomes has not been established, leaving the mechanisms of APOBEC3 mutagenesis poorly understood. Here, to investigate the mechanisms of APOBEC3 mutagenesis, we deleted implicated genes from human cancer cell lines that naturally generate APOBEC3-associated mutational signatures over time4. Analysis of non-clustered and clustered signatures across whole-genome sequences from 251 breast, bladder and lymphoma cancer cell line clones revealed that APOBEC3A deletion diminished APOBEC3-associated mutational signatures. Deletion of both APOBEC3A and APOBEC3B further decreased APOBEC3 mutation burdens, without eliminating them. Deletion of APOBEC3B increased APOBEC3A protein levels, activity and APOBEC3A-mediated mutagenesis in some cell lines. The uracil glycosylase UNG was required for APOBEC3-mediated transversions, whereas the loss of the translesion polymerase REV1 decreased overall mutation burdens. Together, these data represent direct evidence that endogenous APOBEC3 deaminases generate prevalent mutational signatures in human cancer cells. Our results identify APOBEC3A as the main driver of these mutations, indicate that APOBEC3B can restrain APOBEC3A-dependent mutagenesis while contributing its own smaller mutation burdens and dissect mechanisms that translate APOBEC3 activities into distinct mutational signatures.

Mapping clustered mutations in cancer reveals APOBEC3 mutagenesis of ecDNA.

Clustered somatic mutations are common in cancer genomes and previous analyses reveal several types of clustered single-base substitutions, which include doublet- and multi-base substitutions1-5, diffuse hypermutation termed omikli6, and longer strand-coordinated events termed kataegis3,7-9. Here we provide a comprehensive characterization of clustered substitutions and clustered small insertions and deletions (indels) across 2,583 whole-genome-sequenced cancers from 30 types of cancer10. Clustered mutations were highly enriched in driver genes and associated with differential gene expression and changes in overall survival. Several distinct mutational processes gave rise to clustered indels, including signatures that were enriched in tobacco smokers and homologous-recombination-deficient cancers. Doublet-base substitutions were caused by at least 12 mutational processes, whereas most multi-base substitutions were generated by either tobacco smoking or exposure to ultraviolet light. Omikli events, which have previously been attributed to APOBEC3 activity6, accounted for a large proportion of clustered substitutions; however, only 16.2% of omikli matched APOBEC3 patterns. Kataegis was generated by multiple mutational processes, and 76.1% of all kataegic events exhibited mutational patterns that are associated with the activation-induced deaminase (AID) and APOBEC3 family of deaminases. Co-occurrence of APOBEC3 kataegis and extrachromosomal DNA (ecDNA), termed kyklonas (Greek for cyclone), was found in 31% of samples with ecDNA. Multiple distinct kyklonic events were observed on most mutated ecDNA. ecDNA containing known cancer genes exhibited both positive selection and kyklonic hypermutation. Our results reveal the diversity of clustered mutational processes in human cancer and the role of APOBEC3 in recurrently mutating and fuelling the evolution of ecDNA.

Molecular origins of APOBEC-associated mutations in cancer.

The APOBEC family of cytidine deaminases has been proposed to represent a major enzymatic source of mutations in cancer. Here, we summarize available evidence that links APOBEC deaminases to cancer mutagenesis. We also highlight newly identified human cell models of APOBEC mutagenesis, including cancer cell lines with suspected endogenous APOBEC activity and a cell system of telomere crisis-associated mutations. Finally, we draw on recent data to propose potential causes of APOBEC misregulation in cancer, including the instigating factors, the relevant mutator(s), and the mechanisms underlying generation of the genome-dispersed and clustered APOBEC-induced mutations.

Tissue-Biased Expansion of DNMT3A-Mutant Clones in a Mosaic Individual Is Associated with Conserved Epigenetic Erosion.

DNA methyltransferase 3A (DNMT3A) is the most commonly mutated gene in clonal hematopoiesis (CH). Somatic DNMT3A mutations arise in hematopoietic stem cells (HSCs) many years before malignancies develop, but difficulties in comparing their impact before malignancy with wild-type cells have limited the understanding of their contributions to transformation. To circumvent this limitation, we derived normal and DNMT3A mutant lymphoblastoid cell lines from a germline mosaic individual in whom these cells co-existed for nearly 6 decades. Mutant cells dominated the blood system, but not other tissues. Deep sequencing revealed similar mutational burdens and signatures in normal and mutant clones, while epigenetic profiling uncovered the focal erosion of DNA methylation at oncogenic regulatory regions in mutant clones. These regions overlapped with those sensitive to DNMT3A loss after DNMT3A ablation in HSCs and in leukemia samples. These results suggest that DNMT3A maintains a conserved DNA methylation pattern, the erosion of which provides a distinct competitive advantage to hematopoietic cells.

Characterizing Mutational Signatures in Human Cancer Cell Lines Reveals Episodic APOBEC Mutagenesis.

Multiple signatures of somatic mutations have been identified in cancer genomes. Exome sequences of 1,001 human cancer cell lines and 577 xenografts revealed most common mutational signatures, indicating past activity of the underlying processes, usually in appropriate cancer types. To investigate ongoing patterns of mutational-signature generation, cell lines were cultured for extended periods and subsequently DNA sequenced. Signatures of discontinued exposures, including tobacco smoke and ultraviolet light, were not generated in vitro. Signatures of normal and defective DNA repair and replication continued to be generated at roughly stable mutation rates. Signatures of APOBEC cytidine deaminase DNA-editing exhibited substantial fluctuations in mutation rate over time with episodic bursts of mutations. The initiating factors for the bursts are unclear, although retrotransposon mobilization may contribute. The examined cell lines constitute a resource of live experimental models of mutational processes, which potentially retain patterns of activity and regulation operative in primary human cancers.

Somatic mutations reveal asymmetric cellular dynamics in the early human embryo.

Somatic cells acquire mutations throughout the course of an individual's life. Mutations occurring early in embryogenesis are often present in a substantial proportion of, but not all, cells in postnatal humans and thus have particular characteristics and effects. Depending on their location in the genome and the proportion of cells they are present in, these mosaic mutations can cause a wide range of genetic disease syndromes and predispose carriers to cancer. They have a high chance of being transmitted to offspring as de novo germline mutations and, in principle, can provide insights into early human embryonic cell lineages and their contributions to adult tissues. Although it is known that gross chromosomal abnormalities are remarkably common in early human embryos, our understanding of early embryonic somatic mutations is very limited. Here we use whole-genome sequences of normal blood from 241 adults to identify 163 early embryonic mutations. We estimate that approximately three base substitution mutations occur per cell per cell-doubling event in early human embryogenesis and these are mainly attributable to two known mutational signatures. We used the mutations to reconstruct developmental lineages of adult cells and demonstrate that the two daughter cells of many early embryonic cell-doubling events contribute asymmetrically to adult blood at an approximately 2:1 ratio. This study therefore provides insights into the mutation rates, mutational processes and developmental outcomes of cell dynamics that operate during early human embryogenesis.

Genome-wide chemical mutagenesis screens allow unbiased saturation of the cancer genome and identification of drug resistance mutations.

Drug resistance is an almost inevitable consequence of cancer therapy and ultimately proves fatal for the majority of patients. In many cases, this is the consequence of specific gene mutations that have the potential to be targeted to resensitize the tumor. The ability to uniformly saturate the genome with point mutations without chromosome or nucleotide sequence context bias would open the door to identify all putative drug resistance mutations in cancer models. Here, we describe such a method for elucidating drug resistance mechanisms using genome-wide chemical mutagenesis allied to next-generation sequencing. We show that chemically mutagenizing the genome of cancer cells dramatically increases the number of drug-resistant clones and allows the detection of both known and novel drug resistance mutations. We used an efficient computational process that allows for the rapid identification of involved pathways and druggable targets. Such a priori knowledge would greatly empower serial monitoring strategies for drug resistance in the clinic as well as the development of trials for drug-resistant patients.

Understanding mutagenesis through delineation of mutational signatures in human cancer.

Each individual cell within a human body acquires a certain number of somatic mutations during a course of its lifetime. These mutations originate from a wide spectra of both endogenous and exogenous mutational processes that leave distinct patterns of mutations, termed mutational signatures, embedded within the genomes of all cells. In recent years, the vast amount of data produced by sequencing of cancer genomes was coupled with novel mathematical models and computational tools to generate the first comprehensive map of mutational signatures in human cancer. Up to date, >30 distinct mutational signatures have been identified, and etiologies have been proposed for many of them. This review provides a brief historical background on examination of mutational patterns in human cancer, summarizes the knowledge accumulated since introducing the concept of mutational signatures and discusses their future potential applications and perspectives within the field.

Germline TERT promoter mutations are rare in familial melanoma.

Germline CDKN2A mutations occur in 40 % of 3-or-more case melanoma families while mutations of CDK4, BAP1, and genes involved in telomere function (ACD, TERF2IP, POT1), have also been implicated in melanomagenesis. Mutation of the promoter of the telomerase reverse transcriptase (TERT) gene (c.-57 T>G variant) has been reported in one family. We tested for the TERT promoter variant in 675 multicase families wild-type for the known high penetrance familial melanoma genes, 1863 UK population-based melanoma cases and 529 controls. Germline lymphocyte telomere length was estimated in carriers. The c.-57 T>G TERT promoter variant was identified in one 7-case family with multiple primaries and early age of onset (earliest, 15 years) but not among population cases or controls. One family member had multiple primary melanomas, basal cell carcinomas and a bladder tumour. The blood leukocyte telomere length of a carrier was similar to wild-type cases. We provide evidence confirming that a rare promoter variant of TERT (c.-57 T>G) is associated with high penetrance, early onset melanoma and potentially other cancers, and explains <1 % of UK melanoma multicase families. The identification of POT1 and TERT germline mutations highlights the importance of telomere integrity in melanoma biology.

Genome sequencing of normal cells reveals developmental lineages and mutational processes.

The somatic mutations present in the genome of a cell accumulate over the lifetime of a multicellular organism. These mutations can provide insights into the developmental lineage tree, the number of divisions that each cell has undergone and the mutational processes that have been operative. Here we describe whole genomes of clonal lines derived from multiple tissues of healthy mice. Using somatic base substitutions, we reconstructed the early cell divisions of each animal, demonstrating the contributions of embryonic cells to adult tissues. Differences were observed between tissues in the numbers and types of mutations accumulated by each cell, which likely reflect differences in the number of cell divisions they have undergone and varying contributions of different mutational processes. If somatic mutation rates are similar to those in mice, the results indicate that precise insights into development and mutagenesis of normal human cells will be possible.

POT1 loss-of-function variants predispose to familial melanoma.

Deleterious germline variants in CDKN2A account for around 40% of familial melanoma cases, and rare variants in CDK4, BRCA2, BAP1 and the promoter of TERT have also been linked to the disease. Here we set out to identify new high-penetrance susceptibility genes by sequencing 184 melanoma cases from 105 pedigrees recruited in the UK, The Netherlands and Australia that were negative for variants in known predisposition genes. We identified families where melanoma cosegregates with loss-of-function variants in the protection of telomeres 1 gene (POT1), with a proportion of family members presenting with an early age of onset and multiple primary tumors. We show that these variants either affect POT1 mRNA splicing or alter key residues in the highly conserved oligonucleotide/oligosaccharide-binding (OB) domains of POT1, disrupting protein-telomere binding and leading to increased telomere length. These findings suggest that POT1 variants predispose to melanoma formation via a direct effect on telomeres.

Association of a germline copy number polymorphism of APOBEC3A and APOBEC3B with burden of putative APOBEC-dependent mutations in breast cancer.

The somatic mutations in a cancer genome are the aggregate outcome of one or more mutational processes operative through the lifetime of the individual with cancer. Each mutational process leaves a characteristic mutational signature determined by the mechanisms of DNA damage and repair that constitute it. A role was recently proposed for the APOBEC family of cytidine deaminases in generating particular genome-wide mutational signatures and a signature of localized hypermutation called kataegis. A germline copy number polymorphism involving APOBEC3A and APOBEC3B, which effectively deletes APOBEC3B, has been associated with modestly increased risk of breast cancer. Here we show that breast cancers in carriers of the deletion show more mutations of the putative APOBEC-dependent genome-wide signatures than cancers in non-carriers. The results suggest that the APOBEC3A-APOBEC3B germline deletion allele confers cancer susceptibility through increased activity of APOBEC-dependent mutational processes, although the mechanism by which this increase in activity occurs remains unknown.