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Single-cell analysis of gene expression is increasingly important for the analysis of complex tissues, including cancer, developing organs and adult stem cell niches. Here we present a detailed protocol for quantitative gene expression analysis in single cells, by the sequencing of mRNA 5′ ends. In all, 96 cells are lysed, and their mRNA is converted to cDNA. By using a template-switching mechanism, a bar code and an upstream primer-binding sequence are introduced simultaneously with reverse transcription. All cDNA is pooled and then prepared for 5′ end sequencing, including fragmentation, adapter ligation and PCR amplification. The chief advantage of this approach is the great reduction in cost and time, afforded by the early bar-coding strategy. Compared with previous methods, it is more suitable for large-scale quantitative analysis, as well as for the characterization of transcription start sites, but it is unsuitable for the detection of alternatively spliced transcripts. Sample preparation takes 3 d, and two sets of 96 cells can be prepared in parallel. Finally, the sequencing and data analysis can take an additional 4 d altogether.

Metastatic breast cancers are still incurable. Characterizing the evolutionary landscape of these cancers, including the role of metastatic axillary lymph nodes (ALNs) in seeding distant organ metastasis, can provide a rational basis for effective treatments. Here, we have described the genomic analyses of the primary tumors and metastatic lesions from 99 samples obtained from 20 patients with breast cancer. Our evolutionary analyses revealed diverse spreading and seeding patterns that govern tumor progression. Although linear evolution to successive metastatic sites was common, parallel evolution from the primary tumor to multiple distant sites was also evident. Metastatic spreading was frequently coupled with polyclonal seeding, in which multiple metastatic subclones originated from the primary tumor and/or other distant metastases. Synchronous ALN metastasis, a well-established prognosticator of breast cancer, was not involved in seeding the distant metastasis, suggesting a hematogenous route for cancer dissemination. Clonal evolution coincided frequently with emerging driver alterations and evolving mutational processes, notably an increase in apolipoprotein B mRNA-editing enzyme, catalytic polypeptide-like-associated (APOBEC-associated) mutagenesis. Our data provide genomic evidence for a role of ALN metastasis in seeding distant organ metastasis and elucidate the evolving mutational landscape during cancer progression.

Inhibitory signaling during natural killer (NK) cell education translates into increased responsiveness to activation; however, the intracellular mechanism for functional tuning by inhibitory receptors remains unclear. Secretory lysosomes are part of the acidic lysosomal compartment that mediates intracellular signalling in several cell types. Here we show that educated NK cells expressing self-MHC specific inhibitory killer cell immunoglobulin-like receptors (KIR) accumulate granzyme B in dense-core secretory lysosomes that converge close to the centrosome. This discrete morphological phenotype is independent of transcriptional programs that regulate effector function, metabolism and lysosomal biogenesis. Meanwhile, interference of signaling from acidic Ca 2+ stores in primary NK cells reduces target-specific Ca 2+ -flux, degranulation and cytokine production. Furthermore, inhibition of PI(3,5)P 2 synthesis, or genetic silencing of the PI(3,5)P 2 -regulated lysosomal Ca 2+ -channel TRPML1, leads to increased granzyme B and enhanced functional potential, thereby mimicking the educated state. These results indicate an intrinsic role for lysosomal remodeling in NK cell education. Natural killer (NK) cells are functionally calibrated against self MHC during a process termed education. Here the authors show that NK cell education is associated with the accumulation of dense-core secretory lysosomes for expedited release of granzyme B and Ca 2+ flux upon target recognition and NK cell activation.

Background Tumor heterogeneity in breast cancer tumors is today widely recognized. Most of the available knowledge in genetic variation however, relates to the primary tumor while metastatic lesions are much less studied. Many studies have revealed marked alterations of standard prognostic and predictive factors during tumor progression. Characterization of paired primary- and metastatic tissues should therefore be fundamental in order to understand mechanisms of tumor progression, clonal relationship to tumor evolution as well as the therapeutic aspects of systemic disease. Methods We performed full exome sequencing of primary breast cancers and their metastases in a cohort of ten patients and further confirmed our findings in an additional cohort of 20 patients with paired primary and metastatic tumors. Furthermore, we used gene expression from the metastatic lesions and a primary breast cancer data set to study the gene expression of the AKAP gene family. Results We report that somatic mutations in A-kinase anchoring proteins are enriched in metastatic lesions. The frequency of mutation in the AKAP gene family was 10% in the primary tumors and 40% in metastatic lesions. Several copy number variations, including deletions in regions containing AKAP genes were detected and showed consistent patterns in both investigated cohorts. In a second cohort containing 20 patients with paired primary and metastatic lesions, AKAP mutations showed an increasing variant allele frequency after multiple relapses. Furthermore, gene expression profiles from the metastatic lesions ( n  = 120) revealed differential expression patterns of AKAPs relative to the tumor PAM50 intrinsic subtype, which were most apparent in the basal-like subtype. This pattern was confirmed in primary tumors from TCGA ( n  = 522) and in a third independent cohort ( n  = 182). Conclusion Several studies from primary cancers have reported individual AKAP genes to be associated with cancer risk and metastatic relapses as well as direct involvement in cellular invasion and migration processes. Our findings reveal an enrichment of mutations in AKAP genes in metastatic breast cancers and suggest the involvement of AKAPs in the metastatic process. In addition, we report an AKAP gene expression pattern that consistently follows the tumor intrinsic subtype, further suggesting AKAP family members as relevant players in breast cancer biology.

Molecular signatures to guide decisions for adjuvant chemotherapy are recommended in early ER-positive, HER2-negative breast cancer. The objective of this study was to assess what impact gene-expression-based risk testing has had following its recommendation by Swedish national guidelines. Postmenopausal women with ER-positive, HER2-negative and node negative breast cancer at intermediate clinical risk and eligible for chemotherapy were identified retrospectively from five Swedish hospitals. Tumor characteristics, results from Prosigna® test and final treatment decision were available for all patients. Treatment recommendations were compared with the last version of regional guidelines before the introduction of routine risk signature testing. Among the 360 included patients, 41% (n = 148) had a change in decision for adjuvant treatment based on Prosigna® test result. Out of the patients with clinical indication for adjuvant chemotherapy, 52% (n = 118) could avoid treatment based on results from Prosigna® test. On the contrary, 23% (n = 30) of the patients with no indication were escalated to receive adjuvant chemotherapy after testing. Ki67 could not distinguish between the Prosigna® risk groups or intrinsic subtypes and did not significantly differ between patients in which decision for adjuvant therapy was changed based on the test results. In conclusion, we report the first real-world data from implementation of gene-expression-based risk assessment in a Swedish context, which may facilitate the optimization of future versions of the national guidelines.

Cancer forms highly heterogeneous tissues at several molecular levels, genomic, proteomic, transcriptomic and other epigenetic traits. The level of complexity is further augmented by the dynamic nature of tumor progression with cancer cell populations evolving in a clonal manner. The clonal evolution of tumors is shaped by selective pressure exerted by endogenous factors such as intra tumor dynamics between different clones and exogenous factors such as microenvironment and therapeutics. The technical advances in next generation sequencing has accelerated and facilitated a massive acquisition of genomic and transcriptional data from different cancers during the last decade. Despite the increased knowledge in the transcriptional and genomic landscape of tumors, many questions have still not been fully addressed and one of the explanations lies in the heterogenetic nature of cancer tissues that has to be desiccated into its fundamental parts - the single cancer cells. In this thesis the heterogenetic nature of normal- and cancer tissues and the implications to single-cell based methods are discussed. In order to study the transcriptome of single-cells delicate molecular tools needs to be developed. In the two first papers we describe a single-cell RNA sequencing method that is highly sensitive and can produce full gene expression profiles of hundreds of single-cells per sequencing experiment.Several models have been proposed for tumor evolution and one fundamental question is the hierarchical organization of tumor propagating cells. In paper III we studied the tumor progression in myelodysplastic syndromes (MDS), a clonal hematological disorder in which multiple haematological lineages are affected. We found that the putative MDS-stem cell (SC) population is functionally and molecularly distinct from its downstream progenitors and that in MDS, self-renewal and that acquisition of somatic mutations was restricted to the MDS-stem cell population. By targeted sequencing of purified populations and single-cell derived stem cell clones we could track all somatic mutations found in the bone marrow of MDS patients to the distinct MDS-stem cells providing definitive evidence for the existence of rare human MDSSCs in vivo.Another aspect of heterogeneity in solid tumors is the genetic heterogeneity between primary tumor and the metastatic lesions. In paper IV we addressed the heterogeneity in metastatic breast cancer by comparing the genetic profiles of ten pairs of primary tumor to their metastatic lesion obtained from exome sequencing. We found a marked heterogeneity in number of somatic mutations as well as the extent of chromosomal aberrations in the metastatic lesions. We also found a number of mutated genes to be significantly enriched in the metastases. The clinical implications to metastatic heterogeneity is supported by altered receptor status and drug resistance in metastatic lesions and suggest that extended characterization of the metastases is of great importance.In summary, heterogeneity is the main characteristic of both normal and cancer tissues. To resolve the mixed patterns of gene regulation and genomic aberrations, single-cell based approaches needs to be applied and will be a powerful tool to shed light on questions in tumor biology such as transcription dynamics and genetic selection.

Inhibitory signaling during natural killer (NK) cell education translates into increased responsiveness to activation; however the intracellular mechanism for functional tuning by inhibitory receptors remains unclear. We found that educated NK cells expressing self-MHC specific inhibitory killer cell immunoglobulin-like receptors (KIR) show accumulation of granzyme B, localized in dense-core secretory lysosomes, converged close to the centrosome. This discrete morphological phenotype persists in self-KIR+ NK cells independently of transcriptional programs that regulate effector function, metabolism and lysosomal biogenesis. The granzyme-B dense, large secretory lysosomes in self-KIR+ NK cells were efficiently released upon target cell recognition, contributing to their enhanced cytotoxic capacity. Secretory lysosomes are part of the acidic lysosomal compartment, which has been shown to channel calcium and mediate intracellular signalling in several cell types. Interference of signaling from acidic Ca2+ stores in primary NK cells reduced both target-specific Ca2+-flux, degranulation and cytokine production. Furthermore, inhibition of PI(3,5)P2 synthesis or genetic silencing of the PI(3,5)P2-regulated lysosomal Ca2+-channel TRPML1 led to increased levels of granzyme B and enhanced functional potential. These results indicate an intrinsic role for lysosomal homeostasis in NK cell education. Footnotes * Revised for increased clarity with greater focus on the cellular mechanism behind NK cell education. New data added to increase robustness of findings.