Measuring single cell divisions in human tissues from multi-region sequencing data

Both normal tissue development and cancer growth are driven by a branching process of cell division and mutation accumulation that leads to intra-tissue genetic heterogeneity. However, quantifying somatic evolution in humans remains challenging. Here, we show that multi-sample genomic data from a single time point of normal and cancer tissues contains information on single-cell divisions. We present a new theoretical framework that, applied to whole-genome sequencing data of healthy tissue and cancer, allows inferring the mutation rate and the cell survival/death rate per division. On average, we found that cells accumulate 1.14 mutations per cell division in healthy haematopoiesis and 1.37 mutations per division in brain development. In both tissues, cell survival was maximal during early development. Analysis of 131 biopsies from 16 tumours showed 4 to 100 times increased mutation rates compared to healthy development and substantial inter-patient variation of cell survival/death rates

Somatic evolution in healthy and chronically inflamed colon and skin

The human body is made up of trillions of cells which cooperate to reproduce their genetic material. While all the cells are a part of a whole, each is also an individual and will selfishly give rise to a clonal expansion of cells within a tissue given the chance, even to the detriment of the organism. This thesis discusses the evolutionary forces acting on cells within the body, specifically on epithelial cells in the colon and skin. After a general introduction of the evolutionary forces acting on normal cells and the methods used to study them, Chapter 2 focuses specifically on genetic drift within the colon, where clones expand through the process of crypt fission. I apply a statistical framework called Approximate Bayesian Computation to estimate the crypt fission rate in the normal colon and in individuals with Familial adenomatous polyposis (FAP). I estimate the rate of crypt fission to be one every 27 years in the normal colon and one every 13 years in (FAP). In Chapter 3, I describe somatic evolution in the colon under conditions of chronic inflam- mation. I used whole-genome sequencing of individual colonic crypts from patients with inflammatory bowel disease (IBD) to show that the IBD-colon is characterized by a higher mutation burden and larger clonal expansions than the healthy colon. I also show that muta- tions in immune-related genes, including PIGR, ZC3H12A and genes in the interleuking 17 and toll-like receptor pathways, are under positive selection in the colons of IBD patients and may contribute to the disease pathogenesis. In Chapter 4, I focus on the skin. I performed whole-exome sequencing of microbiopsies of epidermis from patients with psoriasis, a second chronic inflammatory disease. In contrast to IBD, I did not find increased mutation burden and clonal spread in psoriasis, except when the skin had been treated with psoralens + UVA (PUVA) phototreatment. The selection landscape of psoriatic skin resembles that of normal skin, and mutations in NOTCH1, FAT1, TP53, PPM1D and NOTCH2 are positively selected. ZFP36L2 was the only gene found to be enriched in mutations that has not been previously reported in normal skin, but it is as yet uncertain if selection of ZFP36L2 mutant cells is a feature specific to psoriatic skin or not. Finally, Chapter 5 discusses my findings in the broader context of cancer and complex-trait genomics. I discuss how a causal relationship between somatic evolution and non-neoplastic diseases may be established and the different ways somatic evolution may affect disease progression for good or ill. I further discuss how to design a study to search for germline determinants of somatic evolution and the need for developing methods to enable such studies to be conducted at scale.Wellcome Trus

Quantifying the pro- and antimutagenic roles of DNA damage and repair

Genome integrity is essential to the survival of any living organism. The genome is constantly challenged by a multitude of endogenous and exogenous mutagenic factors such as environmental exposures or replication errors. Therefore, evolution has supplied cells with a number of repair mechanisms to protect their genetic information; however, excessive exposures or defects in the repair machinery can lead to the accumulation of deleterious mutations which may cause a range of diseases including cancer. Different mutational processes often leave behind characteristic patterns of mutations, so-called mutational signatures. Mutational signature analysis of tumours has gained a lot of attention recently, because it may reveal carcinogenic exposures and also therapeutic vulnerabilities. So far, over 50 mutational signatures have been identified using pattern recognition in large cancer cohorts, reflecting the action of a range of known mutagenic processes, such as UV light, tobacco smoke or mismatch repair deficiency, but for many mutational signatures an underlying generative process is still unknown. The search for the causes behind a given mutational signature is further complicated by the fact that every alteration in the DNA results from failed or incorrect repair of a DNA lesion, hence there are two factors which jointly shape the mutational spectrum of any mutagenic process. In this thesis, I quantify the variability of mutational signatures in model organisms and in human cancer and explore the diversity of DNA damage-repair interactions. Using data from a large mutagenesis screen in C. elegans, including over 50 DNA repair deficient genetic backgrounds, 12 genotoxins and nearly 200 combinations thereof, I characterise the mutational spectra and genomic features of a range of DNA repair deficiencies, and describe the mutational signatures of genotoxins across multiple genetic backgrounds. Importantly, the mutagenic contributions of genetic and mutagenic factors can vary dev pending on the DNA repair components available: over 35% of genotoxin-knockout combinations demonstrated a measurable effect on the mutation rate compared to expected values, and about 10% also presented a new mutational spectrum. Analysis of mutational signatures in cancer exomes demonstrates the relevance of C. elegans results to cancer investigation. Mismatch repair deficiency patterns extracted from C. elegans are comparable to those in gastrointestinal tumours, and help to dissect convoluted mutational processes. The antagonism between DNA damage and repair drives variability in cancer genomes as well: the observed interaction effects were low in magnitude, but evolutionary considerations suggest that cancer risk may be substantially elevated even by small increases in mutagenicity. In summary, this thesis presents the first comprehensive analysis of mutagenic DNA damage-repair interactions using experimental and cancer data. The results show that mutations result from the opposing pro- and anti-mutagenic forces of DNA damage and repair, which shape mutational signatures in highly variable ways. This variation has to be acknowledged and integrated into mutational signature analysis to ensure reliable interpretation and applicability in clinical oncology. Lastly, the cross-species comparison shows that the fundamental laws of mutagenesis are acting similarly across eukaryotic organisms reminding that many mutational processes fuelling tumorigenesis are not exclusive to cancer, but also drive variation and the evolution of species.My PhD studies were funded by the EMBL International PhD Programme

CD8+ cell somatic mutations in multiple sclerosis patients and controls-Enrichment of mutations in STAT3 and other genes implicated in hematological malignancies

Funding Information: This study has been financially supported by research grants from the Helsinki University Hospital, University of Helsinki, the Multiple Sclerosis Foundation of Finland, the Finnish Cultural Foundation, Biogen Finland, Sanofi- Genzyme, Roche and Novartis. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Publisher Copyright: © 2021 Valori et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.Somatic mutations have a central role in cancer but their role in other diseases such as common autoimmune disorders is not clear. Previously we and others have demonstrated that especially CD8+ T cells in blood can harbor persistent somatic mutations in some patients with multiple sclerosis (MS) and rheumatoid arthritis. Here we concentrated on CD8+ cells in more detail and tested (i) how commonly somatic mutations are detectable, (ii) does the overall mutation load differ between MS patients and controls, and (iii) do the somatic mutations accumulate non-randomly in certain genes? We separated peripheral blood CD8+ cells from newly diagnosed relapsing MS patients (n = 21) as well as matched controls (n = 21) and performed next-generation sequencing of the CD8+ cells' DNA, limiting our search to a custom panel of 2524 immunity and cancer related genes, which enabled us to obtain a median sequencing depth of over 2000x. We discovered nonsynonymous somatic mutations in all MS patients' and controls' CD8+ cell DNA samples, with no significant difference in number between the groups (p = 0.60), at a median allelic fraction of 0.5% (range 0.2- 8.6%). The mutations showed statistically significant clustering especially to the STAT3 gene, and also enrichment to the SMARCA2, DNMT3A, SOCS1 and PPP3CA genes. Known activating STAT3 mutations were found both in MS patients and controls and overall 1/5 of the mutations were previously described cancer mutations. The detected clustering suggests a selection advantage of the mutated CD8+ clones and calls for further research on possible phenotypic effects.Peer reviewe

Detection of somatic variants from genomic data and their role in neurodegenerative diseases

[eng] Somatic mutations are those that arise after the zygote is formed and are therefore inherited by a fraction of the cells of an individual. Their relevance in certain skin diseases has been known for almost half a decade and cancer, the most common disease caused by somatic mutations, has been extensively studied. Yet, their prevalence in healthy individuals as well as their putative role in other human disorders such as neurodegenerative diseases are still unanswered questions. Furthermore, accurate detection of somatic variants from bulk sequencing data still poses a technical challenge. This work focuses on detecting and circumventing the biases that hinder their identification. Using this knowledge, we identified somatic point mutations in the exomes of five different tissues from sporadic Parkinson disease patients. We also assessed the detection of somatic copy number variants from array CGH data using two tissues from Alzheimer disease patients. Finally, we participated in the identification of somatic variants in an extensive genomic dataset from a neurotypical individual.[spa] Las mutaciones somáticas son aquellas que surgen tras la formación del cigoto y son, por tanto, heredadas por una fracción de las células de un individuo. Su importancia en algunas enfermedades cutáneas se conoce desde hace casi medio siglo. El cáncer, la enfermedad más común causada por mutaciones somáticas, se ha estudiado extensamente. Sin embargo, su prevalencia en individuos sanos, así como su potencial relevancia en otras afecciones humanas, como las enfermedades neurodegenerativas, son cuestiones todavía por resolver. Asimismo, detectar variantes somáticas con precisión en datos de secuenciación de muestras homogeneizadas sigue siendo complejo técnicamente. Este trabajo se centra en la detección y resolución de los sesgos que dificultan su identificación. Aplicando este conocimiento, identificamos mutaciones somáticas de una sola base en datos de secuenciación del exoma de cinco tejidos diferentes de pacientes de la enfermedad de Parkinson. También evaluamos la detección de variantes de número de copia somáticas en datos de array CGH de dos tejidos de pacientes de Alzheimer. Finalmente, participamos en la identificación de variantes somáticas en un amplio conjunto de datos genómicos de un individuo neurotípico

Hepatocytes undergo punctuated expansion dynamics from a periportal stem cell niche in normal human liver

Background & Aims: While normal human liver is thought to be generally quiescent, clonal hepatocyte expansions have been observed, though neither their cellular source nor their expansion dynamics have been determined. Knowing the hepatocyte cell of origin, and their subsequent dynamics and trajectory within the human liver will provide an important basis to understand disease-associated dysregulation. Methods: Herein, we use in vivo lineage tracing and methylation sequence analysis to demonstrate normal human hepatocyte ancestry. We exploit next-generation mitochondrial sequencing to determine hepatocyte clonal expansion dynamics across spatially distinct areas of laser-captured, microdissected, clones, in tandem with computational modelling in morphologically normal human liver. Results: Hepatocyte clones and rare SOX9+ hepatocyte progenitors commonly associate with portal tracts and we present evidence that clones can lineage-trace with cholangiocytes, indicating the presence of a bipotential common ancestor at this niche. Within clones, we demonstrate methylation CpG sequence diversity patterns indicative of periportal not pericentral ancestral origins, indicating a portal to central vein expansion trajectory. Using spatial analysis of mitochondrial DNA variants by next-generation sequencing coupled with mathematical modelling and Bayesian inference across the portal-central axis, we demonstrate that patterns of mitochondrial DNA variants reveal large numbers of spatially restricted mutations in conjunction with limited numbers of clonal mutations. Conclusions: These datasets support the existence of a periportal progenitor niche and indicate that clonal patches exhibit punctuated but slow growth, then quiesce, likely due to acute environmental stimuli. These findings crucially contribute to our understanding of hepatocyte dynamics in the normal human liver. Impact and implications: The liver is mainly composed of hepatocytes, but we know little regarding the source of these cells or how they multiply over time within the disease-free human liver. In this study, we determine a source of new hepatocytes by combining many different lab-based methods and computational predictions to show that hepatocytes share a common cell of origin with bile ducts. Both our experimental and computational data also demonstrate hepatocyte clones are likely to expand in slow waves across the liver in a specific trajectory, but often lie dormant for many years. These data show for the first time the expansion dynamics of hepatocytes in normal liver and their cell of origin enabling the accurate measurment of changes to their dynamics that may lead to liver disease. These findings are important for researchers determining cancer risk in human liver

The clinical manifestations and molecular mechanisms of mitrochondrial neuro-opthalmological disorders

Autosomal dominant optic atrophy (DOA) classically presents with bilateral, symmetric visual failure in early childhood, with the pathological hallmark being the selective loss of retinal ganglion cells (RGCs). In the first population-based epidemiological study of DOA, we were able to estimate its minimum prevalence at 1 in 35,000 in the North of England. In independent case series from Northern Europe and North America, the majority of families with DOA harboured pathogenic OPA1 mutations (50.0-57.6%), and large-scale OPA1 rearrangements were present in only a small subgroup (11.1-12.9%). We also confirmed that OPA3 mutations were very rare in non-syndromal DOA cases. Visual deterioration was observed in over half (54.2-67.4%) of all patients during long term follow-up, and the rate of visual decline varied markedly both between and within families. In a large multi-centre study of 104 OPA1-positive patients from 45 independent families, we established that additional neuromuscular complications are common in OPA1 disease, affecting up to 20% of all mutational carriers. Bilateral sensorineural deafness beginning in late childhood and early adulthood was a prominent manifestation, followed by a combination of ataxia, myopathy, peripheral neuropathy and chronic progressive external ophthalmoplegia (CPEO) from the third decade of life onwards. We also identified novel clinical presentations with spastic paraparesis mimicking hereditary spastic paraplegia, and a multiple sclerosis-like illness. Patients with these syndromal disease variants (DOA+) had a worse visual prognosis, and this was associated with a more pronounced reduction in retinal nerve fibre layer thickness compared to patients with pure DOA. Interestingly, there was a two- to three-fold increased risk of developing DOA+ features with missense OPA1 mutations and those located within the GTPase domain.EThOS - Electronic Theses Online ServiceMedical Research Council (UK)GBUnited Kingdo

On measuring selection in cancer from subclonal mutation frequencies

Recently available cancer sequencing data have revealed a complex view of the cancer genome containing a multitude of mutations, including drivers responsible for cancer progression and neutral passengers. Measuring selection in cancer and distinguishing drivers from passengers have important implications for development of novel treatment strategies. It has recently been argued that a third of cancers are evolving neutrally, as their mutational frequency spectrum follows a 1/f power law expected from neutral evolution in a particular intermediate frequency range. We study a stochastic model of cancer evolution and derive a formula for the probability distribution of the cancer cell frequency of a subclonal driver, demonstrating that driver frequency is biased towards 0 and 1. We show that it is difficult to capture a driver mutation at an intermediate frequency, and thus the calling of neutrality due to a lack of such driver will significantly overestimate the number of neutrally evolving tumors. Our approach provides quantification of the validity of the 1/f statistic across the entire range of relevant parameter values. We also show that our conclusions remain valid for non-exponential models: spatial 3d model and sigmoidal growth, relevant for early- and late stages of cancer growth

Development of computational tools for variant calling in single-cell RNAseq

Single-cell sequencing technologies have unsurprisingly become a favourable choice for studying key biological questions about cell heterogeneity, rare cell types or lineages. It is only cell-level resolution that allows for an accurate analysis of internal cell processes such as mutagenesis. Eventually, single-cell RNAseq could provide an explanation of mechanisms that lead to the ultimate transformation of healthy tissues into cancerous lesions. One of the main interests of my lab is Barrett’s oesophagus. It is a highly clonal disease and a likely cancer precursor. We decided to take advantage of the single-cell RNAseq technology in order to attempt to identify the tissue of origin of the disease which, despite years of research, still remains unknown. However, the range of methods for identification of mutations in single cells is very limited. In order to address that, we developed our own single-cell RNAseq variant caller. We validated it on a publicly available breast cancer dataset by achieving a reasonable intersection of our results with the output of commonly used bulk tools. Furthermore, we showed that our caller was capable of identifying expected data characteristics such as known breast cancer signatures and mutations in breast cancer genes. We then applied our method to the Barrett’s dataset to investigate connections of Barrett’s with surrounding tissues. Contrary to the previous transcriptomic analysis conducted on the same dataset and indicating a Barrett’s-oesophagus connection, our results revealed a more likely link of Barrett’s with the stomach