Xeroderma pigmentosum (XP) is a rare autosomal recessive congenital syndrome characterized by defective nucleotide excision repair (NER), leading to extreme photosensitivity and a strong predisposition to skin cancer. One of the 8 complementation groups, XP complementation group C (XP-C), is defective specifically in the global genome component of NER and presents not only with increased skin cancer but also with hematologic cancers. Using error-corrected single-molecule sequencing, we show a uniquely high spontaneous somatic mutational load in peripheral blood mononuclear cells (PBMCs) of patients with XP-C but not in those of patients from other XP complementation groups (XP-A, XP-D, and XP-F). The hypermutability observed in XP-C was markedly lower in fibroblasts than in PBMCs. The XP-C mutational profile was characterized by elevated single-nucleotide variants (SNVs) associated with mutational signatures SBS5, SBS8, and SBS32, as well as an enrichment of single-nucleotide cytosine deletions, with SNV profiles closely mirroring those found in XP-C leukemias. These findings indicate that a cancer-like mutation burden is already present in normal lymphocytes before malignant transformation, revealing distinct molecular subtypes within XP defined by spontaneous mutational load in normal blood cells.

Aims: Testing the hypothesis that excess mutations induced in primary fibroblasts by a low dose of N-ethyl-N-nitrosourea (ENU) are inversely correlated with species-specific maximum life span. Methods: To measure excess mutations induced by ENU we treated primary cells of 10 mammalian species, greatly differing in life span. We treated all cells with a low dose, non-toxic dose of ENU (20 ug/ml). We then extracted DNA from all treated and untreated cells and quantified somatic mutation burden by single-molecule sequencing. We measured excessive mutations by calculating the ΔSNVs and we analyzed this across species with linear regression. Results: The average values for ΔSNV were found to range from 0.773 in mice to 0.367 in whale, resulting in a modest inverse correlation with species-specific maximum life span (R2 = 0.2067, P < 0.001). Conclusion: We conclude that DNA repair accuracy, the main determinant of genome sequence integrity, modestly correlates with life span suggesting that longer lived species have better repair capacities compared to shorter-lived species, which is in keeping with genome instability being a primary hallmark of aging and highlights its important role for longevity.

Mutations accumulate with age in most human tissues. While some undergo clonal expansion and contribute to disease, the mutational burden tolerated by a normal cell without functional decline remains unknown. Here, we repeatedly treat proliferating human primary fibroblasts with the point mutagen N-ethyl-N-nitrosourea, and analyze mutation burden by single-cell whole-genome sequencing. Mutation burden increases linearly to ~56,000 single-nucleotide variants per cell, with only a modest reduction in growth rate. We detect negative selection against potentially deleterious coding and noncoding variants, including mutations affecting pathways important for cell growth and maintenance. These findings suggest that selective depletion of harmful variants helps proliferating cells maintain function despite an extreme mutation burden. Because most adult tissues are largely nondividing and cannot remove damaging mutations through a growth disadvantage, somatic mutations that accumulate during aging may have pronounced functional consequences in vivo.

Aneuploidy, an abnormal number of chromosomes, is a hallmark of cancer and has been proposed as an initiating event in tumorigenesis. In glioblastoma (GBM), a highly aggressive brain tumor, cells almost universally display gain of chromosome 7 and loss of chromosome 10. However, it remains unclear whether these alterations arise de novo during malignant transformation or reflect pre-existing chromosomal instability in normal brain tissue. Here, we used single-nucleus whole-genome sequencing (snWGS) on 225 NeuN-negative (non-neuronal) cortical nuclei from 12 healthy individuals and 6 GBM patients, including matched tumor cores and non-tumor brain regions. In healthy brains, approximately 15% of glial nuclei harbored somatic aneuploidies, most often involving chromosome arms, with recurrent 16p alterations detected in up to 3% of nuclei from both healthy controls and GBM non-tumor tissue. These findings establish 16p is a hotspot of structural variation in adult glia. Non-tumor regions in GBM patients closely resembled healthy controls in aneuploidy burden and chromosomal instability metrics and lacked hallmark tumor alterations. In contrast, GBM tumors exhibited significantly elevated aneuploidy (~50%), enrichment for canonical chromosomal instability-driven events, and sex-specific karyotype patterns, consistent with transformation-associated chromosomal instability. Thus, aneuploidy is a recurrent but constrained feature of normal adult glia, whereas chromosome instability and GBM-defining aneuploidies emerge only during malignant transformation.

With advanced sequencing methods, it has now been shown that thousands of mutations accumulate with age in most human tissues. While there is ample evidence that some mutations can clonally amplify and lead to disease, the total burden of mutations a cell can tolerate without functional decline remains unknown. Here we directly addressed this question by exposing serially cultured human primary fibroblasts to multiple, low doses of N-ethyl-N-nitrosourea and analyzing somatic mutation burden using single-cell whole genome sequencing. The results indicate that mitotically active cells can sustain at least ∼56,000 single-nucleotide variants with only a slight adverse effect on growth rate. We provide evidence that such high levels of mutations are tolerated through negative selection against variants in gene coding and non-coding regions, and in sequences associated with genetic pathways for maintaining relevant cellular functions such as growth and cell identity. Since most tissues in adults are non-dividing, these results suggest that somatic mutations in the absence of negative selection may have functionally adverse effects.

Somatic mutation is now recognized as a cause of multiple human diseases other than cancer. Osteoarthritis (OA), a highly prevalent age-related disease, has been associated with increased chromosomal abnormalities in articular cartilage. Here we characterize the somatic mutational landscape of chondrocytes during normal aging and in affected cartilage of patients with OA. We used single-cell whole-genome sequencing to analyze single-nucleotide variants (SNVs) and small insertions and deletions (InDels) in 100 chondrocytes isolated from the cartilage of hip femoral heads of 17 research participants aged 26−90 years, including 9 patients with OA and 8 non-OA donors. Both SNVs and InDels accumulate with age in chondrocytes with a clock-like mutational signature. Surprisingly, the age-related accumulation rate in OA chondrocytes is lower than that in non-OA control chondrocytes. Differences in mutational signatures and Gene Ontology term enrichment were found between OA and non-OA control samples. In this study, to understand the role of somatic mutation in the pathogenesis of OA, we characterized somatic SNV and InDel mutations. With further progress in analytical approaches, structural variations in the chondrocyte genome are also expected to provide valuable information. Somatic mutations accumulate with age and have been linked to functional decline and disease. Single-cell analysis of human cartilage samples from donors with and without osteoarthritis shows that somatic mutations accumulate with age, but, in osteoarthritis, they show distinct mutational patterns and slower accumulation, possibly due to DNA-damage-induced chondrocyte death.

Somatic mutations accumulate with age in human tissues. Clonal amplification of some mutations causes cancers and other diseases. However, it is unclear if random mutation accumulation affects cellular function without clonal amplification. We tested this in cell culture, avoiding the limitation that mutation accumulation in vivo leads to cancer. We performed single-cell whole-genome sequencing of fibroblasts from DNA-mismatch-repair-deficient Msh2 -/- mice and controls after long-term passaging. While maintaining the same growth rates, in the Msh2 -/- fibroblasts, single-nucleotide variants increased up until >50,000 per cell, with small insertions and deletions plateauing at ∼16,000 per cell. We provide evidence for genome-wide negative selection and large-scale mutation-driven population changes, including significant clonal expansion of preexisting mutations and widespread cell-strain-specific hotspots, likely caused by positive selection of mutations in specific genes. Since negative selection to prevent mutations with adverse effects in vivo during aging is difficult to envision, these results suggest a causal role of somatic mutations in age-related cell functional decline.

We discovered a uniquely high spontaneous somatic mutational load in peripheral blood mononuclear cells (PBMCs) of Xeroderma Pigmentosum group C (XP-C) patients, characterized by elevated single nucleotide variants associated with mutation signatures SBS8 and SBS32, as well as an enrichment of single-nucleotide cytosine deletions. This hypermutability was markedly lower in fibroblasts, suggesting a replication-dependent mechanism of mutagenesis due to deficient global genome nucleotide excision repair (GG-NER). Our findings reveal distinct molecular subtypes within XP defined by spontaneous mutational load in normal blood cells and demonstrate that the exceptionally high mutation burden in XP-C leukemia originates from mutations already present prior to malignant transformation.

Aneuploidy is a hallmark of cancers, including high-grade glioma (GBM), one of the most aggressive brain tumors. To assess whether increased aneuploidy already occurs in normal brain tissue of GBM patients, we performed single-nucleus whole-genome sequencing on 225 non-neuronal cortical nuclei from 12 disease-free individuals and 6 GBM patients, in the latter analyzing both tumor and non-tumor distal regions. Somatic aneuploidy was found in approximately 15% of non-neuronal nuclei in the adult human cortex, with recurrent chromosome 16p aneuploidy in up to 4% of nuclei. In contrast, about 51% of GBM tumor nuclei showed frequent aneuploidy of chromosomes 7 and 10, consistent with known GBM profiles. Notably, non-tumor brain regions from GBM patients exhibited aneuploidy frequencies and patterns similar to controls, including recurrent 16p involvement. These findings indicate that somatic aneuploidy in non-neuronal cells is a normal feature of the adult human brain and not linked to increased GBM risk. ### Competing Interest Statement Jan Vijg and Alex Maslov are co-founders of Singulomics Corp and Mutagentech Inc. National Institutes of Health, , AG068908-02, P30CA072720-25, U01CA238726

Somatic mutations are the cause of cancer and have been implicated in other, noncancerous diseases and aging. While clonally expanded mutations can be studied by deep sequencing of bulk DNA, very few somatic mutations expand clonally, and most are unique to each cell. We describe a detailed protocol for single-cell whole-genome sequencing to discover and analyze somatic mutations in tissues and organs. The protocol comprises single-cell multiple displacement amplification (SCMDA), which ensures efficiency and high fidelity in amplification, and the SCcaller software tool to call single-nucleotide variations and small insertions and deletions from the sequencing data by filtering out amplification artifacts. With SCMDA and SCcaller at its core, this protocol describes a complete procedure for the comprehensive analysis of somatic mutations in a single cell, covering (1) single-cell or nucleus isolation, (2) single-cell or nucleus whole-genome amplification, (3) library preparation and sequencing, and (4) computational analyses, including alignment, variant calling, and mutation burden estimation. Methods are also provided for mutation annotation, hotspot discovery and signature analysis. The protocol takes 12–15 h from single-cell isolation to library preparation and 3–7 d of data processing. Compared with other single-cell amplification methods or single-molecular sequencing, it provides high genomic coverage, high accuracy in single-nucleotide variation and small insertions and deletion calling from the same single-cell genome, and fewer processing steps. SCMDA and SCcaller require basic experience in molecular biology and bioinformatics. The protocol can be utilized for studying mutagenesis and genome mosaicism in normal and diseased human and animal tissues under various conditions.

BACKGROUND:The purpose of this study was to (1) perform the first analysis of bone-derived DNA methylation, (2) compare DNA methylation clocks derived from bone with those derived from whole blood, and (3) establish a relationship between DNA methylation age and 1-year mortality within the geriatric hip fracture population. METHODS:Patients ≥65 years old who presented to a Level-I trauma center with a hip fracture were prospectively enrolled from 2020 to 2021. Preoperative whole blood and intraoperative bone samples were collected. Following DNA extraction, RRBS (reduced representation bisulfite sequencing) libraries for methylation clock analysis were prepared. Sequencing data were analyzed using computational algorithms previously described by Horvath et al. to build a regression model of methylation (biological) age for each tissue type. Student t tests were used to analyze differences (Δ) in methylation age versus chronological age. Correlation between blood and bone methylation ages was expressed using the Pearson R coefficient. RESULTS:Blood and bone samples were collected from 47 patients. DNA extraction, sequencing, and methylation analysis were performed on 24 specimens from 12 subjects. Mean age at presentation was 85.4 ± 8.65 years. There was no difference in DNA extraction yield between the blood and bone samples (p = 0.935). The mean follow-up duration was 12.4 ± 4.3 months. The mortality cohort (4 patients, 33%) showed a mean ΔAgeBone of 18.33 ± 6.47 years and mean ΔAgeBlood of 16.93 ± 4.02 years. In comparison, the survival cohort showed a significantly lower mean ΔAgeBone and ΔAgeBlood (7.86 ± 6.7 and 7.31 ± 7.71 years; p = 0.026 and 0.039, respectively). Bone-derived methylation age was strongly correlated with blood-derived methylation age (R = 0.81; p = 0.0016). CONCLUSIONS:Bone-derived DNA methylation clocks were found to be both feasible and strongly correlated with those derived from whole blood within a geriatric hip fracture population. Mortality was independently associated with the DNA methylation age, and that age was approximately 17 years greater than chronological age in the mortality cohort. The results of the present study suggest that prevention of advanced DNA methylation may play a key role in decreasing mortality following hip fracture. LEVEL OF EVIDENCE:Prognostic Level I . See Instructions for Authors for a complete description of levels of evidence.

Substantial numbers of somatic mutations have been found to accumulate with age in different human tissues. Clonal cellular amplification of some of these mutations can cause cancer and other diseases. However, it is as yet unclear if and to what extent an increased burden of random mutations can affect cellular function without clonal amplification. We tested this in cell culture, which avoids the limitation that an increased mutation burden in vivo typically leads to cancer. We performed single-cell whole-genome sequencing of primary fibroblasts from DNA mismatch repair (MMR) deficient Msh2-/- mice and littermate control animals after long-term passaging. Apart from analyzing somatic mutation burden we analyzed clonality, mutational signatures, and hotspots in the genome, characterizing the complete landscape of somatic mutagenesis in normal and MMR-deficient mouse primary fibroblasts during passaging. While growth rate of Msh2-/- fibroblasts was not significantly different from the controls, the number of de novo single-nucleotide variants (SNVs) increased linearly up until at least 30,000 SNVs per cell, with the frequency of small insertions and deletions (INDELs) plateauing in the Msh2-/- fibroblasts to about 10,000 INDELS per cell. We provide evidence for negative selection and large-scale mutation-driven population changes, including significant clonal expansion of preexisting mutations and widespread cell-strain-specific hotspots. Overall, our results provide evidence that increased somatic mutation burden drives significant cell evolutionary changes in a dynamic cell culture system without significant effects on growth. Since similar selection processes against mutations preventing organ and tissue dysfunction during aging are difficult to envision, these results suggest that increased somatic mutation burden can play a causal role in aging and diseases other than cancer.

Thus far, multiple techniques for single cell analysis have been developed, yet we lack a relatively simple tool to assess DNA and RNA from the same cell at whole-transcriptome and whole-genome depths. Here we present an updated method for physical separation of cytoplasmic RNA from the nuclei, which allows for simultaneous studies of DNA and RNA from the same single cell. The method consists of three steps - 1) immobilization of a single cell on solid substrate, 2) hypotonic lysis of immobilized single cell, and 3) separation of cytosol containing aqueous phase and immobilized nucleus. We found that DNA and RNA extracted from single cell using our approach is suitable for downstream sequencing-based applications. We demonstrated that the coverage of transcriptome and genome sequencing data obtained after DNA/RNA separation is similar to that observed without separation. We also showed that the separation procedure does not create any noticeable bias in observed mutational load or mutation spectra. Thus, our method can serve as a tool for simultaneous complex analysis of the genome and transcriptome, providing necessary information on the relationship between somatic mutations and the regulation of gene expression.

Detecting somatic mutations in normal cells and tissues is notoriously challenging due to their low abundance, orders of magnitude below the sequencing error rate. While several techniques, such as single-cell and single-molecule sequencing, have been developed to identify somatic mutations, they are insufficient for detecting genomic structural variants (SVs), which have a significantly greater impact than single-nucleotide variants (SNVs). We introduce Single-Molecule Mutation Sequencing for Structural Variants (SMM-SV-seq), a novel method combining Tn5-mediated, chimera-free library preparation with the precision of error-corrected next-generation sequencing (ecNGS). This approach enhances SV detection accuracy without relying on independent supporting sequencing reads. Our validation studies on human primary fibroblasts treated with varying concentrations of the clastogen bleomycin demonstrated a significant, up to tenfold and dose-dependent, increase in deletions and translocations 24 hours post-treatment. Evaluating SMM-SV-seq’s performance against established computational tools for SV detection, such as Manta and DELLY, using a well-characterized human cell line, SMM-SV-seq showed precision and recall rates of 61.9% and 85.8%, respectively, significantly outperforming Manta (10% precision, 23% recall) and DELLY (15% precision, 32% recall). Using SMM-SV-seq, we documented clear, direct evidence of negative selection against structural variants over time. After a single 2 Gy dose of ionizing radiation, SVs in normal human primary fibroblasts peaked at 24 hours post-intervention and then declined to nearly background levels by day six, highlighting the cellular mechanisms that selectively disadvantage cells harboring these mutations. Additionally, SMM-SV-seq revealed that BRCA1-deficient human breast epithelial cells are more susceptible to the mutagenic effects of ionizing radiation compared to BRCA1-proficient isogenic control cells, suggesting a potential molecular mechanism for increased breast cancer risk in BRCA1 mutation carriers. SMM-SV-seq represents a significant advancement in genomic analysis, enabling the accurate detection of somatic structural variants in normal cells and tissues for the first time. This method complements our previously published Single-Molecule Mutation sequencing (SMM-seq), effective for detecting single-nucleotide variants (SNVs) and small insertions and deletions (INDELs). By addressing challenges such as self-ligation in library preparation and leveraging a powerful ecNGS strategy, SMM-SV-seq enhances the robustness of our genomic analysis toolkit. This breakthrough paves the way for new research into genetic variability and mutation processes, offering deeper insights that could advance our understanding of aging, cancer, and other human diseases. ### Competing Interest Statement A.Y.M. , J.V. and M.L. are co-founders of SingulOmics Corp. A.Y.M. and J.V. are co-founders of Mutagentech Corp.

Somatic mutations accumulate in multiple organs and tissues during aging and are a known cause of cancer. Cellular senescence is a possible cause of functional decline in aging, yet also acts as an anticancer mechanism in vivo. Here, we compared somatic mutation burden between early passage and deeply senescent human fibroblasts using single-cell whole-genome sequencing. The results show that single-nucleotide variants (SNVs) and small insertions and deletions (INDELs) are increased in senescent cells by about twofold but have the same mutational signature as early passage cells. The increase in SNVs and INDELs can be explained by increased replication errors due to the increased number of cell divisions senescent cells are likely to have undergone. By contrast, a stark increase of aneuploidies was observed in deeply senescent cells, with about half of all senescent cells affected but none of the early passage cells analyzed. These results indicate that large chromosomal events rather than small base substitutions or insertions and deletions could be mechanistically linked to cellular senescence.

Retrotransposons are a class of transposable elements capable of self-replication and insertion into new genomic locations. Across species, the mobilization of retrotransposons in somatic cells has been suggested to contribute to the cell and tissue functional decline that occurs during aging. Retrotransposons are broadly expressed across cell types, and de novo insertions have been observed to correlate with tumorigenesis. However, the extent to which new retrotransposon insertions occur during normal aging and their effect on cellular and animal function remains understudied. Here, we use a single nucleus whole genome sequencing approach in Drosophila to directly test whether transposon insertions increase with age in somatic cells. Analyses of nuclei from thoraces and indirect flight muscles using a newly developed pipeline, Retrofind, revealed no significant increase in the number of transposon insertions with age. Despite this, reducing the expression of two different retrotransposons, 412 and Roo, extended lifespan, but did not alter indicators of health such as stress resistance. This suggests a key role for transposon expression and not insertion in regulating longevity. Transcriptomic analyses revealed similar changes to gene expression in 412 and Roo knockdown flies and highlighted changes to genes involved in proteolysis and immune function as potential contributors to the observed changes in longevity. Combined, our data show a clear link between retrotransposon expression and aging.

Postzygotic somatic mutations have been found associated with human disease, including diseases other than cancer. Most information on somatic mutations has come from studying clonally amplified mutant cells, based on a growth advantage or genetic drift. However, almost all somatic mutations are unique for each cell, and the quantitative analysis of these low-abundance mutations in normal tissues remains a major challenge in biology. Here, we introduce single-molecule mutation sequencing (SMM-seq), a novel approach for quantitative identification of point mutations in normal cells and tissues.

Although lung cancer risk among smokers is dependent on smoking dose, it remains unknown if this increased risk reflects an increased rate of somatic mutation accumulation in normal lung cells. Here, we applied single-cell whole-genome sequencing of proximal bronchial basal cells from 33 participants aged between 11 and 86 years with smoking histories varying from never-smoking to 116 pack-years. We found an increase in the frequency of single-nucleotide variants and small insertions and deletions with chronological age in never-smokers, with mutation frequencies significantly elevated among smokers. When plotted against smoking pack-years, mutations followed the linear increase in cancer risk until about 23 pack-years, after which no further increase in mutation frequency was observed, pointing toward individual selection for mutation avoidance. Known lung cancer-defined mutation signatures tracked with both age and smoking. No significant enrichment for somatic mutations in lung cancer driver genes was observed.

Inherited germline mutations in the breast cancer gene 1 (BRCA1) or BRCA2 genes (herein BRCA1/2) greatly increase the risk of breast and ovarian cancer, presumably by elevating somatic mutational errors as a consequence of deficient DNA repair. However, this has never been directly demonstrated by a comprehensive analysis of the somatic mutational landscape of primary, noncancer, mammary epithelial cells of women diagnosed with pathogenic BRCA1/2 germline mutations. Here, we used an accurate, single-cell whole-genome sequencing approach to first show that telomerized primary mammary epithelial cells heterozygous for a highly penetrant BRCA1 variant displayed a robustly elevated mutation frequency as compared with their isogenic control cells. We then demonstrated a small but statistically significant increase in mutation frequency in mammary epithelial cells isolated from the breast of BRCA1/2 mutation carriers as compared with those obtained from age-matched controls with no genetically increased risk for breast cancer.

OBJECTIVE:To establish a workflow for isolating single trophectoderm (TE) and inner cell mass (ICM) cells and to simultaneously evaluate these cells for copy number variation (CNV) as well as methylome development. DESIGN:Experimental. SETTING:Academic medical center. PATIENT(S):Donated genetically abnormal blastocysts. INTERVENTION(S):Single cells were isolated, followed by bisulfite conversion and sequencing to identify CNV and methylome profiles. MAIN OUTCOME MEASURE(S):CNV and methylation profiling. RESULT(S):Two embryos were dissociated, isolating 46 single cells, with 17 ICM and 12 TE cells selected for further downstream analysis. Chromosome ploidies and embryo sex were concordant with the results from conventional aneuploidy testing. In 3 of the 29 cells, additional aneuploidies were discovered, indicating possible mosaicism undetected by routine preimplantation genetic testing for aneuploidy. CpG methylation frequency was higher in ICM cells compared with TE cells (44.3% vs. 32.4%), respectively, while non-CpG methylation frequency was similar among both cell types. CpG methylation levels accurately distinguished ICM from TE cells epigenetically. CONCLUSION(S):We describe an effective workflow for isolating and sequencing single ICM and TE cells from human blastocysts. The use of methylation profiling can help distinguish these two cell populations better then morphologic identification alone. TE cells had significantly lower levels of DNA methylation, which may be explained in part by the fact that these cells have begun the process of differentiation and are transcriptionally more active than ICM. This approach may be used to explore the genetic complexities within human embryos, specifically among the two primary cell types seen at this stage of development.