Genome-scale clustered regularly interspaced short palindromic repeats screen identifies nucleotide metabolism as an actionable therapeutic vulnerability in diffuse large B-cell lymphoma.

Diffuse large B-cell lymphoma (DLBCL) is the most common malignancy that develops in patients with ataxia-telangiectasia, a cancer-predisposing inherited syndrome characterized by inactivating germline ATM mutations. ATM is also frequently mutated in sporadic DLBCL. To investigate lymphomagenic mechanisms and lymphoma-specific dependencies underlying defective ATM, we applied ribonucleic acid (RNA)-seq and genome-scale loss-offunction clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 screens to systematically interrogate B-cell lymphomas arising in a novel murine model (Atm-/-nu-/-) with constitutional Atm loss, thymic aplasia but residual T-cell populations. Atm-/-nu-/-lymphomas, which phenotypically resemble either activated B-cell-like or germinal center Bcell-like DLBCL, harbor a complex karyotype, and are characterized by MYC pathway activation. In Atm-/-nu-/-lymphomas, we discovered nucleotide biosynthesis as a MYCdependent cellular vulnerability that can be targeted through the synergistic nucleotidedepleting actions of mycophenolate mofetil (MMF) and the WEE1 inhibitor, adavosertib (AZD1775). The latter is mediated through a synthetically lethal interaction between RRM2 suppression and MYC dysregulation that results in replication stress overload in Atm-/-nu-/-lymphoma cells. Validation in cell line models of human DLBCL confirmed the broad applicability of nucleotide depletion as a therapeutic strategy for MYC-driven DLBCL independent of ATM mutation status. Our findings extend current understanding of lymphomagenic mechanisms underpinning ATM loss and highlight nucleotide metabolism as a targetable therapeutic vulnerability in MYC-driven DLBCL.

Bi-allelic TTI1 variants cause an autosomal-recessive neurodevelopmental disorder with microcephaly.

Telomere maintenance 2 (TELO2), Tel2 interacting protein 2 (TTI2), and Tel2 interacting protein 1 (TTI1) are the three components of the conserved Triple T (TTT) complex that modulates activity of phosphatidylinositol 3-kinase-related protein kinases (PIKKs), including mTOR, ATM, and ATR, by regulating the assembly of mTOR complex 1 (mTORC1). The TTT complex is essential for the expression, maturation, and stability of ATM and ATR in response to DNA damage. TELO2- and TTI2-related bi-allelic autosomal-recessive (AR) encephalopathies have been described in individuals with moderate to severe intellectual disability (ID), short stature, postnatal microcephaly, and a movement disorder (in the case of variants within TELO2). We present clinical, genomic, and functional data from 11 individuals in 9 unrelated families with bi-allelic variants in TTI1. All present with ID, and most with microcephaly, short stature, and a movement disorder. Functional studies performed in HEK293T cell lines and fibroblasts and lymphoblastoid cells derived from 4 unrelated individuals showed impairment of the TTT complex and of mTOR pathway activity which is improved by treatment with Rapamycin. Our data delineate a TTI1-related neurodevelopmental disorder and expand the group of disorders related to the TTT complex.

Biallelic Mutation of SETX and Additional Likely "In Cis" SETX Sequence Change in Ataxia with Oculomotor Apraxia Type 2.

Ataxia with oculomotor apraxia type 2 (AOA2) is a slowly progressive, autosomal recessive disease characterized by the triad of ataxia, oculomotor apraxia, and sensorimotor neuropathy. The genetic basis of AOA2 is biallelic mutation of the SETX gene, resulting in reduced or absent senataxin, a DNA/RNA repair protein essential for genomic stability. In this case report, we described a case of AOA2 with two clear pathogenic SETX mutations, one of which is novel. We then discussed two further likely "in cis" SETX sequence changes (previously reported in the literature as pathogenic), and presented the case that they are likely benign polymorphisms.

Genotype, extrapyramidal features, and severity of variant ataxia-telangiectasia.

OBJECTIVE: Variant ataxia-telangiectasia is caused by mutations that allow some retained ataxia telangiectasia-mutated (ATM) kinase activity. Here, we describe the clinical features of the largest established cohort of individuals with variant ataxia-telangiectasia and explore genotype-phenotype correlations. METHODS: Cross-sectional data were collected retrospectively. Patients were classified as variant ataxia-telangiectasia based on retained ATM kinase activity. RESULTS: The study includes 57 individuals. Mean age at assessment was 37.5 years. Most had their first symptoms by age 10 (81%). There was a diagnostic delay of more than 10 years in 68% and more than 20 years in one third of probands. Disease severity was mild in one third of patients, and 43% were still ambulant 20 years after disease onset. Only one third had predominant ataxia, and 18% had a pure extrapyramidal presentation. Individuals with extrapyramidal presentations had milder neurological disease severity. There were no significant respiratory or immunological complications, but 25% of individuals had a history of malignancy. Missense mutations were associated with milder neurological disease severity, but with a higher risk of malignancy, compared to leaky splice site mutations. INTERPRETATION: Individuals with variant ataxia-telangiectasia require malignancy surveillance and tailored management. However, our data suggest the condition may sometimes be mis- or underdiagnosed because of atypical features, including exclusive extrapyramidal symptoms, normal eye movements, and normal alpha-fetoprotein levels in some individuals. Missense mutations are associated with milder neurological presentations, but a particularly high malignancy risk, and it is important for clinicians to be aware of these phenotypes. ANN NEUROL 2019;85:170-180.

Degradation of a Novel DNA Damage Response Protein, Tankyrase 1 Binding Protein 1, following Adenovirus Infection.

Infection by most DNA viruses activates a cellular DNA damage response (DDR), which may be to the detriment or advantage of the virus. In the case of adenoviruses, they neutralize antiviral effects of DDR activation by targeting a number of proteins for rapid proteasome-mediated degradation. We have now identified a novel DDR protein, tankyrase 1 binding protein 1 (TNKS1BP1) (also known as Tab182), which is degraded during infection by adenovirus serotype 5 and adenovirus serotype 12. In both cases, degradation requires the action of the early region 1B55K (E1B55K) and early region 4 open reading frame 6 (E4orf6) viral proteins and is mediated through the proteasome by the action of cullin-based cellular E3 ligases. The degradation of Tab182 appears to be serotype specific, as the protein remains relatively stable following infection with adenovirus serotypes 4, 7, 9, and 11. We have gone on to confirm that Tab182 is an integral component of the CNOT complex, which has transcriptional regulatory, deadenylation, and E3 ligase activities. The levels of at least 2 other members of the complex (CNOT3 and CNOT7) are also reduced during adenovirus infection, whereas the levels of CNOT4 and CNOT1 remain stable. The depletion of Tab182 with small interfering RNA (siRNA) enhances the expression of early region 1A proteins (E1As) to a limited extent during adenovirus infection, but the depletion of CNOT1 is particularly advantageous to the virus and results in a marked increase in the expression of adenovirus early proteins. In addition, the depletion of Tab182 and CNOT1 results in a limited increase in the viral DNA level during infection. We conclude that the cellular CNOT complex is a previously unidentified major target for adenoviruses during infection.IMPORTANCE Adenoviruses target a number of cellular proteins involved in the DNA damage response for rapid degradation. We have now shown that Tab182, which we have confirmed to be an integral component of the mammalian CNOT complex, is degraded following infection by adenovirus serotypes 5 and 12. This requires the viral E1B55K and E4orf6 proteins and is mediated by cullin-based E3 ligases and the proteasome. In addition to Tab182, the levels of other CNOT proteins are also reduced during adenovirus infection. Thus, CNOT3 and CNOT7, for example, are degraded, whereas CNOT4 and CNOT1 are not. The siRNA-mediated depletion of components of the complex enhances the expression of adenovirus early proteins and increases the concentration of viral DNA produced during infection. This study highlights a novel protein complex, CNOT, which is targeted for adenovirus-mediated protein degradation. To our knowledge, this is the first time that the CNOT complex has been identified as an adenoviral target.

A Hypomorphic PALB2 Allele Gives Rise to an Unusual Form of FA-N Associated with Lymphoid Tumour Development.

Patients with biallelic truncating mutations in PALB2 have a severe form of Fanconi anaemia (FA-N), with a predisposition for developing embryonal-type tumours in infancy. Here we describe two unusual patients from a single family, carrying biallelic PALB2 mutations, one truncating, c.1676_1677delAAinsG;(p.Gln559ArgfsTer2), and the second, c.2586+1G>A; p.Thr839_Lys862del resulting in an in frame skip of exon 6 (24 amino acids). Strikingly, the affected individuals did not exhibit the severe developmental defects typical of FA-N patients and initially presented with B cell non-Hodgkin lymphoma. The expressed p.Thr839_Lys862del mutant PALB2 protein retained the ability to interact with BRCA2, previously unreported in FA-N patients. There was also a large increased chromosomal radiosensitivity following irradiation in G2 and increased sensitivity to mitomycin C. Although patient cells were unable to form Rad51 foci following exposure to either DNA damaging agent, U2OS cells, in which the mutant PALB2 with in frame skip of exon 6 was induced, did show recruitment of Rad51 to foci following damage. We conclude that a very mild form of FA-N exists arising from a hypomorphic PALB2 allele.

Very mild presentation in adult with classical cellular phenotype of ataxia telangiectasia.

BACKGROUND: The major clinical feature of ataxia telangiectasia (A-T) is severe progressive neurodegeneration with onset in infancy. This classical A-T phenotype is caused by biallelic null mutations in the ATM gene, leading to the absence of ATM protein and increased cellular radiosensitivity. We report an unusual case of A-T in a 41-year-old mother, A-T210, who had very mild neurological symptoms despite complete loss of ATM protein. METHODS: A neurological examination was performed, cellular radiosensitivity was assessed, and the ATM gene was sequenced. Skin fibroblasts and a lymphoblastoid cell line (LCL) were assayed for ATM protein expression and kinase activity. RESULTS: Patient A-T210 showed mild chorea, dystonia, and gait ataxia, walked independently, and drove a car. LCL and skin fibroblasts were radiosensitive and did not express ATM protein. Two ATM-null mutations were identified. CONCLUSIONS: The severe neurodegeneration resulting from loss of ATM can be mitigated in some circumstances.

Identification of the first ATRIP-deficient patient and novel mutations in ATR define a clinical spectrum for ATR-ATRIP Seckel Syndrome.

A homozygous mutational change in the Ataxia-Telangiectasia and RAD3 related (ATR) gene was previously reported in two related families displaying Seckel Syndrome (SS). Here, we provide the first identification of a Seckel Syndrome patient with mutations in ATRIP, the gene encoding ATR-Interacting Protein (ATRIP), the partner protein of ATR required for ATR stability and recruitment to the site of DNA damage. The patient has compound heterozygous mutations in ATRIP resulting in reduced ATRIP and ATR expression. A nonsense mutational change in one ATRIP allele results in a C-terminal truncated protein, which impairs ATR-ATRIP interaction; the other allele is abnormally spliced. We additionally describe two further unrelated patients native to the UK with the same novel, heterozygous mutations in ATR, which cause dramatically reduced ATR expression. All patient-derived cells showed defective DNA damage responses that can be attributed to impaired ATR-ATRIP function. Seckel Syndrome is characterised by microcephaly and growth delay, features also displayed by several related disorders including Majewski (microcephalic) osteodysplastic primordial dwarfism (MOPD) type II and Meier-Gorlin Syndrome (MGS). The identification of an ATRIP-deficient patient provides a novel genetic defect for Seckel Syndrome. Coupled with the identification of further ATR-deficient patients, our findings allow a spectrum of clinical features that can be ascribed to the ATR-ATRIP deficient sub-class of Seckel Syndrome. ATR-ATRIP patients are characterised by extremely severe microcephaly and growth delay, microtia (small ears), micrognathia (small and receding chin), and dental crowding. While aberrant bone development was mild in the original ATR-SS patient, some of the patients described here display skeletal abnormalities including, in one patient, small patellae, a feature characteristically observed in Meier-Gorlin Syndrome. Collectively, our analysis exposes an overlapping clinical manifestation between the disorders but allows an expanded spectrum of clinical features for ATR-ATRIP Seckel Syndrome to be defined.

Serotype-specific inactivation of the cellular DNA damage response during adenovirus infection.

Adenovirus type 5 (Ad5) inactivates the host cell DNA damage response by facilitating the degradation of Mre11, DNA ligase IV, and p53. In the case of p53, this is achieved through polyubiquitylation by Ad5E1B55K and Ad5E4orf6, which recruit a Cul5-based E3 ubiquitin ligase. Recent evidence indicates that this paradigm does not apply to other adenovirus serotypes, since Ad12, but not Ad5, causes the degradation of TOPBP1 through the action of E4orf6 alone and a Cul2-based E3 ubiquitin ligase. We now have extended these studies to adenovirus groups A to E. While infection by Ad4, Ad5, and Ad12 (groups E, C, and A, respectively) cause the degradation of Mre11, DNA ligase IV, and p53, infection with Ad3, Ad7, Ad9, and Ad11 (groups B1, B1, D, and B2, respectively) only affects DNA ligase IV levels. Indeed, Ad3, Ad7, and Ad11 cause the marked accumulation of p53. Despite this, MDM2 levels were very low following infection with all of the viruses examined here, regardless of whether they increase p53 expression. In addition, we found that only Ad12 causes the degradation of TOPBP1, and, like Ad5, Ad4 recruits a Cul5-based E3 ubiquitin ligase to degrade p53. Surprisingly, Mre11 and DNA ligase IV degradation do not appear to be significantly affected in Ad4-, Ad5-, or Ad12-infected cells depleted of Cul2 or Cul5, indicating that E1B55K and E4orf6 recruit multiple ubiquitin ligases to target cellular proteins. Finally, although Mre11 is not degraded by Ad3, Ad7, Ad9, and Ad11, no viral DNA concatemers could be detected. We suggest that group B and D adenoviruses have evolved mechanisms based on the loss of DNA ligase IV and perhaps other unknown molecules to disable the host cell DNA damage response to promote viral replication.

Modeling ATM mutant proteins from missense changes confirms retained kinase activity.

Ataxia-telangiectasia mutated (ATM) is the gene mutated in the cancer-predisposing disorder ataxia-telangiectasia (A-T). We modeled ATM sequence variants identified in UK A-T patients to determine the stability and kinase activity of the resulting proteins as well as the distribution of these mutations across the coding region. Of 20 missense changes modeled, 10 proteins showed ATM kinase activity and 10 showed none. In the majority of cases the mutant ATM protein was unstable, although this was variable. Reduction in ATM kinase activity can result either from the presence of low levels of unstable mutant protein with relatively normal specific kinase activity or from stable mutant protein with deficient ATM kinase activation. Indeed, ATM mutant proteins without kinase activity toward downstream targets were still able to autophosphorylate on serine 1981, although in a much less efficient manner, suggesting that this was not sufficient for ATM activation. In terms of function, green fluorescent protein (GFP)-tagged kinase inactive ATM proteins could form ionizing radiation (IR)-induced foci (IRIF), at least temporarily, which colocalized with the DNA double-strand break (DSB) marker gammaH2AX. Consistent with this, both kinase active and inactive mutant ATM proteins were able to interfere with phosphorylation of targets by endogenous ATM. Since the majority of missense mutations occurred C-terminal to aa1966, including all 10 mutations with absence of kinase activity, the implication was that mutations N-terminal to this, with exceptions, are less likely to result in loss of kinase activity and therefore, are less likely to be identified in A-T patients.

The RIDDLE syndrome protein mediates a ubiquitin-dependent signaling cascade at sites of DNA damage.

The biological response to DNA double-strand breaks acts to preserve genome integrity. Individuals bearing inactivating mutations in components of this response exhibit clinical symptoms that include cellular radiosensitivity, immunodeficiency, and cancer predisposition. The archetype for such disorders is Ataxia-Telangiectasia caused by biallelic mutation in ATM, a central component of the DNA damage response. Here, we report that the ubiquitin ligase RNF168 is mutated in the RIDDLE syndrome, a recently discovered immunodeficiency and radiosensitivity disorder. We show that RNF168 is recruited to sites of DNA damage by binding to ubiquitylated histone H2A. RNF168 acts with UBC13 to amplify the RNF8-dependent histone ubiquitylation by targeting H2A-type histones and by promoting the formation of lysine 63-linked ubiquitin conjugates. These RNF168-dependent chromatin modifications orchestrate the accumulation of 53BP1 and BRCA1 to DNA lesions, and their loss is the likely cause of the cellular and developmental phenotypes associated with RIDDLE syndrome.

Pathogenic ATM mutations occur rarely in a subset of multiple myeloma patients.

Ataxia Telangiectasia (A-T) patients have biallelic inactivation of the ATM gene and exhibit a 200-fold-increased frequency of lymphoid tumours. ATM mutations have been found in a number of adult lymphoid malignancies but there is no data on the occurrence of ATM mutations in multiple myeloma tumours. The purpose of our work was to investigate the occurrence of ATM mutations in multiple myeloma and to this end we screened 45 sporadic cases for ATM mutations using denaturing high-performance liquid chromatography analysis and DNA sequencing. Pathogenic ATM mutations were identified in 2/45 of the myelomas compared with a published estimate of ATM mutant allele frequency in the UK population of 2/521 (P = 0.033). One was the missense mutation 7181C>T which was then modelled in an expression system and the S2394L protein shown to have no ATM kinase activity. The second myeloma had the pathogenic ATM splice site mutation IVS40-1G>C leading to loss of exon 41. We also report a 48-year-old ataxia telangiectasia patient who developed multiple myeloma. Taken together our study suggests that ATM mutation may play a role in the pathogenesis of a subset of multiple myelomas.

RIDDLE immunodeficiency syndrome is linked to defects in 53BP1-mediated DNA damage signaling.

Cellular DNA double-strand break-repair pathways have evolved to protect the integrity of the genome from a continual barrage of potentially detrimental insults. Inherited mutations in genes that control this process result in an inability to properly repair DNA damage, ultimately leading to developmental defects and also cancer predisposition. Here, we describe a patient with a previously undescribed syndrome, which we have termed RIDDLE syndrome (radiosensitivity, immunodeficiency, dysmorphic features and learning difficulties), whose cells lack an ability to recruit 53BP1 to sites of DNA double-strand breaks. As a consequence, cells derived from this patient exhibit a hypersensitivity to ionizing radiation, cell cycle checkpoint abnormalities, and impaired end-joining in the recombined switch regions. Sequencing of TP53BP1 and other genes known to regulate ionizing radiation-induced 53BP1 foci formation in this patient failed to detect any mutations. Therefore, these data indicate the existence of a DNA double-strand break-repair protein that functions upstream of 53BP1 and contributes to the normal development of the human immune system.

Atypical presentation of ataxia-oculomotor apraxia type 1.

A subgroup of autosomal recessive cerebellar ataxias (ARCAs) associated with oculomotor apraxia (OMA) and other variable features has been reported. Ataxia-oculomotor apraxia types 1 and 2 (AOA1 and AOA2) belong to this subgroup and have been described in adults with early onset cerebellar ataxia. AOA1 is associated with oculomotor apraxia, severe sensorimotor neuropathy, choreiform movements, cognitive impairment, and cerebellar atrophy at an early age. We describe a male child with AOA1 who is homozygous for the G837A (W279X) mutation in the APTX gene. He presented at the age of 3 years 6 months with some atypical features including absence of OMA, chorea, and cerebellar atrophy. These manifestations, in addition to peripheral neuropathy, appeared at 8 years of age. We highlight the importance of considering the diagnosis of AOA1 in children with early-onset cerebellar ataxia, once other well-known disorders such as Friedreich's ataxia and ataxia-telangiectasia have been excluded.

The interaction of the hnRNP family member E1B-AP5 with p53.

Adenovirus early region 1B-associated protein 5, E1B-AP5, a member of the heterogeneous nuclear ribonucleoprotein (hnRNP) family, was originally isolated on the basis of its ability to bind to the adenovirus 5 early region1B55K protein. Here, it has been demonstrated that E1B-AP5 interacts with mutant and wild-type p53 from human cells in pull-down assays using GST-E1B-AP5. This interaction has been confirmed by co-immunoprecipitation studies and pull-down experiments with in vitro translated E1B-AP5 and GST-p53. The binding site for E1B-AP5 has been mapped to the C-terminal region of p53. In reciprocal experiments, it has been shown that several regions of E1B-AP5 bound to p53 although it is probable that a major site of interaction is located between amino acids 395 and 732 of E1B-AP5. In reporter assays, E1B-AP5 inhibited p53 transcriptional activity although not as efficiently as the Ad5E1B55K protein. Transfection of E1B-AP5 into human tumour cells affected the cellular response to UV radiation, such that, although p53 expression was induced, little change in the level of p53-inducible genes could be observed.

The ataxia-oculomotor apraxia 1 gene product has a role distinct from ATM and interacts with the DNA strand break repair proteins XRCC1 and XRCC4.

Ataxia-oculomotor apraxia 1 (AOA1) is an autosomal recessive neurodegenerative disease that is reminiscent of ataxia-telangiectasia (A-T). AOA1 is caused by mutations in the gene encoding aprataxin, a protein whose physiological function is currently unknown. We report here that, in contrast to A-T, AOA1 cell lines exhibit neither radioresistant DNA synthesis nor a reduced ability to phosphorylate downstream targets of ATM following DNA damage, suggesting that AOA1 lacks the cell cycle checkpoint defects that are characteristic of A-T. In addition, AOA1 primary fibroblasts exhibit only mild sensitivity to ionising radiation, hydrogen peroxide, and methyl methanesulphonate (MMS). Strikingly, however, aprataxin physically interacts in vitro and in vivo with the DNA strand break repair proteins XRCC1 and XRCC4. Aprataxin possesses a divergent forkhead associated (FHA) domain that closely resembles the FHA domain present in polynucleotide kinase, and appears to mediate the interactions with CK2-phosphorylated XRCC1 and XRCC4 through this domain. Aprataxin is therefore physically associated with both the DNA single-strand and double-strand break repair machinery, raising the possibility that AOA1 is a novel DNA damage response-defective disease.

Adult-onset ataxia telangiectasia due to ATM 5762ins137 mutation homozygosity.

Ataxia telangiectasia (A-T) is an autosomal recessive neurodegenerative disorder that arises because of mutations in the ATM gene. The 5762ins137 A-->G point mutation activates a cryptic splice donor site resulting in a 137 bp intronic insert being aberrantly spliced into the ATM transcript. However, normal ATM transcript also is produced from this affected allele, albeit at significantly reduced levels. An exceptionally mild A-T phenotype occurs as a result of homozygosity for the 5762ins137 mutation because of relative preservation of ATM protein expression/kinase activity.

Microarray analysis reveals that TP53- and ATM-mutant B-CLLs share a defect in activating proapoptotic responses after DNA damage but are distinguished by major differences in activating prosurvival responses.

The ATM/p53-dependent DNA damage response pathway plays an important role in the progression of lymphoid tumors. Inactivation of the ATM or TP53 gene is frequent in B-cell lymphocytic leukemia (B-CLL) and leads to aggressive disease. Although the ATM and p53 pathways overlap, they are not congruent, and it is unclear how the mechanism of tumor progression differs between ATM- and p53-deficient tumors. Using microarray analysis of ATM-mutant, TP53-mutant, and ATM/TP53 wild-type B-CLLs, we show that after exposure to DNA damage transcriptional responses are entirely dependent on ATM function. The p53 proapoptotic responses comprise only a part of ATM-regulated transcription; additionally, ATM regulates prosurvival responses independently of p53. Consequently, the greater severity of the TP53-mutant B-CLLs compared with ATM-mutant B-CLLs is consistent with the additive effect of defective apoptotic and elevated survival responses after DNA damage in these tumors. We also show that transcription expression profiles of ATM-deficient, TP53-deficient, and wild-type B-CLLs are indistinguishable before irradiation. Therefore, damage-induced transcriptional fingerprinting can be used to stratify tumors according to their biologic differences and simultaneously identify potential targets for treating refractory tumors.

Ataxia telangiectasia mutated-deficient B-cell chronic lymphocytic leukemia occurs in pregerminal center cells and results in defective damage response and unrepaired chromosome damage.

B-cell chronic lymphocytic leukemia (B-CLL) is a heterogeneous disease involving more than one molecular mechanism that leads to the transformation of CD5(+) B cells at either the pregerminal or postgerminal center stage of differentiation. It was previously demonstrated that ataxia telangiectasia mutated (ATM) gene mutations can occur in B-CLL and cause a defect in the p53 pathway. Here the role of ATM mutations in the pathogenesis of B-CLL is addressed. Of 50 B-CLL tumors with fully analyzed ATM and TP53, 16 had ATM mutations. Six of 50 B-CLLs showed mutations in TP53 and the remaining 28 tumors had wild-type ATM or TP53. No tumor had both ATM and TP53 mutations. Remarkably, all 16 ATM mutant B-CLLs showed the absence of somatic variable region heavy chain hypermutation indicating a pregerminal center cell origin and a common pathogenesis for these tumors. Furthermore, in 5 of the 16 B-CLLs, ATM mutation preceded the transformation stage of differentiation. At the cellular level, ATM mutant tumors exhibited a deficient ATM-dependent p53 response to gamma irradiation, failure to up-regulate TRAIL-R2, a downstream target that links irradiation-induced p53 response with apoptosis, and an inability to repair induced chromosome breaks. Mantle cell lymphoma (MCL) is also of pregerminal center origin and ATM mutations are frequent in this malignancy. It is concluded that ATM is likely to play an important role at the pregerminal center stage and a model is proposed where loss of ATM function during B-cell ontogeny drives B-CLL tumorigenesis in pregerminal B cells by a dual defect in p53 damage response and repair of chromosome breaks.