Pericentric Inversion Research Articles

Neo-sex chromosome evolution and phenotypic differentiation across an elevational gradient in horned larks (Eremophila alpestris).

Genetic structure and phenotypic variation among populations are affected by both geographic distance and environmental variation across species' distributions. Understanding the relative contributions of isolation by distance (IBD) and isolation by environment (IBE) is important for elucidating population dynamics across habitats and ecological gradients. In this study, we compared phenotypic and genetic variation among Horned Lark (Eremophila alpestris) populations from 10 sites encompassing an elevational gradient from low-elevation desert scrub in Death Valley (285a.s.l.) to high-elevation meadows in the White Mountains of the Sierra Nevada of California (greater than 3000m a.s.l.). Using a ddRAD data set of 28,474 SNPs aligned to a high-quality reference genome, we compared genetic structure with elevational, environmental, and spatial distance to quantify how different aspects of the landscape drive genomic and phenotypic differentiation in Horned Larks. We found larger-bodied birds were associated with sites that had less seasonality and higher annual precipitation, and longer spurs occurred in soils with more clay and silt content, less sand, and finer fragments. Larks have large neo-sex chromosomes, and we found that associations with elevation and environmental variation were much stronger among neo-sex chromosomes compared to autosomes. Furthermore, we found that putative chromosomal translocations, fusions, and inversions were associated with elevation and may underlie local adaptation across an elevational gradient in Horned Larks. Our results suggest that genetic variation in Horned Larks is affected more by IBD than IBE, but specific phenotypes and genomic regions-particually on neo-sex chromosomes-bear stronger associations with the environment.

Cytogenetics of Black Fly Larvae (Diptera: Simuliidae) from the Logan River, Utah, the Type Locality of Simulium brevicercum

I analyzed 221 black fly larvae of the Simulium arcticum complex from the Logan River, Logan Canyon, Utah, for larval sexual dimorphism, chromosomal inversions, morphology of the centromere band of chromosome IIL, presence of B chromosomes, and frequency of the IS-1 autosomal polymorphism. All 2275 larvae collected at the Logan River were of the S. arcticum complex with one exception, which was S. canadense. I found a significant association between larval color and sex, but I did not find such an association between larval head patterns and sex. Nearly all chromosomally analyzed larvae had standard, noninverted IIL chromosomes in both sexes. However, 70 of the 117 noninverted IIL chromosome males had the enhanced/thin (Ce Ct) centromeric dimorphism. Thus, there are at least two Y chromosomes in the Logan River population (Y0 and Y1). I also found four females and three males that had different chromosomal inversions at the base of chromosome IIL, but each of these was a singleton. Of the 120 males analyzed, six larvae had one acrocentric B chromosome and one larva had two acrocentric B chromosomes. None of the larvae at the Logan River was polymorphic for the IS-1 autosomal inversion. The presence of Ce Ct individuals that are also st/st for the IIL arm suggests that some of these larvae at the Logan River may be a new taxon. This observation is significant since some populations defined as Simulium brevicercum have only the IIL-st/st, Ce Ce genotype, while others have IIL-st/st larvae that have both Ce Ce and Ce Ct centromeric bands.

Pericentric inversion of chromosome 6 and male fertility problems.

As a significant chromosomal structural abnormality, chromosomal inversion is closely related to male infertility. For inversion carriers, the interchromosomal effect explains male infertility, but its specific mechanism remains unclear. Additionally, inversion carriers with different chromosomes have different clinical manifestations. Therefore, genetic counseling is difficult in clinical practice. Herein, four male carriers of pericentric inversion in chromosome 6 have been described. Two patients showed asthenospermia, one showed azoospermia, and the wife of the remaining patient had recurrent miscarriages. Through a literature search, the association between the breakpoint of pericentric inversion in chromosome 6 and male fertility problems are also discussed in this study. Overall, important genes related to asthenospermia in chromosome 6p21 were found, which may be related to the clinical phenotype. These results suggest that physicians should focus on the breakpoints of inversion in genetic counseling.

Highlights from Recent Cancer Literature

Highlights from Recent Cancer Literature

Pericentric inversion of chromosome 9: an abnormal phenotype

Pericentric inversion of chromosome 9 is considered a ba­lanced structural anomaly. More often cited as a rela­tive­ly common part of the normal human karyotype, this chromosomal rearrangement could correlate with in­fer­ti­lity, abortions and different abnormal clinical conditions. A 5-month-old infant was admitted to our hospital for poor feeding and loose stools. The physical examination revealed an underweight boy (under the fifth percentile) with pla­gio­ce­pha­ly, dysmorphic and low-set ears, hypertelorism, sad­dle nose and micrognathia. The abnormal phenotype also contained long limbs, arachnodactyly, pilonidal sinus and a systolic ejection murmur in the pulmonic area. He was unresponsive to auditory stimuli. The ENT exa­mi­na­tion described bilateral sensorineural hearing loss. Based on dysmorphic appearance and karyotyping of pe­ri­phe­ral, the case was classified as a pericentric inversion of chro­mo­some 9 (p11q13). Despite being categorized as a clinically in­si­gni­fi­cant variant to a cytogenetic finding, further research is still needed for the few cases where specific pathologies were associated with pericentric inversion of chromosome 9.

Evolutionary genetics: Inversions — Do not quail but go big!

How local phenotypic variation is maintained in highly mobile organisms, like birds, is an open question. A new study shows that a massive chromosomal inversion underlies a polymorphism in common quail.

Stable inversion clines in a grasshopper species group despite complex geographical history.

Chromosomal inversions are known to play roles in adaptation and differentiation in many species. They involve clusters of correlated genes (i.e., loci in linkage disequilibrium, LD) possibly associated with environmental variables. The grasshopper "species complex" Trimerotropis pallidipennis comprises several genetic lineages distributed from North to South America in arid and semi-arid high-altitude environments. The southernmost lineage, Trimerotropis sp., segregates for four to seven putative inversions that display clinal variation, possibly through adaptation to temperate environments. We analysed chromosomal, mitochondrial and genome-wide single nucleotide polymorphism data in 19Trimerotropis sp. populations mainly distributed along two altitudinal gradients (MS and Ju). Populations across Argentina comprise two main chromosomally and genetically differentiated lineages: one distributed across the southernmost border of the "Andes Centrales," adding evidence for a differentiation hotspot in this area; and the other widely distributed in Argentina. Within the latter, network analytical approaches to LD found three clusters of correlated loci (LD-clusters), with inversion karyotypes explaining >79% of the genetic variation. Outlier loci associated with environmental variables mapped to two of these LD-clusters. Furthermore, despite the complex geographical history indicated by population genetic analyses, the clines in inversion karyotypes have remained stable for more than 20generations, implicating their role in adaptation and differentiation within this lineage. We hypothesize that these clines could be the consequence of a coupling between extrinsic postzygotic barriers and spatially varying selection along environmental gradients resulting in a hybrid zone. These results provide a framework for future investigations about candidate genes implicated in rapid adaptation to new environments.

Rare Case of Pericentric Inversion, Inv(9)(p13q34) of the Der(9)t(9;22)(q34;q11) in A Patient with Chronic Myeloid Leukemia – A Case Report

As we know, the Philadelphia chromosome (Ph) is a highly specific marker for chronic myeloid leukemia (CML). This hematological disease is characterised by the formation of the BCR/ABL1 fusion gene, usually with typical translocation pattern including 9q34 and 22q11. In this paper we describe a 55 years old female patient with typical clinical and haematological signs of CML and a chromosome 9 differing from that which normally participates in translocation t(9;22). The karyotype of this Ph positive patient is characterised by pericentric inv(9)(p13q34) of the der(9)t(9;22)(q34;q11). Reverse transcriptase-polymerase chain reaction revealed a e14a2 type of BCR/ABL1 fusion transcript. As a consequence of this unusual translocation, FISH also found the separation of the ABL1/BCR1 fusion gene on chromosome 9. On reviewing the literature, to date only 10 Ph-positive leukemia patients have been noticed to have pericentric inversion inv(9)(p22q34)der(9)t(9;22)(q34;q11). No one case has been described with pericentric inversion inv(9)(p13q34) of the der(9)t(9;22)(q34;q11). This indicate that pericentric inv(9)(p13q34) of the der(9)t(9;22)(q34;q11) is a novel, rare, chromosomal abnormality in Ph-positive CML.

Comparative cytogenetic patterns in Carangidae fishes in association with their distribution range.

Carangidae are an important and widespreaded family of pelagic predatory fishes that inhabit reef regions or open ocean areas, some species occupying a vast circumglobal distribution. Cytogenetic comparisons among representatives of its different tribes help to understand the process of karyotype divergence in marine ecosystems due to the variable migratory ability of species. In this sense, conventional cytogenetic investigations (Giemsa staining, Ag-NORs, and C-banding), GC base-specific fluorochrome staining and FISH mapping of ribosomal DNAs were performed. Four species, Elagatisbipinnulata (Quoy et Gaimard, 1825) and Seriolarivoliana (Valenciennes, 1883) (Naucratini), with circumtropical distributions, Gnathanodonspeciosus (Forsskål, 1775) (Carangini), widely distributed in the tropical and subtropical waters of the Indian and Pacific oceans, and Trachinotuscarolinus (Linnaeus, 1766) (Trachinotini), distributed along the western Atlantic Ocean, were analyzed, thus encompassing representatives of three out its four tribes. All species have diploid chromosome number 2n = 48, with karyotypes composed mainly by acrocentric chromosomes (NF = 50–56). The 18S rDNA/Ag-NORs/GC+ and 5S rDNA loci were located on chromosomes likely homeologs. Karyotypes showed a pattern considered basal for the family or with small variations in their structures, apparently due to pericentric inversions. The migratory capacity of large pelagic swimmers, in large distribution areas, likely restricts the fixation of chromosome changes in Carangidae responsible for a low level of karyotype diversification.

A large chromosomal inversion shapes gene expression in seaweed flies (Coelopa frigida).

Inversions often underlie complex adaptive traits, but the genic targets inside them are largely unknown. Gene expression profiling provides a powerful way to link inversions with their phenotypic consequences. We examined the effects of the Cf‐Inv(1) inversion in the seaweed fly Coelopa frigida on gene expression variation across sexes and life stages. Our analyses revealed that Cf‐Inv(1) shapes global expression patterns, most likely via linked variation, but the extent of this effect is variable, with much stronger effects in adults than larvae. Furthermore, within adults, both common as well as sex‐specific patterns were found. The vast majority of these differentially expressed genes mapped to Cf‐Inv(1). However, genes that were differentially expressed in a single context (i.e., in males, females, or larvae) were more likely to be located outside of Cf‐Inv(1). By combining our findings with genomic scans for environmentally associated SNPs, we were able to pinpoint candidate variants in the inversion that may underlie mechanistic pathways that determine phenotypes. Together the results of this study, combined with previous findings, support the notion that the polymorphic Cf‐Inv(1) inversion in this species is a major factor shaping both coding and regulatory variation resulting in highly complex adaptive effects.

Genetic markers associated with insecticide resistance and resting behaviour in Anopheles gambiae mosquitoes in selected sites in Kenya

BackgroundMolecular diagnostic tools have been incorporated in insecticide resistance monitoring programmes to identify underlying genetic basis of resistance and develop early warning systems of vector control failure. Identifying genetic markers of insecticide resistance is crucial in enhancing the ability to mitigate potential effects of resistance. The knockdown resistance (kdr) mutation associated with resistance to DDT and pyrethroids, the acetylcholinesterase-1 (ace-1R) mutation associated with resistance to organophosphates and carbamates and 2La chromosomal inversion associated with indoor resting behaviour, were investigated in the present study.MethodsAnopheles mosquitoes sampled from different sites in Kenya and collected within the context of malaria vector surveillance were analysed. Mosquitoes were collected indoors using light traps, pyrethrum spray and hand catches between August 2016 and November 2017. Mosquitoes were identified using morphological keys and Anopheles gambiae sensu lato (s.l.) mosquitoes further identified into sibling species by the polymerase chain reaction method following DNA extraction by alcohol precipitation. Anopheles gambiae and Anopheles arabiensis were analysed for the presence of the kdr and ace-1R mutations, while 2La inversion was only screened for in An. gambiae where it is polymorphic. Chi-square statistics were used to determine correlation between the 2La inversion karyotype and kdr-east mutation.ResultsThe kdr-east mutation occurred at frequencies ranging from 0.5 to 65.6% between sites. The kdr-west mutation was only found in Migori at a total frequency of 5.3% (n = 124). No kdr mutants were detected in Tana River. The ace-1R mutation was absent in all populations. The 2La chromosomal inversion screened in An. gambiae occurred at frequencies of 87% (n = 30), 80% (n = 10) and 52% (n = 50) in Baringo, Tana River and Migori, respectively. A significant association between the 2La chromosomal inversion and the kdr-east mutation was found.ConclusionThe significant association between the 2La inversion karyotype and kdr-east mutation suggests that pyrethroid resistant An. gambiae continue to rest indoors regardless of the presence of treated bed nets and residual sprays, a persistence further substantiated by studies documenting continued mosquito abundance indoors. Behavioural resistance by which Anopheles vectors prefer not to rest indoors may, therefore, not be a factor of concern in this study’s malaria vector populations.

A rare interstitial deletion (2)(p11.2p13) in a child with pericentric inversion (2)(p11.2q13) of paternal origin

An unbalanced 46,XY,der(2)del(2)(p11.2p13)inv(2)(p11.2q13) karyotype was found in a phenotypically abnormal child. A de novo interstitial deletion resulting in loss of band 2p12 was present on a chromosome 2 with an inv(2)(p11.2q13) inherited from the father. The inv(2) is generally considered a benign familial variant without significant reproductive consequences. However, our findings led us to consider a previously proposed mechanism of unequal meiotic crossing over at the base of a parental inversion loop, which could lead to either duplication or deletion of a segment adjacent to the inverted region in offspring. This phenomenon has been reported in association with inversions of chromosomes 7, 13, 15 and 17 and may explain origin of the deletion in our case. Alternatively, repetitive sequences might be present around such inversions which could predispose to de novo deletions independently of the inversion.Although the mechanism of origin of the deletion in our proband cannot be proven, our review of similar imbalances with other inversions and the findings in the family described in this report suggest there could be a small risk for a related imbalance in offspring of couples with an inv(2)(p11q13). Further monitoring of pregnancy outcome in families with the inversion should be considered. However, at this point, it seems premature to recommend prenatal diagnosis for all couples in this situation.For del(2)(p11.2p13), which is rare, an emerging phenotype is proposed. Our patient shared several features with four previously published cases, namely a broad nasal bridge, abnormal ears, highly-arched palate, psychomotor retardation, and micrognathia. However, our patient also had sensorineural hearing loss and significant hypotonia which have not been previously reported.

Mad2 Induced Aneuploidy Contributes to Eml4-Alk Driven Lung Cancer by Generating an Immunosuppressive Environment

Simple SummaryIn tissue homeostasis, aneuploid cells have been suggested to be recognized and eliminated by immune cells. However, these initiating cancer cells can evade the immune system and ultimately derivate a tumor. To better understand how aneuploidy might contribute to tumor initiation and/or progression, we used two lung cancer models: one in which cancer cells are aneuploid and the surrounding normal epithelial cells are diploid, and a second one in which both tumor and normal cells are aneuploid. We show that aneuploid cells surrounding the tumor generate an immunosuppressive environment that contributes to lung cancer initiation.Aneuploidy, an imbalance number of chromosomes, is frequently observed in lung cancer and inversely correlates with patient survival. Paradoxically, an aneuploid karyotype has detrimental consequences on cellular fitness, and it has been proposed that aneuploid cells, at least in vitro, generate signals for their own elimination by NK cells. However, how aneuploidy affects tumor progression as well as the interplay between aneuploid tumor cells and the tumor microenvironment is still unclear. We generated a new mouse model in which overexpression of Mad2 was almost entirely restricted to normal epithelial cells of the lung, and combined it with an oncogenic Eml4-Alk chromosome inversion. This combination resulted in a higher tumor burden and an increased number of tumor nodules compared to control Eml4-Alk mice alone. The FISH analysis detected significant differences in the aneuploidy levels in the non-tumor regions of Eml4-Alk+Mad2 compared to Eml4-Alk alone, although both tumor groups presented similar levels of aneuploidy. We further show that aneuploid cells in the non-tumor areas adjacent to lung tumors recruit immune cells, such as tumor-associated macrophages. In fact, these areas presented an increase in alveolar macrophages, neutrophils, decreased cytotoxic CD8+ T cells, and IFN-γ, suggesting that aneuploid cells in the surrounding tumor areas create an immunosuppressive signature that might contribute to lung tumor initiation and progression.

An accurate assignment test for extremely low-coverage whole-genome sequence data.

Genomic assignment tests can provide important diagnostic biological characteristics, such as population of origin or ecotype. Yet, assignment tests often rely on moderate- to high-coverage sequence data that can be difficult to obtain for fields such as molecular ecology and ancient DNA. We have developed a novel approach that efficiently assigns biologically relevant information (i.e., population identity or structural variants such as inversions) in extremely low-coverage sequence data. First, we generate databases from existing reference data using a subset of diagnostic single nucleotide polymorphisms (SNPs) associated with a biological characteristic. Low-coverage alignment files are subsequently compared to these databases to ascertain allelic state, yielding a joint probability for each association. To assess the efficacy of this approach, we assigned haplotypes and population identity in Heliconius butterflies, Atlantic herring, and Atlantic cod using chromosomal inversion sites and whole-genome data. We scored both modern and ancient specimens, including the first whole-genome sequence data recovered from ancient Atlantic herring bones. The method accurately assigns biological characteristics, including population membership, using extremely low-coverage data (as low as 0.0001x) based on genome-wide SNPs. This approach will therefore increase the number of samples in evolutionary, ecological and archaeological research for which relevant biological information can be obtained.

Genome structure and evolution in the cruciferous tribe Thlaspideae (Brassicaceae).

Whole-genome duplications (WGDs) and chromosome rearrangements (CRs) play the key role in driving the diversification and evolution of plant lineages. Although the direct link between WGDs and plant diversification is well documented, relatively few studies focus on the evolutionary significance of CRs. The cruciferous tribe Thlaspideae represents an ideal model system to address the role of large-scale chromosome alterations in genome evolution, as most Thlaspideae species share the same diploid chromosome number (2n=2x=14). Here we constructed the genome structure in 12 Thlaspideae species, including field pennycress (Thlaspi arvense) and garlic mustard (Alliaria petiolata). We detected and precisely characterized genus- and species-specific CRs, mostly pericentric inversions, as the main genome-diversifying drivers in the tribe. We reconstructed the structure of seven chromosomes of an ancestral Thlaspideae genome, identified evolutionary stable chromosomes versus chromosomes prone to CRs, estimated the rate of CRs, and uncovered an allohexaploid origin of garlic mustard from diploid taxa closely related to A. petiolata and Parlatoria cakiloidea. Furthermore, we performed detailed bioinformatic analysis of the Thlaspideae repeatomes, and identified repetitive elements applicable as unique species- and genus-specific barcodes and chromosome landmarks. This study deepens our general understanding of the evolutionary role of CRs, particularly pericentric inversions, in plant genome diversification, and provides a robust base for follow-up whole-genome sequencing efforts.

In vitro fertilization and preimplantation genetic testing methods in infertility treatment of a woman with karyotype 46,XX,ins(13;4)(q34;p14p15.3),inv(4)(p14q12). Case report

The frequency of structural chromosomal transpositions can range from 1.8 to 8% among patients with reproductive disorders. There are several types of the rarest chromosomal abnormalities: insertion (insertion of a chromosomal region) and inversion (rotation of a chromosome region). This article describes a clinical case of the infertility treatment using assisted reproductive technologies in a woman with a rare chromosomal abnormality: simultaneous insertion and inversion of chromosomes 46, XX, ins (13;4)(q34;p14p15.3), inv(4)(p14q12). The structure and frequency of chromosomal aberrations were determined by high-throughput sequencing in preimplantation embryos. The result of the sequencing analysis showed that unbalanced variants for a known pathology were detected in 9 (56.3%) out of 16 observations, while in 6 (37%) only for a pathology known in the karyotype and in 3 (19%) they were presented simultaneously with the pathology of other chromosomes or with mosaicism. According to the results of the study, in preimplantation embryos, where one of the parents had chromosomal abnormalities, in addition to unbalanced variants, there is aneuploidy of other chromosomes not involved in the known pathology. They are described in 3 (21%) out of 14 observations of all identified pathology. In this regard, patients with aberrations in the karyotype are recommended, whenever possible, to carry out preimplantation genetic testing of structural rearrangements by methods allowed to analyze all chromosomes simultaneously. For example, high-throughput sequencing on the Illumina platform may become an alternative for prenatal diagnostics, which is performed in fertile couples with high risk of having a child with hereditary or congenital disorders. In the case of detection of chromosomal changes in the fetus, patients are faced with a number of ethical issues related to the necessity for medical abortion, which may contradict their religious and moral convictions.

Chromosome-scale genome assembly of the high royal jelly-producing honeybees

A high royal jelly-producing strain of honeybees (HRJHB) has been obtained by successive artificial selection of Italian honeybees (Apis mellifera ligustica) in China. The HRJHB can produce amounts of royal jelly that are dozens of times greater than their original counterparts, which has promoted China to be the largest producer of royal jelly in the world. In this study, we generated a chromosome-scale of the genome sequence for the HRJHB using PacBio long reads and Hi-C technique. The genome consists of 16 pseudo-chromosomes that contain 222 Mb of sequence, with a scaffold N50 of 13.6 Mb. BUSCO analysis yielded a completeness score of 99.3%. The genome has 12,288 predicted protein-coding genes and a rate of 8.11% of repetitive sequences. One chromosome inversion was identified between the HRJHB and the closely related Italian honeybees through whole-genome alignment analysis. The HRJHB’s genome sequence will be an important resource for understanding the genetic basis of high levels of royal jelly production, which may also shed light on the evolution of domesticated insects.

A supergene underlies linked variation in color and morphology in a Holarctic songbird

The genetic architecture of a phenotype can have considerable effects on the evolution of a trait or species. Characterizing genetic architecture provides insight into the complexity of a given phenotype and, potentially, the role of the phenotype in evolutionary processes like speciation. We use genome sequences to investigate the genetic basis of phenotypic variation in redpoll finches (Acanthis spp.). We demonstrate that variation in redpoll phenotype is broadly controlled by a ~55-Mb chromosomal inversion. Within this inversion, we find multiple candidate genes related to melanogenesis, carotenoid coloration, and bill shape, suggesting the inversion acts as a supergene controlling multiple linked traits. A latitudinal gradient in ecotype distribution suggests supergene driven variation in color and bill morphology are likely under environmental selection, maintaining supergene haplotypes as a balanced polymorphism. Our results provide a mechanism for the maintenance of ecotype variation in redpolls despite a genome largely homogenized by gene flow.

Fine Breakpoint Mapping by Genome Sequencing Reveals the First Large X Inversion Disrupting the NHS Gene in a Patient with Syndromic Cataracts

Inversions are structural variants that are generally balanced. However, they could lead to gene disruptions or have positional effects leading to diseases. Mutations in the NHS gene cause Nance-Horan syndrome, an X-linked disorder characterised by congenital cataracts and dental anomalies. Here, we aimed to characterise a balanced pericentric inversion X(p22q27), maternally inherited, in a child with syndromic bilateral cataracts by breakpoint mapping using whole-genome sequencing (WGS). 30× Illumina paired-end WGS was performed in the proband, and breakpoints were confirmed by Sanger sequencing. EdU assays and FISH analysis were used to assess skewed X-inactivation patterns. RNA expression of involved genes in the breakpoint boundaries was evaluated by droplet-digital PCR. We defined the breakpoint position of the inversion at Xp22.13, with a 15 bp deletion, disrupting the unusually large intron 1 of the canonical NHS isoform, and also perturbing topologically-associated domains (TADs). Moreover, a microhomology region of 5 bp was found on both sides. RNA analysis confirmed null and reduced NHS expression in the proband and his unaffected mother, respectively. In conclusion, we report the first chromosomal inversion disrupting NHS, fine-mapped by WGS. Our data expand the clinical spectrum and the pathogenic mechanisms underlying the NHS defects.

A recurrent chromosomal inversion suffices for driving escape from oncogene-induced senescence via subTAD reorganization

Oncogene-induced senescence (OIS) is an inherent and important tumor suppressor mechanism. However, if not removed timely via immune surveillance, senescent cells also have detrimental effects. Although this has mostly been attributed to the senescence-associated secretory phenotype (SASP) of these cells, we recently proposed that "escape" from the senescent state is another unfavorable outcome. The mechanism underlying this phenomenon remains elusive. Here, we exploit genomic and functional data from a prototypical human epithelial cell model carrying an inducible CDC6 oncogene to identify an early-acquired recurrent chromosomal inversion that harbors a locus encoding the circadian transcription factor BHLHE40. This inversion alone suffices for BHLHE40 activation upon CDC6 induction and driving cell cycle re-entry of senescent cells, and malignant transformation. Ectopic overexpression of BHLHE40 prevented induction of CDC6-triggered senescence. We provide strong evidence in support of replication stress-induced genomic instability being a causative factor underlying "escape" from oncogene-induced senescence.