Association Between Germline Mutations in BRF1, a Subunit of the RNA Polymerase III Transcription Complex, and Hereditary Colorectal Cancer

MAF1 is a predictive biomarker in HER2 positive breast cancer

RNA polymerase III transcription is pivotal in regulating cellular growth and frequently deregulated in various cancers. MAF1 negatively regulates RNA polymerase III transcription. Currently, it is unclear if MAF1 is universally deregulated in human cancers. Recently, MAF1 expression has been demonstrated to be altered in colorectal and liver carcinomas and Luminal B breast cancers. In this study, we analyzed clinical breast cancer datasets to determine if MAF1 alterations correlate with clinical outcomes in HER2-positive breast cancer. Using various bioinformatics tools, we screened breast cancer datasets for alterations in MAF1 expression. We report that MAF1 is amplified in 39% of all breast cancer sub-types, and the observed amplification co-occurs with MYC. MAF1 amplification correlated with increased methylation of the MAF1 promoter and MAF1 protein expression is significantly decreased in luminal, HER2-positive, and TNBC breast cancer subtypes. MAF1 protein expression is also significantly reduced in stage 2 and 3 breast cancer compared to normal and significantly decreased in all breast cancer patients, regardless of race and age. In SKBR3 and BT474 breast cancer cell lines treated with anti-HER2 therapies, MAF1 mRNA expression is significantly increased. In HER2-positive breast cancer patients, MAF1 expression significantly increases and correlates with five years of relapse-free survival in response to trastuzumab treatment, suggesting MAF1 is a predictive biomarker in breast cancer. These data suggest a role for MAF1 alterations in HER2-positive breast cancer. More extensive studies are warranted to determine if MAF1 serves as a predictive and prognostic biomarker in breast cancer.

BDP1 as a biomarker in serous ovarian cancer

BACKGROUND

TFIIIB, an RNA polymerase III specific transcription factor has been found to be deregulated in human cancers with much of the research focused on the TBP, BRF1, and BRF2 subunits. To date, the TFIIIB specific subunit BDP1 has not been investigated in ovarian cancer but has previously been shown to be deregulated in neuroblastoma, breast cancer, and Non-Hodgkins lymphoma.

RESULTS

Using in silico analysis of clinically derived platforms, we report a decreased BDP1 expression as a result of deletion in serous ovarian cancer and a correlation with higher and advanced ovarian stages. Further analysis in the context of TP53 mutations, a major contributor to ovarian tumorigenesis, suggests that high BDP1 expression is unfavorable for overall survival and high BDP1 expression occurs in stages 2, 3 and 4 serous ovarian cancer. Additionally, high BDP1 expression is disadvantageous and unfavorable for progression-free survival. Lastly, BDP1 expression significantly decreased in patients treated with first-line chemotherapy, platin and taxane, at twelve-month relapse-free survival.

CONCLUSIONS

Taken together with a ROC analysis, the data suggest BDP1 could be of clinical relevance as a predictive biomarker in serous ovarian cancer. Lastly, this study further demonstrates that both the over- and under expression of BDP1 warrants further investigation and suggests BDP1 may exhibit dual function in the context of tumorigenesis.

Transcriptional factor III A promotes colorectal cancer progression by upregulating cystatin A

BACKGROUND

Advanced colorectal cancer (CRC) generally has poor outcomes and high mortality rates. Clarifying the molecular mechanisms underlying CRC progression is necessary to develop new diagnostic and therapeutic strategies to improve CRC outcome and decrease mortality. Transcriptional factor III A (GTF3A), an RNA polymerase III transcriptional factor, is a critical driver of tumorgenesis and aggravates CRC cell growth.

AIM

To confirm whether GTF3A promotes CRC progression by regulating the expression of cystatin A (Csta) gene and investigate whether GTF3A can serve as a prognostic biomarker and therapeutic target for patients with CRC.

METHODS

Human tissue microarrays containing 90 pairs of CRC tissues and adjacent non-tumor tissues, and human tissue microarrays containing 20 pairs of CRC tissues, adjacent non-tumor tissues, and metastatic tissues were examined for GTF3A expression using immunohistochemistry. The survival rates of patients were analyzed. Short hairpin GTF3As and CSTAs were designed and packaged into the virus to block the expression of Gtf3a and Csta genes, respectively. In vivo tumor growth assays were performed to confirm whether GTF3A promotes CRC cell proliferation in vivo. Electrophoretic mobility shift assay and fluorescence in situ hybridization assay were used to detect the interaction of GTF3A with Csta, whereas luciferase activity assay was used to evaluate the expression of the Gtf3a and Csta genes. RNA-Sequencing (RNA-Seq) and data analyses were used to screen for target genes of GTF3A.

RESULTS

The expression of GTF3A was higher in CRC tissues and lymph node metastatic tissues than in the adjacent normal tissues. GTF3A was associated with CRC prognosis, and knockdown of the Gtf3a gene impaired CRC cell proliferation, invasion, and motility in vitro and in vivo. Moreover, RNA-Seq analysis revealed that GTF3A might upregulate the expression of Csta, whereas the luciferase activity assay showed that GTF3A bound to the promoter of Csta gene and increased Csta transcription. Furthermore, CSTA regulated the expression of epithelial-mesenchymal transition (EMT) markers.

CONCLUSION

GTF3A increases CSTA expression by binding to the Csta promoter, and increased CSTA level promotes CRC progression by regulating the EMT. Inhibition of GTF3A prevents CRC progression. Therefore, GTF3A is a potential novel therapeutic target and biomarker for CRC.

BDP1 Alterations Correlate with Clinical Outcomes in Breast Cancer

Simple Summary

Breast cancer accounts for 30% of all new cancer diagnoses in the United States. The most common type of breast cancer is invasive breast cancer. A hallmark trait of breast cancer is uncontrolled cell growth due to genetic alterations. TFIIIB-mediated RNA polymerase III transcription is specifically deregulated in human cancers. The TFIIIB BDP1 subunit is not well characterized in human cancer. The objective of this study was to analyze publicly available clinical cancer datasets to determine if BDP1 alterations correlate with clinical outcomes in available breast cancer datasets. BDP1 copy number and expression negatively correlate with breast cancer outcomes, including stage, grade, and mortality.

Abstract

TFIIIB is deregulated in a variety of cancers. However, few studies investigate the TFIIIB subunit BDP1 in cancer. BDP1 has not been studied in breast cancer patients. Herein, we analyzed clinical breast cancer datasets to determine if BDP1 alterations correlate with clinical outcomes. BDP1 copy number (n = 1602; p = 8.03 × 10⁻⁹) and mRNA expression (n = 130; p = 0.002) are specifically decreased in patients with invasive ductal carcinoma (IDC). In IDC, BDP1 copy number negatively correlates with high grade (n = 1992; p = 2.62 × 10⁻¹⁹) and advanced stage (n = 1992; p = 0.005). BDP1 mRNA expression also negatively correlated with high grade (n = 55; p = 6.81 × 10⁻⁴) and advanced stage (n = 593; p = 4.66 × 10⁻⁴) IDC. Decreased BDP1 expression correlated with poor clinical outcomes (n = 295 samples): a metastatic event at three years (p = 7.79 × 10⁻⁷) and cancer reoccurrence at three years (p = 4.81 × 10⁻⁷) in IDC. Decreased BDP1 mRNA correlates with patient death at three (p = 9.90 × 10⁻⁶) and five (p = 1.02 × 10⁻⁶) years. Both BDP1 copy number (n = 3785; p = 1.0 × 10⁻¹⁴) and mRNA expression (n = 2434; p = 5.23 × 10⁻⁶) are altered in triple-negative invasive breast cancer (TNBC). Together, these data suggest a role for BDP1 as potential biomarker in breast cancer and additional studies are warranted.

From APC to the genetics of hereditary and familial colon cancer syndromes

Hereditary colorectal cancer (CRC) syndromes attributable to high penetrance mutations represent 9-26% of young-onset CRC cases. The clinical significance of many of these mutations is understood well enough to be used in diagnostics and as an aid in patient care. However, despite the advances made in the field, a significant proportion of familial and early-onset cases remains molecularly uncharacterized and extensive work is still needed to fully understand the genetic nature of CRC susceptibility. With the emergence of next-generation sequencing and associated methods, several predisposition loci have been unraveled, but validation is incomplete. Individuals with cancer-predisposing mutations are currently enrolled in life-long surveillance, but with the development of new treatments, such as cancer vaccinations, this might change in the not so distant future for at least some individuals. For individuals without a known cause for their disease susceptibility, prevention and therapy options are less precise. Herein, we review the progress achieved in the last three decades with a focus on how CRC predisposition genes were discovered. Furthermore, we discuss the clinical implications of these discoveries and anticipate what to expect in the next decade.

Germline mutations in a DNA repair pathway are associated with familial colorectal cancer

Aiming to identify rare high-penetrance mutations in new genes for the underlying predisposition in familial colorectal cancer (CRC), we performed whole-exome sequencing in 24 familial CRCs. Mutations in genes that regulate DNA repair (RMI1, PALB2, FANCI) were identified that were related to the Fanconi anemia DNA repair pathway. In one pedigree, we found a nonsense mutation in CHEK2. CHEK2 played an essential role in cell cycle and DNA damage repair. Somatic mutation analysis in CHEK2 variant carriers showed mutations in TP53, APC, and FBXW7. Loss of heterozygosity was found in carcinoma of CHEK2 variant carrier, and IHC showed loss of Chk2 expression in cancer tissue. We identified a second variant in CHEK2 in 126 sporadic CRCs. A KO cellular model for CHEK2 (CHEK2^KO) was generated by CRISPR/Cas9. Functional experiments demonstrated that CHEK2^KO cells showed defective cell cycle arrest and apoptosis, as well as reduced p53 phosphorylation, upon DNA damage. We associated germline mutations in genes that regulate the DNA repair pathway with the development of CRC. We identified CHEK2 as a regulator of DNA damage response and perhaps as a gene involved in CRC germline predisposition. These findings link CRC predisposition to the DNA repair pathway, supporting the connection between genome integrity and cancer risk.

A Transcriptomic Approach Reveals Selective Ribosomal Remodelling in the Tumour Versus the Stromal Compartment of Metastatic Colorectal Cancer

Simple Summary

In this study, we analyzed a cohort of six colorectal cancer patients harboring KRAS mutations and with wild-type BRAF from a transcriptional perspective, with the aim of elucidating the role of the stromal cells in tumor progression. Specifically, paraffin-embedded specimens were subjected to microdissection and hybridized on Agilent-026652 microarrays to compare the gene expression of tumor samples composed of neoplastic epithelial samples against the neighboring stromal tissue. A paired rank-product test led to the detection of 193 differentially expressed genes. Subsequent functional enrichment analysis pointed to extracellular matrix constituents, angiogenesis, and cell migration as the main biological processes enhanced in stromata, while the tumor compartment was characterized by an overexpression of many ribosomal protein genes. A further gene set enrichment analysis against a comprehensive ribosomal protein gene set finally revealed that only cytosolic ribosomes (80S) were affected by such upregulation, while mitochondrial ribosomes were virtually unaltered.

Abstract

Because of its high incidence and poor prognosis, colorectal cancer (CRC) represents an important health issue in several countries. As with other carcinomas, the so-called tumour microenvironment (TME) has been shown to play key roles in CRC progression and related therapeutical outcomes, even though a deeper understanding of the underlying molecular mechanisms is needed to devise new treatment strategies. For some years now, omics technologies and consolidated bioinformatics pipelines have allowed scientists to access large amounts of biologically relevant information, even when starting from small tissue samples; thus, in order to shed new light upon the role of the TME in CRC, we compared the gene expression profiles of 6 independent tumour tissues (all progressed towards metastatic disease) to the expression profile of the surrounding stromata. To do this, paraffin-embedded whole tissues were first microdissected to obtain samples enriched with tumour and stromal cells, respectively. Afterwards, RNA was extracted and analysed using a microarray-based approach. A thorough bioinformatics analysis was then carried out to identify transcripts differentially expressed between the two groups and possibly enriched functional terms. Overall, 193 genes were found to be significantly downregulated in tumours compared to the paired stromata. The functional analysis of the downregulated gene list revealed three principal macro areas of interest: the extracellular matrix, cell migration, and angiogenesis. Conversely, among the upregulated genes, the main alterations detected by the functional annotation were related to the ribosomal proteins (rProteins) of both the large (60S) and small (40S) subunits of the cytosolic ribosomes. Subsequent gene set enrichment analysis (GSEA) confirmed the massive overexpression of most cytosolic—but not mitochondrial—ribosome rProteins.

Alternative splicing of mRNA in colorectal cancer: new strategies for tumor diagnosis and treatment

Alternative splicing (AS) is an important event that contributes to posttranscriptional gene regulation. This process leads to several mature transcript variants with diverse physiological functions. Indeed, disruption of various aspects of this multistep process, such as cis- or trans- factor alteration, promotes the progression of colorectal cancer. Therefore, targeting some specific processes of AS may be an effective therapeutic strategy for treating cancer. Here, we provide an overview of the AS events related to colorectal cancer based on research done in the past 5 years. We focus on the mechanisms and functions of variant products of AS that are relevant to malignant hallmarks, with an emphasis on variants with clinical significance. In addition, novel strategies for exploiting the therapeutic value of AS events are discussed.

Candidate Gene Discovery in Hereditary Colorectal Cancer and Polyposis Syndromes-Considerations for Future Studies

To discover novel high-penetrant risk loci for hereditary colorectal cancer (hCRC) and polyposis syndromes many whole-exome and whole-genome sequencing (WES/WGS) studies have been performed. Remarkably, these studies resulted in only a few novel high-penetrant risk genes. Given this observation, the possibility and strategy to identify high-penetrant risk genes for hCRC and polyposis needs reconsideration. Therefore, we reviewed the study design of WES/WGS-based hCRC and polyposis gene discovery studies (n = 37) and provide recommendations to optimize discovery and validation strategies. The group of genetically unresolved patients is phenotypically heterogeneous, and likely composed of distinct molecular subtypes. This knowledge advocates for the screening of a homogeneous, stringently preselected discovery cohort and obtaining multi-level evidence for variant pathogenicity. This evidence can be collected by characterizing the molecular landscape of tumors from individuals with the same affected gene or by functional validation in cell-based models. Together, the combined approach of a phenotype-driven, tumor-based candidate gene search might elucidate the potential contribution of novel genetic predispositions in genetically unresolved hCRC and polyposis.

RNA polymerase III transcription as a disease factor

In this review, Yeganeh et al. summarize different human diseases that have been linked to defects in the Pol III transcription apparatus or to Pol III products imbalance and discuss the possible underlying mechanisms.

RNA polymerase (Pol) III is responsible for transcription of different noncoding genes in eukaryotic cells, whose RNA products have well-defined functions in translation and other biological processes for some, and functions that remain to be defined for others. For all of them, however, new functions are being described. For example, Pol III products have been reported to regulate certain proteins such as protein kinase R (PKR) by direct association, to constitute the source of very short RNAs with regulatory roles in gene expression, or to control microRNA levels by sequestration. Consistent with these many functions, deregulation of Pol III transcribed genes is associated with a large variety of human disorders. Here we review different human diseases that have been linked to defects in the Pol III transcription apparatus or to Pol III products imbalance and discuss the possible underlying mechanisms.

BRIP1, a Gene Potentially Implicated in Familial Colorectal Cancer Type X

Familial colorectal cancer Type X (FCCTX) comprises a heterogeneous group of families with an increased risk of developing colorectal cancer and other related tumors, but with mismatch repair-proficient, microsatellite-stable (MSS) tumors. Unfortunately, the genetic basis underlying their cancer predisposition remains unknown. Although pathogenic germline variants in BRIP1 increase the risk of developing hereditary ovarian cancer, the involvement of BRIP1 in hereditary colorectal cancer is still not well known. In order to identify new BRIP1 variants associated with inherited colorectal cancer, affected and nonaffected individuals from 18 FCCTX or high-risk MSS colorectal cancer families were evaluated by whole-exome sequencing, and another 62 colorectal cancer patients from FCCTX or high-risk MSS colorectal cancer families were screened by a next-generation sequencing (NGS) multigene panel. The families were recruited at the Genetic Counseling Unit of Hospital Clínico San Carlos of Madrid. A total of three different BRIP1 mutations in three unrelated families were identified. Among them, there were two frameshift variants [c.1702_1703del, p.(Asn568TrpfsTer9) and c.903del, p.(Leu301PhefsTer2)] that result in the truncation of the protein and are thus classified as pathogenic (class 5). The remaining was a missense variant [c.2220G>T, p.(Gln740His)] considered a variant of uncertain significance (class 3). The segregation and loss-of-heterozygosity studies provide evidence linking the two BRIP1 frameshift variants to colorectal cancer risk, with suggestive but not definitive evidence that the third variant may be benign. The results here presented suggest that germline BRIP1 pathogenic variants could be associated with hereditary colorectal cancer predisposition.Prevention Relevance: We suggest that BRIP1 pathogenic germline variants may have a causal role in CRC as moderate cancer susceptibility alleles and be associated with hereditary CRC predisposition. A better understanding of hereditary CRC may provide important clues to disease predisposition and could contribute to molecular diagnostics, improved risk stratification, and targeted therapeutic strategies.

Ribosome Biogenesis Alterations in Colorectal Cancer

Many studies have focused on understanding the regulation and functions of aberrant protein synthesis in colorectal cancer (CRC), leaving the ribosome, its main effector, relatively underappreciated in CRC. The production of functional ribosomes is initiated in the nucleolus, requires coordinated ribosomal RNA (rRNA) processing and ribosomal protein (RP) assembly, and is frequently hyperactivated to support the needs in protein synthesis essential to withstand unremitting cancer cell growth. This elevated ribosome production in cancer cells includes a strong alteration of ribosome biogenesis homeostasis that represents one of the hallmarks of cancer cells. None of the ribosome production steps escape this cancer-specific dysregulation. This review summarizes the early and late steps of ribosome biogenesis dysregulations described in CRC cell lines, intestinal organoids, CRC stem cells and mouse models, and their possible clinical implications. We highlight how this cancer-related ribosome biogenesis, both at quantitative and qualitative levels, can lead to the synthesis of ribosomes favoring the translation of mRNAs encoding hyperproliferative and survival factors. We also discuss whether cancer-related ribosome biogenesis is a mere consequence of cancer progression or is a causal factor in CRC, and how altered ribosome biogenesis pathways can represent effective targets to kill CRC cells. The association between exacerbated CRC cell growth and alteration of specific steps of ribosome biogenesis is highlighted as a key driver of tumorigenesis, providing promising perspectives for the implementation of predictive biomarkers and the development of new therapeutic drugs.

Germline mutational profile of Chinese patients under 70 years old with colorectal cancer

BACKGROUND

Inherited susceptibility accounts for nearly one-third of colorectal cancer (CRC) predispositions and has an 80%-100% lifetime risk of this disease. However, there are few data about germline mutations of hereditary CRC-related genes in Chinese patients with CRC. This study aimed to assess the prevalence of gene mutations related to cancer susceptibility among Chinese patients with CRC, differences between Chinese and Western patients, and the phenotype-genotype correlation.

METHODS

We retrospectively collected tumor samples from 526 patients with CRC under 70 years old who underwent hereditary CRC genetic testing. A series of bioinformatic analyses, as well as statistical comparisons, were performed.

RESULTS

We found that 77 patients (14.6%) harbored functional variants of the 12 genes. The mutation frequencies of the top 5 mutated genes were 6.5% for MutL homolog 1 (MLH1), 5.1% for MutS homolog 2 (MSH2), 1.0% for MSH6, 0.8% for PMS1 homolog 2 (PMS2), and 0.8% for APC regulator of the WNT signaling pathway (APC). Our data showed much higher rates of mutations of MSH6 and PMS2 genes among all mismatch repair (MMR) genes as compared with those in Western populations. Mutations in MLH1, MSH2, and MSH6 were found to be mutually exclusive. Patients with MLH1 or MSH2 mutations had higher frequencies of personal history of cancer (MLH1: 20.6% vs. 8.7%; MSH2: 25.9% vs. 8.6%) and family history of cancer than those without these mutations (MLH1: 73.5% vs. 48.4%; MSH2: 70.4% vs. 48.9%), and the lesions were more prone to occur on the right side of the colon than on the left side (MLH1: 73.5% vs. 29.3%; MSH2: 56.0% vs. 31.0%). The proportion of stage I/II disease was higher in patients with MLH1 mutations than in those without MLH1 mutations (70.6% vs. 50.7%), and the rate of polyps was higher in patients with APC mutations than in those with wild-type APC (75.0% vs. 17.4%).

CONCLUSION

These results provide a full-scale landscape of hereditary susceptibility over 12 related genes in CRC patients and suggest that a comprehensive multi-gene panel testing for hereditary CRC predisposition could be a helpful analysis in clinical practice.

Expanding the phenotype of cerebellar-facial-dental syndrome: Two siblings with a novel variant in BRF1

Cerebellofaciodental syndrome (MIM #616202) is an autosomal recessive condition characterized by intellectual disability, microcephaly, cerebellar hypoplasia, dysmorphic features, and short stature. To date, eight patients carrying biallelic BRF1 variants have been reported. Here, we describe two siblings with congenital microcephaly and corpus callosum hypoplasia, pre and postnatal growth retardation, congenital heart defect and severe global developmental delay. We also detected additional findings not previously reported in this syndrome, including bilateral sensorineural hearing impairment and inner ear malformation. Whole exome sequencing identified a novel homozygous missense variant (c.654G>C, p.[Trp218Cys]) in BRF1, predicted to affect the protein structure. Expression assessment showed extremely low BRF1 protein expression caused by the identified variant, supporting its causal involvement. The description of new patients with cerebellofaciodental syndrome is essential to better delineate the phenotypic and genotypic spectrum of the disease.

Updates in the field of hereditary nonpolyposis colorectal cancer

INTRODUCTION

Up to one third of colorectal cancers show familial clustering and 5% are hereditary single-gene disorders. Hereditary non-polyposis colorectal cancer comprises DNA mismatch repair-deficient and -proficient subsets, represented by Lynch syndrome (LS) and familial colorectal cancer type X (FCCTX), respectively. Accurate knowledge of molecular etiology and genotype-phenotype correlations are critical for tailored cancer prevention and treatment.

AREAS COVERED

The authors highlight advances in the molecular dissection of hereditary non-polyposis colorectal cancer, based on recent literature retrieved from PubMed. Future possibilities for novel gene discoveries are discussed.

EXPERT COMMENTARY

LS is molecularly well established, but new information is accumulating of the associated clinical and tumor phenotypes. FCCTX remains poorly defined, but several promising candidate genes have been discovered and share some preferential biological pathways. Multi-level characterization of specimens from large patient cohorts representing multiple populations, combined with proper bioinformatic and functional analyses, will be necessary to resolve the outstanding questions.

Germline Mutations in FAF1 Are Associated With Hereditary Colorectal Cancer

BACKGROUND & AIMS

A significant proportion of colorectal cancer (CRC) cases have familial aggregation but little is known about the genetic factors that contribute to these cases. We performed an exhaustive functional characterization of genetic variants associated with familial CRC.

METHODS

We performed whole-exome sequencing analyses of 75 patients from 40 families with a history of CRC (including early-onset cases) of an unknown germline basis (discovery cohort). We also sequenced specific genes in DNA from an external replication cohort of 473 families, including 488 patients with colorectal tumors that had normal expression of mismatch repair proteins (validation cohort). We disrupted the Fas-associated factor 1 gene (FAF1) in DLD-1 CRC cells using CRISPR/Cas9 gene editing; some cells were transfected with plasmids that express FAF1 missense variants. Cells were analyzed by immunoblots, quantitative real-time polymerase chain reaction, and functional assays monitoring apoptosis, proliferation, and assays for Wnt signaling or nuclear factor (NF)-kappa-B activity.

RESULTS

We identified predicted pathogenic variant in the FAF1 gene (c.1111G>A; p.Asp371Asn) in the discovery cohort; it was present in 4 patients of the same family. We identified a second variant in FAF1 in the validation cohort (c.254G>C; p.Arg85Pro). Both variants encoded unstable FAF1 proteins. Expression of these variants in CRC cells caused them to become resistant to apoptosis, accumulate beta-catenin in the cytoplasm, and translocate NF-kappa-B to the nucleus.

CONCLUSIONS

In whole-exome sequencing analyses of patients from families with a history of CRC, we identified variants in FAF1 that associate with development of CRC. These variants encode unstable forms of FAF1 that increase resistance of CRC cells to apoptosis and increase activity of beta-catenin and NF-kappa-B.

Dominantly Inherited Hereditary Nonpolyposis Colorectal Cancer Not Caused by MMR Genes

In the past two decades, multiple studies have been undertaken to elucidate the genetic cause of the predisposition to mismatch repair (MMR)-proficient nonpolyposis colorectal cancer (CRC). Here, we present the proposed candidate genes according to their involvement in specific pathways considered relevant in hereditary CRC and/or colorectal carcinogenesis. To date, only pathogenic variants in RPS20 may be convincedly linked to hereditary CRC. Nevertheless, accumulated evidence supports the involvement in the CRC predisposition of other genes, including MRE11, BARD1, POT1, BUB1B, POLE2, BRF1, IL12RB1, PTPN12, or the epigenetic alteration of PTPRJ. The contribution of the identified candidate genes to familial/early onset MMR-proficient nonpolyposis CRC, if any, is extremely small, suggesting that other factors, such as the accumulation of low risk CRC alleles, shared environmental exposures, and/or gene-environmental interactions, may explain the missing heritability in CRC.

A Systematic Literature Review of Whole Exome and Genome Sequencing Population Studies of Genetic Susceptibility to Cancer

The application of next-generation sequencing (NGS) technologies in cancer research has accelerated the discovery of somatic mutations; however, progress in the identification of germline variation associated with cancer risk is less clear. We conducted a systematic literature review of cancer genetic susceptibility studies that used NGS technologies at an exome/genome-wide scale to obtain a fuller understanding of the research landscape to date and to inform future studies. The variability across studies on methodologies and reporting was considerable. Most studies sequenced few high-risk (mainly European) families, used a candidate analysis approach, and identified potential cancer-related germline variants or genes in a small fraction of the sequenced cancer cases. This review highlights the importance of establishing consensus on standards for the application and reporting of variants filtering strategies. It also describes the progress in the identification of cancer-related germline variation to date. These findings point to the untapped potential in conducting studies with appropriately sized and racially diverse families and populations, combining results across studies and expanding beyond a candidate analysis approach to advance the discovery of genetic variation that accounts for the unexplained cancer heritability.

Brf1 loss and not overexpression disrupts tissues homeostasis in the intestine, liver and pancreas

RNA polymerase III (Pol-III) transcribes tRNAs and other small RNAs essential for protein synthesis and cell growth. Pol-III is deregulated during carcinogenesis; however, its role in vivo has not been studied. To address this issue, we manipulated levels of Brf1, a Pol-III transcription factor that is essential for recruitment of Pol-III holoenzyme at tRNA genes in vivo. Knockout of Brf1 led to embryonic lethality at blastocyst stage. In contrast, heterozygous Brf1 mice were viable, fertile and of a normal size. Conditional deletion of Brf1 in gastrointestinal epithelial tissues, intestine, liver and pancreas, was incompatible with organ homeostasis. Deletion of Brf1 in adult intestine and liver induced apoptosis. However, Brf1 heterozygosity neither had gross effects in these epithelia nor did it modify tumorigenesis in the intestine or pancreas. Overexpression of BRF1 rescued the phenotypes of Brf1 deletion in intestine and liver but was unable to initiate tumorigenesis. Thus, Brf1 and Pol-III activity are absolutely essential for normal homeostasis during development and in adult epithelia. However, Brf1 overexpression or heterozygosity are unable to modify tumorigenesis, suggesting a permissive, but not driving role for Brf1 in the development of epithelial cancers of the pancreas and gut.

Identification of specific modules and significant genes associated with colon cancer by weighted gene co‑expression network analysis

Colon cancer is one of the most commonly diagnosed malignancies and is a leading cause of cancer‑associated mortality. The aim of the present study was to investigate the molecular mechanisms underlying colon cancer and identify potentially significant genes associated with the disease using weighted gene co‑expression network analysis (WGCNA). The test datasets used were downloaded from The Cancer Genome Atlas (TCGA) database. WGCNA was applied to analyze microarray data obtained from colon adenocarcinoma samples to identify significant modules and highly associated genes. A gene co‑expression network was constructed and different gene modules were selected. Functional and pathway enrichment analyses were performed to investigate the molecular mechanisms of colon cancer. In addition, highly connected hub genes associated with the most significant module were selected for further analysis. Nine specific modules associated with colon cancer were identified, of which the turquoise module was observed to exhibit the greatest association with the disease. Pathway enrichment analysis of the turquoise module suggested that genes in the turquoise module were associated with 'RNA polymerase' and 'purine metabolism'. Furthermore, gene ontology enrichment analysis revealed the top 30 hub genes with a higher degree in the turquoise module, such as σ‑non‑opioid intracellular receptor 1, transmembrane protein 147  TMEM147) and carbamoyl‑phosphate synthetase 2, aspartate transcarbamylase, and dihydroorotase, were predominantly enriched in the biological processes 'translation' and 'gene expression'. Experimental verification demonstrated that the expression of TMEM147 in colon cancer was significantly increased compared with the control. Therefore, the results suggested that genes associated with RNA polymerase and the purine metabolic pathways may be substantially involved in the pathogenesis of colon cancer. Furthermore, TMEM147 may represent a biomarker for colon cancer.

Approaches to functionally validate candidate genetic variants involved in colorectal cancer predisposition

Most next generation sequencing (NGS) studies identified candidate genetic variants predisposing to colorectal cancer (CRC) but do not tackle its functional interpretation to unequivocally recognize a new hereditary CRC gene. Besides, germline variants in already established hereditary CRC-predisposing genes or somatic variants share the same need when trying to categorize those with relevant significance. Functional genomics approaches have an important role in identifying the causal links between genetic architecture and phenotypes, in order to decipher cellular function in health and disease. Therefore, functional interpretation of identified genetic variants by NGS platforms is now essential. Available approaches nowadays include bioinformatics, cell and molecular biology and animal models. Recent advances, such as the CRISPR-Cas9, ZFN and TALEN systems, have been already used as a powerful tool with this objective. However, the use of cell lines is of limited value due to the CRC heterogeneity and its close interaction with microenvironment. Access to tridimensional cultures or organoids and xenograft models that mimic the in vivo tissue architecture could revolutionize functional analysis. This review will focus on the application of state-of-the-art functional studies to better tackle new genes involved in germline predisposition to this neoplasm.

Genetic predisposition to colorectal cancer: syndromes, genes, classification of genetic variants and implications for precision medicine

This article reviews genes and syndromes associated with predisposition to colorectal cancer (CRC), with an overview of gene variant classification. We include updates on the application of preventive and therapeutic measures, focusing on the use of non-steroidal anti-inflammatory drugs (NSAIDs) and immunotherapy. Germline pathogenic variants in genes conferring high or moderate risk to cancer are detected in 6-10% of all CRCs and 20% of those diagnosed before age 50. CRC syndromes can be subdivided into nonpolyposis and polyposis entities, the most common of which are Lynch syndrome and familial adenomatous polyposis, respectively. In addition to known and novel genes associated with highly penetrant CRC risk, identification of pathogenic germline variants in genes associated with moderate-penetrance cancer risk and/or hereditary cancer syndromes not traditionally linked to CRC may have an impact on genetic testing, counseling, and surveillance. The use of multigene panels in genetic testing has exposed challenges in the classification of variants of uncertain significance. We provide an overview of the main classification systems and strategies for improving these. Finally, we highlight approaches for integrating chemoprevention in the care of individuals with genetic predisposition to CRC and use of targeted agents and immunotherapy for treatment of mismatch repair-deficient and hypermutant tumors. Copyright © 2018 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Update on genetic predisposition to colorectal cancer and polyposis

The present article summarizes recent developments in the characterization of genetic predisposition to colorectal cancer (CRC). The main themes covered include new hereditary CRC and polyposis syndromes, non-CRC hereditary cancer genes found mutated in CRC patients, strategies used to identify novel causal genes, and review of candidate genes that have been proposed to predispose to CRC and/or colonic polyposis. We provide an overview of newly described genes and syndromes associated with predisposition to CRC and polyposis, including: polymerase proofreading-associated polyposis, NTHL1-associated polyposis, mismatch repair gene biallelic inactivation-related adenomatous polyposis (including MSH3- and MLH3-associated polyposes), GREM1-associated mixed polyposis, RNF43-associated serrated polyposis, and RPS20 mutations as a rare cause of hereditary nonpolyposis CRC. The implementation of next generation sequencing approaches for genetic testing has exposed the presence of pathogenic germline variants in genes associated with hereditary cancer syndromes not traditionally linked to CRC, which may have an impact on genetic testing, counseling and surveillance. The identification of new hereditary CRC and polyposis genes has not deemed an easy endeavor, even though known CRC-related genes explain a small proportion of the estimated familial risk. Whole-genome sequencing may offer a technology for increasing this proportion, particularly if applied on pedigree data allowing linkage type of analysis. The final section critically surveys the large number of candidate genes that have been recently proposed for CRC predisposition.

Germline variation in O6-methylguanine-DNA methyltransferase (MGMT) as cause of hereditary colorectal cancer

Somatic epigenetic inactivation of the DNA repair protein O6-methylguanine DNA methyltransferase (MGMT) is frequent in colorectal cancer (CRC); however, its involvement in CRC predisposition remains unexplored. We assessed the role and relevance of MGMT germline mutations and epimutations in familial and early-onset CRC. Mutation and promoter methylation screenings were performed in 473 familial and/or early-onset mismatch repair-proficient nonpolyposis CRC cases. No constitutional MGMT inactivation by promoter methylation was observed. Of six rare heterozygous germline variants identified, c.346C > T (p.H116Y) and c.476G > A (p.R159Q), detected in three and one families respectively, affected highly conserved residues and showed segregation with cancer in available family members. In vitro, neither p.H116Y nor p.R159Q caused statistically significant reduction of MGMT repair activity. No evidence of somatic second hits was found in the studied tumors. Case-control data showed over-representation of c.346C > T (p.H116Y) in familial CRC compared to controls, but no overall association of MGMT mutations with CRC predisposition. In conclusion, germline mutations and constitutional epimutations in MGMT are not major players in hereditary CRC. Nevertheless, the over-representation of c.346C > T (p.H116Y) in our familial CRC cohort warrants further research.

Early-onset colorectal cancer in young individuals

Treatment of young adults with colorectal cancer (CRC) represents an unmet clinical need, especially as diagnosis in this population might lead to the greatest loss of years of life. Since 1994, CRC incidence in individuals younger than 50 years has been increasing by 2% per year. The surge in CRC incidence in young adults is particularly alarming as the overall CRC frequency has been decreasing. Early-onset CRC are characterized by a more advanced stage at diagnosis, poorer cell differentiation, higher prevalence of signet ring cell histology, and left colon-sided location of the primary tumor. Among EO-CRC, approximately 30% of patients are affected by tumors harboring mutations causing hereditary cancer predisposing syndromes, and 20% have familial CRC. Most notably, the remaining 50% of EO-CRC patients have neither hereditary syndromes nor familial CRC, thus representing a formidable challenge for research. In this review article we summarize epidemiology, clinical and molecular features, heredity and outcome of treatments of EO-CRC, and provide considerations for future perspectives.

Germline variation in the oxidative DNA repair genes NUDT1 and OGG1 is not associated with hereditary colorectal cancer or polyposis

The causal association of NUDT1 (=MTH1) and OGG1 with hereditary colorectal cancer (CRC) remains unclear. Here, we sought to provide additional evidence for or against the causal contribution of NUDT1 and OGG1 mutations to hereditary CRC and/or polyposis. Mutational screening was performed using pooled DNA amplification and targeted next-generation sequencing in 529 families (441 uncharacterized MMR-proficient familial nonpolyposis CRC and 88 polyposis cases). Cosegregation, in silico analyses, in vitro functional assays, and case-control associations were carried out to characterize the identified variants. Five heterozygous carriers of novel (n = 1) or rare (n = 4) NUDT1 variants were identified. In vitro deleterious effects were demonstrated for c.143G>A p.G48E (catalytic activity and protein stability) and c.403G>T p.G135W (protein stability), although cosegregation data in the carrier families were inconclusive or nonsupportive. The frequency of missense, loss-of-function, and splice-site NUDT1 variants in our familial CRC cohort was similar to the one observed in cancer-free individuals, suggesting lack of association with CRC predisposition. No OGG1 pathogenic mutations were identified. Our results suggest that the contribution of NUDT1 and OGG1 germline mutations to hereditary CRC and to polyposis is inexistent or, at most, negligible. The inclusion of these genes in routine genetic testing is not recommended.