Cosmic Whirl: Navigating the Comet Trail in DNA: H2AX Phosphorylation and the Enigma of Uncertain Significance Variants

Pathogenic variations in the BRCA2 gene have been detected with the development of next-generation sequencing (NGS)-based hereditary cancer panel testing technology. It also reveals an increasing number of variants of uncertain significance (VUSs). Well-established functional tests are crucial to accurately reclassifying VUSs for effective diagnosis and treatment. We retrospectively analyzed the multi-gene cancer panel results of 922 individuals and performed in silico analysis following ClinVar classification. Then, we selected five breast cancer-diagnosed patients' missense BRCA2 VUSs (T1011R, T1104P/M1168K, R2027K, G2044A, and D2819) for reclassification. The effects of VUSs on BRCA2 function were analyzed using comet and H2AX phosphorylation (γH2AX) assays before and after the treatment of peripheral blood mononuclear cells (PBMCs) of subjects with the double-strand break (DSB) agent doxorubicin (Dox). Before and after Dox-induction, the amount of DNA in the comet tails was similar in VUS carriers; however, notable variations in γH2AX were observed, and according to combined computational and functional analyses, we reclassified T1001R as VUS-intermediate, T1104P/M1168K and D2819V as VUS (+), and R2027K and G2044A as likely benign. These findings highlight the importance of the variability of VUSs in response to DNA damage before and after Dox-induction and suggest that further investigation is needed to understand the underlying mechanisms.

Expression of human BRCA2 in Saccharomyces cerevisiae complements the loss of RAD52 in double-strand break repair

BRCA2 is a tumor-suppressor gene that is normally expressed in the breast and ovarian tissue of mammals. The BRCA2 protein mediates the repair of double-strand breaks (DSBs) using homologous recombination, which is a conserved pathway in eukaryotes. Women who express missense mutations in the BRCA2 gene are predisposed to an elevated lifetime risk for both breast cancer and ovarian cancer. In the present study, the efficiency of human BRCA2 (hBRCA2) in DSB repair was investigated in the budding yeast Saccharomyces cerevisiae. While budding yeast does not possess a true BRCA2 homolog, they have a potential functional homolog known as Rad52, which is an essential repair protein involved in mediating homologous recombination using the same mechanism as BRCA2 in humans. Therefore, to examine the functional overlap between Rad52 in yeast and hBRCA2, we expressed the wild-type hBRCA2 gene in budding yeast with or without Rad52 and monitored ionizing radiation resistance and DSB repair efficiency. We found that the expression of hBRCA2 in rad52 mutants increases both radiation resistance and DSB repair frequency compared to cells not expressing BRCA2. Specifically, BRCA2 improved the protection against ionizing radiation by at least 1.93-fold and the repair frequency by 6.1-fold. In addition, our results show that homology length influences repair efficiency in rad52 mutant cells, which impacts BRCA2 mediated repair of DSBs. This study provides evidence that S. cerevisiae could be used to monitor BRCA2 function, which can help in understanding the genetic consequences of BRCA2 variants and how they may contribute to cancer progression.

Saccharomyces cerevisiae as a Model System for Eukaryotic Cell Biology, from Cell Cycle Control to DNA Damage Response

The yeast Saccharomyces cerevisiae has been used for bread making and beer brewing for thousands of years. In addition, its ease of manipulation, well-annotated genome, expansive molecular toolbox, and its strong conservation of basic eukaryotic biology also make it a prime model for eukaryotic cell biology and genetics. In this review, we discuss the characteristics that made yeast such an extensively used model organism and specifically focus on the DNA damage response pathway as a prime example of how research in S. cerevisiae helped elucidate a highly conserved biological process. In addition, we also highlight differences in the DNA damage response of S. cerevisiae and humans and discuss the challenges of using S. cerevisiae as a model system.

Yeast as a Model to Unravel New BRCA2 Functions in Cell Metabolism

Mutations in BRCA2 gene increase the risk for breast cancer and for other cancer types, including pancreatic and prostate cancer. Since its first identification as an oncosupressor in 1995, the best-characterized function of BRCA2 is in the repair of DNA double-strand breaks (DSBs) by homologous recombination. BRCA2 directly interacts with both RAD51 and single-stranded DNA, mediating loading of RAD51 recombinase to sites of single-stranded DNA. In the absence of an efficient homologous recombination pathway, DSBs accumulate resulting in genome instability, thus supporting tumorigenesis. Yet the precise mechanism by which BRCA2 exerts its tumor suppressor function remains unclear. BRCA2 has also been involved in other biological functions including protection of telomere integrity and stalled replication forks, cell cycle progression, transcriptional control and mitophagy. Recently, we and others have reported a role of BRCA2 in modulating cell death programs through a molecular mechanism conserved in yeast and mammals. Here we hypothesize that BRCA2 is a multifunctional protein which exerts specific functions depending on cell stress response pathway. Based on a differential RNA sequencing analysis carried out on yeast cells either growing or undergoing a regulated cell death process, either in the absence or in the presence of BRCA2, we suggest that BRCA2 causes central carbon metabolism reprogramming in response to death stimuli and encourage further investigation on the role of metabolic reprogramming in BRCA2 oncosuppressive function.

Imprecise Medicine: BRCA2 Variants of Uncertain Significance (VUS), the Challenges and Benefits to Integrate a Functional Assay Workflow with Clinical Decision Rules

Pathological mutations in homology-directed repair (HDR) genes impact both future cancer risk and therapeutic options for patients. HDR is a high-fidelity DNA repair pathway for resolving DNA double-strand breaks throughout the genome. BRCA2 is an essential protein that mediates the loading of RAD51 onto resected DNA breaks, a key step in HDR. Germline mutations in BRCA2 are associated with an increased risk for breast, ovarian, prostate, and pancreatic cancer. Clinical findings of germline or somatic BRCA2 mutations in tumors suggest treatment with platinum agents or PARP inhibitors. However, when genetic analysis reveals a variant of uncertain significance (VUS) in the BRCA2 gene, precision medicine-based decisions become complex. VUS are genetic changes with unknown pathological impact. Current statistics indicate that between 10–20% of BRCA sequencing results are VUS, and of these, more than 50% are missense mutations. Functional assays to determine the pathological outcome of VUS are urgently needed to provide clinical guidance regarding cancer risk and treatment options. In this review, we provide a brief overview of BRCA2 functions in HDR, describe how BRCA2 VUS are currently assessed in the clinic, and how genetic and biochemical functional assays could be integrated into the clinical decision process. We suggest a multi-step workflow composed of robust and accurate functional assays to correctly evaluate the potential pathogenic or benign nature of BRCA2 VUS. Success in this precision medicine endeavor will offer actionable information to patients and their physicians.

Globally Rare BRCA2 Variants With Founder Haplotypes in the South African Population: Implications for Point-of-Care Testing Based on a Single-Institution BRCA1/2 Next-Generation Sequencing Study

Breast cancer patients historically benefitted from population-based genetic research performed in South Africa, which led to the development of founder-based BRCA1/2 diagnostic tests. With the advent of next-generation sequencing (NGS) technologies, the clinical utility of limited, targeted genetic assays were questioned. The study focused on mining NGS data obtained from an extensive single-institution NGS series (n=763). The aims were to determine (i) the prevalence of the most common recurrent/founder variants in patients referred for NGS directly; and (ii) to explore the data for inferred haplotypes associated with previous and potential new recurrent/founder variants. The identification of additional founder variants was essential for promoting and potentially advancing to rapid founder-based BRCA1/2 point-of-care (POC) technology as a time- and cost-effective alternative. NGS revealed actionable BRCA1/2 variants in 11.1% of patients tested (BRCA1 - 4.7%; BRCA2 - 6.4%), of which 22.4% represented variants currently screened for using first-tier targeted genetic testing. A retrospective investigation into the overall mutation-positive rate for an extended cohort (n=1906), which included first-tier test results, revealed that targeted genetic testing identified 74% of all pathogenic variants. This percentage justified the use of targeted genetic testing as a first-tier assay. Inferred haplotype analysis confirmed the founder status of BRCA2 c.5771_5774del (rs80359535) and c.7934del (rs80359688) and revealed an additional African founder variant (BRCA2 c.582G>A - rs80358810). A risk-benefit analysis using a questionnaire-based survey was performed in parallel to determine genetic professionals' views regarding POC testing. This was done to bridge the clinical implementation gap between haplotype analysis and POC testing as a first-tier screen during risk stratification of breast and ovarian cancer patients. The results reflected high acceptance (94%) of BRCA1/2 POC testing when accompanied by genetic counselling. Establishing the founder status for several recurrent BRCA2 variants across ethnic groups supports unselected use of the BRCA POC assay in all SA breast/ovarian cancer patients by recent local and international public health recommendations. Incorporating POC genotyping into the planned NGS screening algorithm of the Department of Health will ensure optimal use of the country's recourses to adhere to the set standards for optimal care and management for all breast cancer patients.

Molecular contribution of BRCA1 and BRCA2 to genome instability in breast cancer patients: review of radiosensitivity assays

BACKGROUND

DNA repair pathways, cell cycle arrest checkpoints, and cell death induction are present in cells to process DNA damage and prevent genomic instability caused by various extrinsic and intrinsic ionizing factors. Mutations in the genes involved in these pathways enhances the ionizing radiation sensitivity, reduces the individual's capacity to repair DNA damages, and subsequently increases susceptibility to tumorigenesis.

BODY

BRCA1 and BRCA2 are two highly penetrant genes involved in the inherited breast cancer and contribute to different DNA damage pathways and cell cycle and apoptosis cascades. Mutations in these genes have been associated with hypersensitivity and genetic instability as well as manifesting severe radiotherapy complications in breast cancer patients. The genomic instability and DNA repair capacity of breast cancer patients with BRCA1/2 mutations have been analyzed in different studies using a variety of assays, including micronucleus assay, comet assay, chromosomal assay, colony-forming assay, γ -H2AX and 53BP1 biomarkers, and fluorescence in situ hybridization. The majority of studies confirmed the enhanced spontaneous & radiation-induced radiosensitivity of breast cancer patients compared to healthy controls. Using G2 micronucleus assay and G2 chromosomal assay, most studies have reported the lymphocyte of healthy carriers with BRCA1 mutation are hypersensitive to invitro ionizing radiation compared to non-carriers without a history of breast cancer. However, it seems this approach is not likely to be useful to distinguish the BRCA carriers from non-carrier with familial history of breast cancer.

CONCLUSION

In overall, breast cancer patients are more radiosensitive compared to healthy control; however, inconsistent results exist about the ability of current radiosensitive techniques in screening BRCA1/2 carriers or those susceptible to radiotherapy complications. Therefore, developing further radiosensitivity assay is still warranted to evaluate the DNA repair capacity of individuals with BRCA1/2 mutations and serve as a predictive factor for increased risk of cancer mainly in the relatives of breast cancer patients. Moreover, it can provide more evidence about who is susceptible to manifest severe complication after radiotherapy.

Stimulation of homologous recombination in plants expressing heterologous recombinases

BACKGROUND

Current excitement about the opportunities for gene editing in plants have been prompted by advances in CRISPR/Cas and TALEN technologies. CRISPR/Cas is widely used to knock-out or modify genes by inducing targeted double-strand breaks (DSBs) which are repaired predominantly by error-prone non-homologous end-joining or microhomology-mediated end joining resulting in mutations that may alter or abolish gene function. Although such mutations are random, they occur at sufficient frequency to allow useful mutations to be routinely identified by screening. By contrast, gene knock-ins to replace entire genes with alternative alleles or copies with specific characterised modifications, is not yet routinely possible. Gene replacement (or gene targeting) by homology directed repair occurs at extremely low frequency in higher plants making screening for useful events unfeasible. Homology directed repair might be increased by inhibiting non-homologous end-joining and/or stimulating homologous recombination (HR). Here we pave the way to increasing gene replacement efficiency by evaluating the effect of expression of multiple heterologous recombinases on intrachromosomal homologous recombination (ICR) in Nicotiana tabacum plants.

RESULTS

We expressed several bacterial and human recombinases in different combinations in a tobacco transgenic line containing a highly sensitive β-glucuronidase (GUS)-based ICR substrate. Coordinated simultaneous expression of multiple recombinases was achieved using the viral 2A translational recoding system. We found that most recombinases increased ICR dramatically in pollen, where HR will be facilitated by the programmed DSBs that occur during meiosis. DMC1 expression produced the greatest stimulation of ICR in primary transformants, with one plant showing a 1000-fold increase in ICR frequency. Evaluation of ICR in homozygous T2 plant lines revealed increases in ICR of between 2-fold and 380-fold depending on recombinase(s) expressed. By comparison, ICR was only moderately increased in vegetative tissues and constitutive expression of heterologous recombinases also reduced plant fertility.

CONCLUSION

Expression of heterologous recombinases can greatly increase the frequency of HR in plant reproductive tissues. Combining such recombinase expression with the use of CRISPR/Cas9 to induce DSBs could be a route to radically improving gene replacement efficiency in plants.

Yeast-based assays for the functional characterization of cancer-associated variants of human DNA repair genes

Technological advances are continuously revealing new genetic variants that are often difficult to interpret. As one of the most genetically tractable model organisms, yeast can have a central role in determining the consequences of human genetic variation. DNA repair gene mutations are associated with many types of cancers, therefore the evaluation of the functional impact of these mutations is crucial for risk assessment and for determining therapeutic strategies. Owing to the evolutionary conservation of DNA repair pathways between human cells and the yeast Saccharomyces cerevisiae, several functional assays have been developed. Here, we describe assays for variants of human genes belonging to the major DNA repair pathways divided in functional assays for human genes with yeast orthologues and human genes lacking a yeast orthologue. Human genes with orthologues can be studied by introducing the correspondent human mutations directly in the yeast gene or expressing the human gene carrying the mutations; while the only possible approach for human genes without a yeast orthologue is the heterologous expression. The common principle of these approaches is that the mutated gene determines a phenotypic alteration that can vary according to the gene studied and the domain of the protein. Here, we show how the versatility of yeast can help in classifying cancer-associated variants.

6-Thioguanine and Its Analogs Promote Apoptosis of Castration-Resistant Prostate Cancer Cells in a BRCA2-Dependent Manner

Background: Mutations in the oncosuppressor gene BReast CAncer susceptibility gene 2 (BRCA2) predispose to aggressive forms of prostate cancer which show poor response to taxane-based therapy, the standard treatment for castration-resistant, aggressive prostate cancer. Herein, we addressed the question whether changes in BRCA2 expression, a potential surrogate marker for BRCA2 activity, may affect the response of castration-resistant prostate cancer cells to 6-thioguanine (6-TG), a thiopurine used in the treatment of haematological malignancies. Methods: Yeast, normal prostate cells and castration-resistant prostate cancer cells were treated with 6-TG or its analogues, in presence or absence of paclitaxel, or with olaparib, a poly-(ADP-ribose) polymerase (PARP) inhibitor currently in clinical trials for treatment of metastatic castration-resistant prostate cancer, and cell proliferation, apoptosis and androgen receptor (AR) levels were measured. Results: 6-TG inhibited cell proliferation in yeast, normal and castration-resistant prostate cancer cells but promoted apoptosis only in cancer cells. Suppression of BRCA2 expression by siRNA or shRNA increased the sensitivity to 6-TG- and olaparib-induced apoptosis but did not affect cancer cell response to taxane. Intriguingly, 6-TG reduced AR expression levels independently on BRCA2 expression. Instead, olaparib decreased AR levels only in BRCA2-knockdown prostate cancer cells. Notably, overexpression of BRCA2 resulted in resistance of castration-resistant prostate cancer cells to 6-TG-, taxane- and olaparib-based treatment but promoted sensitivity to apoptosis induced by 2-amino-6-bromopurine and 2,6-dithiopurine, two 6-TG analogues. Conclusions: Our results provide a pre-clinical rationale for the use of 6-TG in the treatment of BRCA2-deficient castration-resistant prostate cancers, and of certain 6-TG analogues for treatment of BRCA2-proficient prostate cancers.

Modeling human disease in yeast: recreating the PI3K-PTEN-Akt signaling pathway in Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae is a model organism that has been thoroughly exploited to understand the universal mechanisms that govern signaling pathways. Due to its ease of manipulation, humanized yeast models that successfully reproduce the function of human genes permit the development of highly efficient genetic approaches for molecular studies. Of special interest are those pathways related to human disease that are conserved from yeast to mammals. However, it is also possible to engineer yeast cells to implement functions that are naturally absent in fungi. Along the years, we have reconstructed several aspects of the mammalian phosphatidylinositol 3-kinase (PI3K) pathway in S. cerevisiae. Here, we briefly review the use of S. cerevisiae as a tool to study human oncogenes and tumor suppressors, and we present an overview of the models applied to the study of the PI3K oncoproteins, the tumor suppressor PTEN, and the Akt protein kinase. We discuss the application of these models to study the basic functional properties of these signaling proteins, the functional assessment of their clinically relevant variants, and the design of feasible platforms for drug discovery.

CRIMEtoYHU: a new web tool to develop yeast-based functional assays for characterizing cancer-associated missense variants

Evaluation of the functional impact of cancer-associated missense variants is more difficult than for protein-truncating mutations and consequently standard guidelines for the interpretation of sequence variants have been recently proposed. A number of algorithms and software products were developed to predict the impact of cancer-associated missense mutations on protein structure and function. Importantly, direct assessment of the variants using high-throughput functional assays using simple genetic systems can help in speeding up the functional evaluation of newly identified cancer-associated variants. We developed the web tool CRIMEtoYHU (CTY) to help geneticists in the evaluation of the functional impact of cancer-associated missense variants. Humans and the yeast Saccharomyces cerevisiae share thousands of protein-coding genes although they have diverged for a billion years. Therefore, yeast humanization can be helpful in deciphering the functional consequences of human genetic variants found in cancer and give information on the pathogenicity of missense variants. To humanize specific positions within yeast genes, human and yeast genes have to share functional homology. If a mutation in a specific residue is associated with a particular phenotype in humans, a similar substitution in the yeast counterpart may reveal its effect at the organism level. CTY simultaneously finds yeast homologous genes, identifies the corresponding variants and determines the transferability of human variants to yeast counterparts by assigning a reliability score (RS) that may be predictive for the validity of a functional assay. CTY analyzes newly identified mutations or retrieves mutations reported in the COSMIC database, provides information about the functional conservation between yeast and human and shows the mutation distribution in human genes. CTY analyzes also newly found mutations and aborts when no yeast homologue is found. Then, on the basis of the protein domain localization and functional conservation between yeast and human, the selected variants are ranked by the RS. The RS is assigned by an algorithm that computes functional data, type of mutation, chemistry of amino acid substitution and the degree of mutation transferability between human and yeast protein. Mutations giving a positive RS are highly transferable to yeast and, therefore, yeast functional assays will be more predictable. To validate the web application, we have analyzed 8078 cancer-associated variants located in 31 genes that have a yeast homologue. More than 50% of variants are transferable to yeast. Incidentally, 88% of all transferable mutations have a reliability score >0. Moreover, we analyzed by CTY 72 functionally validated missense variants located in yeast genes at positions corresponding to the human cancer-associated variants. All these variants gave a positive RS. To further validate CTY, we analyzed 3949 protein variants (with positive RS) by the predictive algorithm PROVEAN. This analysis shows that yeast-based functional assays will be more predictable for the variants with positive RS. We believe that CTY could be an important resource for the cancer research community by providing information concerning the functional impact of specific mutations, as well as for the design of functional assays useful for decision support in precision medicine.

A comprehensive analysis of BRCA2 gene: focus on mechanistic aspects of its functions, spectrum of deleterious mutations, and therapeutic strategies targeting BRCA2-deficient tumors

BRCA2is the main susceptibility gene known to be involved in the pathogenesis of breast cancer. It plays an important role in maintaining the genome stability by homologous recombination through DNA double-strand breaks repairing, by interacting with various other proteins including RAD51, DSS1, RPA, MRE11, PALB2, and p53. BRCA2-deficient cells show the abnormalities of chromosome number. BRCA2 is also found to be involved in centrosome duplication specifically in the metaphase to anaphase transition. Inactivation or depletion of BRCA2 leads to centrosome amplification that results in unequal separation of chromosomes. BRCA2 localizes with central spindle and midbody during telophase and cytokinesis. Inactivation or depletion of BRCA2 leads to multinucleation of cell. Around 2000 mutations have been reported in BRCA2 gene. BRCA2-deficient tumors are being taking into consideration for targeted cancer therapy by using different inhibitors like poly ADP-ribose polymerase and thymidylate synthase. The present review focusses on the role of BRCA2 in various critical cellular processes based on the mechanistic approaches. Mutations reported in the BRCA2 gene in various ethnic groups till date have also been compiled with an insight into the functional aspects of these alterations. The therapeutic strategies for targeting BRCA2-deficient tumors have also been targeted.

DNA repair-related functional assays for the classification of BRCA1 and BRCA2 variants: a critical review and needs assessment

Mutation of BRCA1 and BRCA2 is the most common cause of inherited breast and ovarian cancer. Genetic screens to detect carriers of variants can aid in cancer prevention by identifying individuals with a greater cancer risk and can potentially be used to predict the responsiveness of tumours to therapy. Frequently, classification cannot be performed based on traditional approaches such as segregation analyses, including for many missense variants, which are therefore referred to as variants of uncertain significance (VUS). Functional assays provide an important alternative for classification of BRCA1 and BRCA2 VUS. As reviewed here, both of these tumour suppressors promote the maintenance of genome stability via homologous recombination. Thus, related assays may be particularly relevant to cancer risk. Progress in implementing functional assays to assess missense variants of BRCA1 and BRCA2 is considered here, along with current limitations and the path to more impactful assay systems. While functional assays have been developed to independently evaluate BRCA1 and BRCA2 VUS, high-throughput assays with sufficient sensitivity to characterise the large number of identified variants are lacking. Additionally, because of relatively low conservation of certain domains of BRCA1, and of BRCA2, between humans and rodents, heterologous expression in rodent cells may have limited reliability or capacity to assess variants present throughout either protein. Moving forward, it will be important to perform assays in human cell lines with relevance to particular tumour types, and to strengthen risk predictions based on multifactorial statistical analyses that also include available data on cosegregation and tumour pathology.

Multiple gene sequencing for risk assessment in patients with early-onset or familial breast cancer

Since BRCA mutations are only responsible for 10–20% of cases of breast cancer in patients with early-onset or a family history and since next-generation sequencing technology allows the simultaneous sequencing of a large number of target genes, testing for multiple cancer-predisposing genes is now being considered, but its significance in clinical practice remains unclear. We then developed a sequencing panel containing 68 genes that had cancer risk association for patients with early-onset or familial breast cancer. A total of 133 patients were enrolled and 30 (22.6%) were found to carry germline deleterious mutations, 9 in BRCA1, 11 in BRCA2, 2 in RAD50, 2 in TP53 and one each in ATM, BRIP1, FANCI, MSH2, MUTYH, and RAD51C. Triple-negative breast cancer (TNBC) was associated with the highest mutation rate (45.5%, p = 0.025). Seven of the 9 BRCA1 mutations and the single FANCI mutation were in the TNBC group; 9 of the 11 BRCA2, 1 of the 2 RAD50 as well as BRIP1, MSH2, MUTYH, and RAD51C mutations were in the hormone receptor (HR)(+)Her2(−) group, and the other RAD50, ATM, and TP53 mutations were in the HR(+)Her2(+) group. Mutation carriers were considered as high-risk to develop malignancy and advised to receive cancer screening. Screening protocols of non-BRCA genes were based on their biologic functions; for example, patients carrying RAD51C mutation received a screening protocol similar to that for BRCA, since BRCA and RAD51C are both involved in homologous recombination. In conclusion, we consider that multiple gene sequencing in cancer risk assessment is clinically valuable.

Functional assays for analysis of variants of uncertain significance in BRCA2

Missense variants in the BRCA2 gene are routinely detected during clinical screening for pathogenic mutations in patients with a family history of breast and ovarian cancer. These subtle changes frequently remain of unknown clinical significance because of the lack of genetic information that may help establish a direct correlation with cancer predisposition. Therefore, alternative ways of predicting the pathogenicity of these variants are urgently needed. Since BRCA2 is a protein involved in important cellular mechanisms such as DNA repair, replication, and cell cycle control, functional assays have been developed that exploit these cellular activities to explore the impact of the variants on protein function. In this review, we summarize assays developed and currently utilized for studying missense variants in BRCA2. We specifically depict details of each assay, including variants of uncertain significance analyzed, and describe a validation set of (genetically) proven pathogenic and neutral missense variants to serve as a golden standard for the validation of each assay. Guidelines are proposed to enable implementation of laboratory-based methods to assess the impact of the variant on cancer risk.

The expanding role of yeast in cancer research and diagnosis: insights into the function of the oncosuppressors p53 and BRCA1/2

When the glucose supply is high, despite the presence of oxygen, Saccharomyces cerevisiae uses fermentation as its main metabolic pathway and switches to oxidative metabolism only when this carbon source is limited. There are similarities between glucose-induced repression of oxidative metabolism of yeast and metabolic reprogramming of tumor cells. The glucose-induced repression of oxidative metabolism is regulated by oncogene homologues in yeast, such as RAS and Sch9p, the yeast homologue of Akt. Yeast also undergoes an apoptosis-like programmed cell death process sharing several features with mammalian apoptosis, including oxidative stress and a major role played by mitochondria. Evasion of apoptosis and sustained proliferative signaling are hallmarks of cancer. This, together with the possibility of heterologous expression of human genes in yeast, has allowed new insights to be obtained into the function of mammalian oncogenes/oncosuppressors. Here, we elaborate on the similarities between tumor and yeast cells underpinning the use of this model organism in cancer research. We also review the achievements obtained through heterologous expression in yeast of p53, BRCA1, and BRCA2, which are among the best-known cancer-susceptibility genes, with the aim of understanding their role in tumorigenesis. Yeast-cell-based functional assays for cancer genetic testing will also be dealt with.