Natural selection and mammalian BRCA1 sequences: elucidating functionally important sites relevant to breast cancer susceptibility in humans

Pathogenic variants in human DNA damage repair genes mostly arose after the latest human out-of-Africa migration

Introduction

The DNA damage repair (DDR) system in human genome is pivotal in maintaining genomic integrity. Pathogenic variation (PV) in DDR genes impairs their function, leading to genome instability and increased susceptibility to diseases, especially cancer. Understanding the evolution origin and arising time of DDR PV is crucial for comprehending disease susceptibility in modern humans.

Methods

We used big data approach to identify the PVs in DDR genes in modern humans. We mined multiple genomic databases derived from 251,214 modern humans of African and non-Africans. We compared the DDR PVs between African and non-African. We also mined the DDR PVs in the genomic data derived from 5,031 ancient humans. We used the DDR PVs from ancient humans as the intermediate to further the DDR PVs between African and non-African.

Results and discussion

We identified 1,060 single-base DDR PVs across 77 DDR genes in modern humans of African and non-African. Direct comparison of the DDR PVs between African and non-African showed that 82.1% of the non-African PVs were not present in African. We further identified 397 single-base DDR PVs in 56 DDR genes in the 5,031 ancient humans dated between 45,045 and 100 years before present (BP) lived in Eurasian continent therefore the descendants of the latest out-of-Africa human migrants occurred 50,000-60,000 years ago. By referring to the ancient DDR PVs, we observed that 276 of the 397 (70.3%) ancient DDR PVs were exclusive in non-African, 106 (26.7%) were shared between non-African and African, and only 15 (3.8%) were exclusive in African. We further validated the distribution pattern by testing the PVs in BRCA and TP53, two of the important genes in genome stability maintenance, in African, non-African, and Ancient humans. Our study revealed that DDR PVs in modern humans mostly emerged after the latest out-of-Africa migration. The data provides a foundation to understand the evolutionary basis of disease susceptibility, in particular cancer, in modern humans.

OrthoMaM v12: a database of curated single-copy ortholog alignments and trees to study mammalian evolutionary genomics

To date, the databases built to gather information on gene orthology do not provide end-users with descriptors of the molecular evolution information and phylogenetic pattern of these orthologues. In this context, we developed OrthoMaM, a database of ORTHOlogous MAmmalian Markers describing the evolutionary dynamics of coding sequences in mammalian genomes. OrthoMaM version 12 includes 15,868 alignments of orthologous coding sequences (CDS) from the 190 complete mammalian genomes currently available. All annotations and 1-to-1 orthology assignments are based on NCBI. Orthologous CDS can be mined for potential informative markers at the different taxonomic levels of the mammalian tree. To this end, several evolutionary descriptors of DNA sequences are provided for querying purposes (e.g. base composition and relative substitution rate). The graphical web interface allows the user to easily browse and sort the results of combined queries. The corresponding multiple sequence alignments and ML trees, inferred using state-of-the art approaches, are available for download both at the nucleotide and amino acid levels. OrthoMaM v12 can be used by researchers interested either in reconstructing the phylogenetic relationships of mammalian taxa or in understanding the evolutionary dynamics of coding sequences in their genomes. OrthoMaM is available for browsing, querying and complete or filtered download at https://orthomam.mbb.cnrs.fr/.

Oncogenic BRCA1,2 Mutations in the Human Lineage-A By-Product of Sexual Selection?

In this review, we discuss the long-known problem of tissue-specific carcinogenesis in BRCA1 and BRCA2 mutation carriers: while the genes are expressed ubiquitously, increased cancer risk is observed mostly in the breast and ovaries, and to a much lesser extent, in some other tissues such as the prostate or pancreas. We reevaluate hypotheses on the evolutionary origin of these mutations in humans. Also, we align together the reports that at least some great apes have much lower risks of epithelial cancers in general and breast cancer in particular with the fact that humans have more voluminous breast tissue as compared to their closest extant relatives, particularly chimpanzees and bonobos. We conjecture that this disparity may be a consequence of sexual selection, augmented via selection for enhanced lactation. Further, we argue that there is an organ-specific enigma similar to the Peto paradox: breast cancer risk in humans is only minimally correlated with breast size. These considerations lead to the hypothesis that, along with the evolutionary development of larger breasts in humans, additional changes have played a balancing role in suppressing breast cancer. These yet-to-be-discovered mechanisms, while purely speculative, may be valuable to understanding human breast cancer, though they may not be exclusive to the mammary gland epithelial cells. Combining these themes, we review some anti-carcinogenesis preventive strategies and prospects of new interventions against breast cancer.

An efficient qPCR assay for the quantification of human cells in preclinical animal models by targeting human specific DNA in the intron of BRCA1

BACKGROUND

Precise quantification of grafted human cells in preclinical animal models such as non-human primates, rodents and rabbits is needed for the evaluations of the safety and efficacy of cell therapy. Quantitative PCR (qPCR) as a swift, sensitive and powerful assay is suitable for human cell quantification. However, it is a formidable challenge due to that the genome of non-human primates share more than 95% of similarity as human.

METHODS

In the present study, we developed a probe-based quantitative PCR (qPCR) assay for the quantification of human cells in preclinical animal models via targeting human specific DNA in the intron of BRCA1 (termed BRCA1-qPCR). The 5' and 3' end of BRCA1-qPCR probe was conjugated with FAM and non-fluorescent quencher-minor groove binder (NFQ-MGB), respectively. 1 µg of genomic DNA from human and preclinical animal models including rhesus monkeys, cynomolgus monkeys, New Zealand white rabbits, SD rats, C57BL/6 and BALB/c mice were used for determining the specificity and sensitivity of the BRCA1-qPCR assay. A calibration curve was generated by BRCA1-qPCR analysis of linearized plasmid containing targeted human specific DNA in BRCA1. The BRCA1-qPCR assay was validated by analysis of 0.003%, 0.03% and 0.3% of human leukocytes mixed within murine leukocytes.

RESULTS

The BRCA1-qPCR assay detected human DNA rather than DNA from tested species. The amplification efficiency of the BRCA1-qPCR assay was 95.4% and the linearity of the calibration curve was R2 = 0.9997. The BRCA1-qPCR assay detected as low as 5 copies of human specific DNA and is efficient to specially amplify 30 pg human DNA in the presence of 1 µg of genomic DNA from tested species, respectively. The BRCA1-qPCR assay was able to quantify as low as 0.003% of human cells within murine leukocytes.

CONCLUSION

The BRCA1-qPCR assay is efficient for the quantification of human cells in preclinical animal models.

A global perspective on the ethnic-specific BRCA variation and its implication in clinical application

Pathogenic BRCA1 and BRCA2 (BRCA) variation is the genetic predisposition for high cancer risk affecting mostly breast and ovarian. BRCA variation information is widely used in clinical diagnosis, treatment, and prevention of BRCA-related cancer. The positive selection imposed on human BRCA leads to highly ethnic-specific BRCA variation to adapt different living environment on earth. Most of the human BRCA variants identified so far were from the European descendant populations and used as the standard reference for global human populations, whereas BRCA variation in other ethnic populations remains poorly characterized. This review addresses the origin of ethnic-specific BRCA variation, the importance of ethnic-specific BRCA variation in clinical application, the limitation of current BRCA variation data, and potential solutions to fill the gap.

Human BRCA pathogenic variants were originated during recent human history

BRCA1 and BRCA2 (BRCA) play essential roles in maintaining genome stability. BRCA germline pathogenic variants increase cancer risk. However, the evolutionary origin of human BRCA pathogenic variants remains largely elusive. We tested the 2,972 human BRCA1 and 3,652 human BRCA2 pathogenic variants from ClinVar database in 100 vertebrates across eight clades, but failed to find evidence to show cross-species evolution conservation as the origin; we searched the variants in 2,792 ancient human genome data, and identified 28 BRCA1 and 22 BRCA2 pathogenic variants in 44 cases dated from 45,000 to 300 yr ago; we analyzed the haplotype-dated human BRCA pathogenic founder variants, and observed that they were mostly arisen within the past 3,000 yr; we traced ethnic distribution of human BRCA pathogenic variants, and found that the majority were present in single or a few ethnic populations. Based on the data, we propose that human BRCA pathogenic variants were highly likely arisen in recent human history after the latest out-of-Africa migration, and the expansion of modern human population could largely increase the variation spectrum.

DNA damage repair system in C57BL/6 J mice is evolutionarily stable

Background

DNA damage repair (DDR) system is vital in maintaining genome stability and survival. DDR consists of over 160 genes in 7 different pathways to repair specific type of DNA damage caused by external and internal damaging factors. The functional importance of DDR system implies that evolution could play important roles in maintaining its functional intactness to perform its function. Indeed, it has been observed that positive selection is present in BRCA1 and BRCA2 (BRCA), which are key genes in homologous recombination pathway of DDR system, in the humans and its close relatives of chimpanzee and bonobos. Efforts have been made to investigate whether the same selection could exist for BRCA in other mammals but found no evidence so far. However, as most of the studies in non-human mammals analyzed only a single or few individuals in the studied species, the observation may not reflect the true status in the given species. Furthermore, few studies have studied evolution selection in other DDR genes except BRCA. In current study, we used laboratory mouse C57BL/6 J as a model to address evolution selection on DDR genes in non-primate mammals by dynamically monitoring genetic variation across 30 generations in C57BL/6 J.

Results

Using exome sequencing, we collected coding sequences of 169 DDR genes from 44 C57BL/6 J individual genomes in 2018. We compared the coding sequences with the mouse reference genome sequences derived from 1998 C57BL/6 J DNA, and with the mouse Eve6B reference genome sequences derived from 2003 C57BL/6 J DNA, covering 30 generations of C57BL/6 J from 1998 to 2018. We didn’t identify meaningful coding variation in either Brca1 or Brca2, or in 167 other DDR genes across the 30 generations. In the meantime, we did identify 812 coding variants in 116 non-DNA damage repair genes during the same period, which served as a quality control to validate the reliability of our analytic pipeline and the negative results in DDR genes.

Conclusions

DDR genes in laboratory mouse strain C57BL/6 J were not under positive selection across its 30-generation period, highlighting the possibility that DDR system in rodents could be evolutionarily stable.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07983-7.

Reinterpretation of BRCA1 and BRCA2 variants of uncertain significance in patients with hereditary breast/ovarian cancer using the ACMG/AMP 2015 guidelines

BACKGROUND

Although BRCA1 or BRCA2 (BRCA1/2) genetic testing plays an important role in determining treatment modalities in patients with hereditary breast and ovarian cancer, sequence variants with unknown clinical significance or variant of uncertain significance (VUS) have limited use in medical decision-making. With vast quantities of gene-related data being updated, the clinical significance of VUS may change over time. We reinterpreted the sequence variant previously reported as BRCA1/2 VUS results in patients with breast or ovarian cancer and assessed whether the clinical significance of VUS was changed.

METHODS

We retrospectively reviewed medical records of 423 breast or ovarian cancer patients who underwent BRCA1/2 genetic testing from 2010 to 2017. The VUSs in BRCA1/2 were reanalyzed using the 2015 American College of Medical Genetics and Genomics and the Association for Molecular Pathology standards and guidelines (ACMG/AMP 2015 guidelines) and the VUS was reclassified into five categories: "pathogenic", "likely pathogenic", "VUS", "likely benign", and "benign".

RESULTS

A total of 75 patients (48 sequence types of VUS) were identified as carrying either one or more VUS in BRCA1/2. Among the 75 patients, two patients (2.7%) were reclassified as "likely pathogenic", 30 patients (40.0%) were reclassified as either "benign" or "likely benign", and the remaining 43 patients (57.3%) were still classified as VUS category.

CONCLUSIONS

Since the clinical significance of VUS in BRCA1/2 may vary from time to time, reinterpretation of the VUS results could contribute to clinical decision-making.

BRCA1 homozygous unclassified variant in a patient with non-Fanconi anemia: A case report

The present case report discusses a woman affected by chronic lymphatic leukemia and breast cancer with a familial history of breast cancer; suspected to be hereditary breast and ovarian cancer (HBOC) syndrome. The patient underwent BRCA1 and BRCA2 genetic testing. Sequencing of BRCA1 revealed the presence of the variant of unknown significance (VUS) c.3082C>T (p.Arg1028Cys) at homozygous state, whereas no mutations were detected in BRCA2. Multiplex ligation-dependent probe amplification confirmed the presence of two alleles. Although consanguineity between her parents was reported, which therefore supported the molecular data, her clinical phenotype was not suggestive of typical Fanconi anemia (FA), particularly of a BRCA1-linked FA. In the two cases reported in the literature, carriers of biallelic BRCA1 mutation present a severe and quite typical phenotype. For this reason, the patient was offered a diepoxybutane test, where neither complex rearrangements nor multiradial formation were detected. We were therefore inclined to consider that BRCA1 VUS as of little clinical significance.

Bat Biology, Genomes, and the Bat1K Project: To Generate Chromosome-Level Genomes for All Living Bat Species

Bats are unique among mammals, possessing some of the rarest mammalian adaptations, including true self-powered flight, laryngeal echolocation, exceptional longevity, unique immunity, contracted genomes, and vocal learning. They provide key ecosystem services, pollinating tropical plants, dispersing seeds, and controlling insect pest populations, thus driving healthy ecosystems. They account for more than 20% of all living mammalian diversity, and their crown-group evolutionary history dates back to the Eocene. Despite their great numbers and diversity, many species are threatened and endangered. Here we announce Bat1K, an initiative to sequence the genomes of all living bat species (n∼1,300) to chromosome-level assembly. The Bat1K genome consortium unites bat biologists (>148 members as of writing), computational scientists, conservation organizations, genome technologists, and any interested individuals committed to a better understanding of the genetic and evolutionary mechanisms that underlie the unique adaptations of bats. Our aim is to catalog the unique genetic diversity present in all living bats to better understand the molecular basis of their unique adaptations; uncover their evolutionary history; link genotype with phenotype; and ultimately better understand, promote, and conserve bats. Here we review the unique adaptations of bats and highlight how chromosome-level genome assemblies can uncover the molecular basis of these traits. We present a novel sequencing and assembly strategy and review the striking societal and scientific benefits that will result from the Bat1K initiative.

BRCA1 Gene Mutation Screening for the Hereditary Breast and/or Ovarian Cancer Syndrome in Breast Cancer Cases: a First High Resolution DNA Melting Analysis in Indonesia

Specific patterns of the hereditary breast and ovarian cancer (HBOC) syndrome are related to mutations in the BRCA1 gene. One hundred unrelated breast cancer patients were interviewed to obtain clinical symptoms and signs, pedigree and familial history of HBOC syndrome related cancer. Subsequently, data were calculated using the Breast and Ovarian Analysis of Disease Incidence and Carrier Estimation Algorithm (BOADICEA) risk prediction model. Patients with high score of BOADICEA were offered genetic testing. Eleven patients with high score of BOADICEA, 2 patients with low score of BOADICEA, 2 patient's family members and 15 controls underwent BRCA1 genetic testing. Mutation screening using PCR-HRM was carried out in 22 exons (41 amplicons) of BRCA1 gene. Sanger sequencing was subjected in all samples with aberrant graph. This study identified 10 variants in the BRCA1 gene, consisting of 6 missense mutations (c.1480C>A, c.2612C>T, c.2566T>C, c.3113A>G, c.3548 A>G, c.4837 A>G), 3 synonymous mutations (c.2082 C> T, c.2311 T> C and c.4308T>C) and one intronic mutation (c.134+35 G>T). All variants tend to be polymorphisms and unclassified variants. However, no known pathogenic mutations were found.

Comparative sequence analyses of rhodopsin and RPE65 reveal patterns of selective constraint across hereditary retinal disease mutations

Retinitis pigmentosa (RP) comprises several heritable diseases that involve photoreceptor, and ultimately retinal, degeneration. Currently, mutations in over 50 genes have known links to RP. Despite advances in clinical characterization, molecular characterization of RP remains challenging due to the heterogeneous nature of causal genes, mutations, and clinical phenotypes. In this study, we compiled large datasets of two important visual genes associated with RP: rhodopsin, which initiates the phototransduction cascade, and the retinoid isomerase RPE65, which regenerates the visual cycle. We used a comparative evolutionary approach to investigate the relationship between interspecific sequence variation and pathogenic mutations that lead to degenerative retinal disease. Using codon-based likelihood methods, we estimated evolutionary rates (dN/dS) across both genes in a phylogenetic context to investigate differences between pathogenic and nonpathogenic amino acid sites. In both genes, disease-associated sites showed significantly lower evolutionary rates compared to nondisease sites, and were more likely to occur in functionally critical areas of the proteins. The nature of the dataset (e.g., vertebrate or mammalian sequences), as well as selection of pathogenic sites, affected the differences observed between pathogenic and nonpathogenic sites. Our results illustrate that these methods can serve as an intermediate step in understanding protein structure and function in a clinical context, particularly in predicting the relative pathogenicity (i.e., functional impact) of point mutations and their downstream phenotypic effects. Extensions of this approach may also contribute to current methods for predicting the deleterious effects of candidate mutations and to the identification of protein regions under strong constraint where we expect pathogenic mutations to occur.

Evolutionary analysis of selective constraints identifies ameloblastin (AMBN) as a potential candidate for amelogenesis imperfecta

Background

Ameloblastin (AMBN) is a phosphorylated, proline/glutamine-rich protein secreted during enamel formation. Previous studies have revealed that this enamel matrix protein was present early in vertebrate evolution and certainly plays important roles during enamel formation although its precise functions remain unclear. We performed evolutionary analyses of AMBN in order to (i) identify residues and motifs important for the protein function, (ii) predict mutations responsible for genetic diseases, and (iii) understand its molecular evolution in mammals.

Results

In silico searches retrieved 56 complete sequences in public databases that were aligned and analyzed computationally. We showed that AMBN is globally evolving under moderate purifying selection in mammals and contains a strong phylogenetic signal. In addition, our analyses revealed codons evolving under significant positive selection. Evidence for positive selection acting on AMBN was observed in catarrhine primates and the aye-aye. We also found that (i) an additional translation initiation site was recruited in the ancestral placental AMBN, (ii) a short exon was duplicated several times in various species including catarrhine primates, and (iii) several polyadenylation sites are present.

Conclusions

AMBN possesses many positions, which have been subjected to strong selective pressure for 200 million years. These positions correspond to several cleavage sites and hydroxylated, O-glycosylated, and phosphorylated residues. We predict that these conserved positions would be potentially responsible for enamel disorder if substituted. Some motifs that were previously identified as potentially important functionally were confirmed, and we found two, highly conserved, new motifs, the function of which should be tested in the near future. This study illustrates the power of evolutionary analyses for characterizing the functional constraints acting on proteins with yet uncharacterized structure.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-015-0431-0) contains supplementary material, which is available to authorized users.

Rapid evolution of BRCA1 and BRCA2 in humans and other primates

Background

The maintenance of chromosomal integrity is an essential task of every living organism and cellular repair mechanisms exist to guard against insults to DNA. Given the importance of this process, it is expected that DNA repair proteins would be evolutionarily conserved, exhibiting very minimal sequence change over time. However, BRCA1, an essential gene involved in DNA repair, has been reported to be evolving rapidly despite the fact that many protein-altering mutations within this gene convey a significantly elevated risk for breast and ovarian cancers.

Results

To obtain a deeper understanding of the evolutionary trajectory of BRCA1, we analyzed complete BRCA1 gene sequences from 23 primate species. We show that specific amino acid sites have experienced repeated selection for amino acid replacement over primate evolution. This selection has been focused specifically on humans and our closest living relatives, chimpanzees (Pan troglodytes) and bonobos (Pan paniscus). After examining BRCA1 polymorphisms in 7 bonobo, 44 chimpanzee, and 44 rhesus macaque (Macaca mulatta) individuals, we find considerable variation within each of these species and evidence for recent selection in chimpanzee populations. Finally, we also sequenced and analyzed BRCA2 from 24 primate species and find that this gene has also evolved under positive selection.

Conclusions

While mutations leading to truncated forms of BRCA1 are clearly linked to cancer phenotypes in humans, there is also an underlying selective pressure in favor of amino acid-altering substitutions in this gene. A hypothesis where viruses are the drivers of this natural selection is discussed.

A phylomedicine approach to understanding the evolution of auditory sensory perception and disease in mammals

Hereditary deafness affects 0.1% of individuals globally and is considered as one of the most debilitating diseases of man. Despite recent advances, the molecular basis of normal auditory function is not fully understood and little is known about the contribution of single-nucleotide variations to the disease. Using cross-species comparisons of 11 ‘deafness’ genes (Myo15, Ush1 g, Strc, Tecta, Tectb, Otog, Col11a2, Gjb2, Cldn14, Kcnq4, Pou3f4) across 69 evolutionary and ecologically divergent mammals, we elucidated whether there was evidence for: (i) adaptive evolution acting on these genes across mammals with similar hearing capabilities; and, (ii) regions of long-term evolutionary conservation within which we predict disease-associated mutations should occur. We find evidence of adaptive evolution acting on the eutherian mammals in Myo15, Otog and Tecta. Examination of selection pressures in Tecta and Pou3f4 across a taxonomic sample that included a wide representation of auditory specialists, the bats, did not uncover any evidence for a role in echolocation. We generated ‘conservation indices’ based on selection estimates at nucleotide sites and found that known disease mutations fall within sites of high evolutionary conservation. We suggest that methods such as this, derived from estimates of evolutionary conservation using phylogenetically divergent taxa, will help to differentiate between deleterious and benign mutations.

Selection and the cell cycle: positive Darwinian selection in a well-known DNA damage response pathway

Cancer is a common occurrence in multi-cellular organisms and is not strictly limited to the elderly in a population. It is therefore possible that individuals with genotypes that protect against early onset cancers have a selective advantage. In this study the patterns of mutation in the proteins of a well-studied DNA damage response pathway have been examined for evidence of adaptive evolutionary change. Using a maximum likelihood framework and the mammalian species phylogeny, together with codon models of evolution, selective pressure variation across the interacting network of proteins has been detected. The presence of signatures of adaptive evolution in BRCA1 and BRCA2 has already been documented but the effect on the entire network of interacting proteins in this damage response pathway has, until now, been unknown. Positive selection is evident throughout the network with a total of 11 proteins out of 15 examined displaying patterns of substitution characteristic of positive selection. It is also shown here that modern human populations display evidence of an ongoing selective sweep in 9 of these DNA damage repair proteins. The results presented here provide the community with new residues that may be relevant to cancer susceptibility while also highlighting those proteins where human and mouse have undergone lineage-specific functional shift. An understanding of this damage response pathway from an evolutionary perspective will undoubtedly contribute to future cancer treatment approaches.

The molecular evolution of acrosin in placental mammals

Acrosin is thought to fulfill several different roles in fertilization including that of a serine protease and in secondary zona pellucida (ZP) binding. However, acrosin's importance as a fertilization protein has been questioned. Especially since it was discovered that acrosin knockout mice are fertile. In this study, we explored the sites involved in serine protease activity and secondary binding. We also assessed conservation in functional sites across species and examined whether amino acid changes present in the human population have the potential to affect fertility. In addition, since many mammalian reproduction proteins have been found to evolve rapidly, we tested for positive selection. Sequences from 43 mammals from all 19 placental orders, which included a total of 828 nucleotides from acrosin exons 2, 3, 4, and a portion of exon 5, were obtained. We found that all sites of the serine catalytic triad as well as three other sites linked to catalytic activity were completely conserved. Five of six sites proposed to play a role in secondary binding were 100% conserved as basic residues. These results support an evolutionary conserved role for acrosin as a serine protease and secondary binding protein across placental mammals. We found statistically significant support for positive selection within acrosin, but no single amino acid site reached the significance level of P > 0.95 for inclusion within the category omega > 1. Based upon two amino acid mutation scoring systems, three out of seven human residue changing single nucleotide polymorphisms (SNPs) were found to be potentially protein-altering mutations.

UniPrime: a workflow-based platform for improved universal primer design

UniPrime is an open-source software (http://uniprime.batlab.eu), which automatically designs large sets of universal primers by simply inputting a gene ID reference. UniPrime automatically retrieves and aligns homologous sequences from GenBank, identifies regions of conservation within the alignment and generates suitable primers that can amplify variable genomic regions. UniPrime differs from previous automatic primer design programs in that all steps of primer design are automated, saved and are phylogenetically limited. We have experimentally verified the efficiency and success of this program by amplifying and sequencing four diverse genes (AOF2, EFEMP1, LRP6 and OAZ1) across multiple Orders of mammals. UniPrime is an experimentally validated, fully automated program that generates successful cross-species primers that take into account the biological aspects of the PCR.

Evaluation of unclassified variants in the breast cancer susceptibility genes BRCA1 and BRCA2 using five methods: results from a population-based study of young breast cancer patients

INTRODUCTION

Efforts are ongoing to determine the significance of unclassified variants (UVs) in the breast cancer susceptibility genes BRCA1/BRCA2, but no study has systematically assessed whether women carrying the suspected deleterious UVs have characteristics commonly seen among women carrying known deleterious or disease-causing mutations in BRCA1/BRCA2.

METHODS

We sequenced BRCA1/BRCA2 in 1,469 population-based female breast cancer patients diagnosed between the ages of 20 and 49 years. We used existing literature to classify variants into known deleterious mutations, polymorphic variants, and UVs. The UVs were further classified as high risk or low risk based on five methods: allele frequency, Polyphen algorithm, sequence conservation, Grantham matrix scores, and a combination of the Grantham matrix score and sequence conservation. Furthermore, we examined whether patients who carry the variants classified as high risk using these methods have risk characteristics similar to patients with known deleterious BRCA1/BRCA2 mutations (early age at diagnosis, family history of breast cancer or ovarian cancer, and negative estrogen receptor/progesterone receptor).

RESULTS

We identified 262 distinct BRCA1/BRCA2 variants, including 147 UVs, in our study population. The BRCA1 UV carriers, but not the BRCA2 UV carriers, who were classified as high risk using each classification method were more similar to the deleterious mutation carriers with respect to family history than those carriers classified as low risk. For example, the odds ratio of having a first-degree family history for the high-risk women classified using Polyphen was 3.39 (95% confidence interval = 1.16 to 9.94) compared with normal/polymorphic BRCA1 carriers. The corresponding odds ratio of low-risk women was 1.53 (95% confidence interval = 1.07 to 2.18). The odds ratio for high-risk women defined by allele frequency was 2.00 (95% confidence interval = 1.14 to 3.51), and that of low-risk women was 1.30 (95% confidence interval = 0.87 to 1.93).

CONCLUSION

The results suggest that the five classification methods yielded similar results. Polyphen was particularly better at isolating BRCA1 UV carriers likely to have a family history of breast cancer or ovarian cancer, and may therefore help to classify BRCA1 UVs. Our study suggests that these methods may not be as successful in classifying BRCA2 UVs.

Mammalian evolution and biomedicine: new views from phylogeny

Recent progress resolving the phylogenetic relationships of the major lineages of mammals has had a broad impact in evolutionary biology, comparative genomics and the biomedical sciences. Novel insights into the timing and historical biogeography of early mammalian diversification have resulted from a new molecular tree for placental mammals coupled with dating approaches that relax the assumption of the molecular clock. We highlight the numerous applications to come from a well-resolved phylogeny and genomic prospecting in multiple lineages of mammals, from identifying regulatory elements in mammalian genomes to assessing the functional consequences of mutations in human disease loci and those driving adaptive evolution.

Common variation in the BRCA1 gene and prostate cancer risk

Rare inactivating mutations in the BRCA1 gene seem to play a limited role in prostate cancer. To our knowledge, however, no study has comprehensively assessed the role of other BRCA1 sequence variations (e.g., missense mutations) in prostate cancer. In a study of 817 men with and without prostate cancer from 323 familial and early-onset prostate cancer families, we used family-based association tests and conditional logistic regression to investigate the association between prostate cancer and single nucleotide polymorphisms (SNPs) tagging common haplotype variation in a 200-kb region surrounding (and including) the BRCA1 gene. We also used the Genotype-Identity-by-Descent Sharing Test to determine whether our most strongly associated SNP could account for prostate cancer linkage to chromosome 17q21 in a sample of 154 families from our previous genome-wide linkage study. The strongest evidence for prostate cancer association was for a glutamine-to-arginine substitution at codon 356 (Gln(356)Arg) in exon 11 of the BRCA1 gene. The minor (Arg) allele was preferentially transmitted to affected men (P = 0.005 for a dominant model), with an estimated odds ratio of 2.25 (95% confidence interval, 1.21-4.20). Notably, BRCA1 Gln(356)Arg is not in strong linkage disequilibrium with other BRCA1 coding SNPs or any known HapMap SNP on chromosome 17. In addition, Genotype-Identity-by-Descent Sharing Test results suggest that Gln(356)Arg accounts (in part) for our prior evidence of prostate cancer linkage to chromosome 17q21 (P = 0.022). Thus, we have identified a common, nonsynonymous substitution in the BRCA1 gene that is associated with and linked to prostate cancer.