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Paolo Casali

Publications and source records attributed to Paolo Casali.

At least 19 recordsLinked to original sources
PubMed2006

DNA repair in antibody somatic hypermutation.

Somatic hypermutation (SHM) underlies the generation of a diverse repertoire of high-affinity antibodies. It is effected by a two-step process: (i) DNA lesions initiated by activation-induced cytidine deaminase (AID), and (ii) lesion repair by the combined intervention of DNA replication and repair factors that include mismatch repair (MMR) proteins and translesion DNA synthesis (TLS) polymerases. AID and TLS polymerases that are crucial to SHM, namely polymerase (pol) theta, pol zeta and pol eta, are induced in B cells by the stimuli that are required to trigger this process: B-cell receptor crosslinking and CD40 engagement by CD154. These polymerases, together with MMR proteins and other DNA replication and repair factors, could assemble to form a multimolecular complex ("mutasome") at the site of DNA lesions. Molecular interactions in the mutasome would result in a "polymerase switch", that is, the substitution of the high-fidelity replicative pol delta and pol epsilon with the TLS pol theta, pol eta, Rev1, pol zeta and, perhaps, pol iota, which are error-prone and crucially insert mismatches or mutations while repairing DNA lesions. Here, we place these concepts in the context of the existing in vivo and in vitro findings, and discuss an integrated mechanistic model of SHM.

Animals↗
PubMed2006

A role for the MutL mismatch repair Mlh3 protein in immunoglobulin class switch DNA recombination and somatic hypermutation.

Class switch DNA recombination (CSR) and somatic hypermutation (SHM) are central to the maturation of the Ab response. Both processes involve DNA mismatch repair (MMR). MMR proteins are recruited to dU:dG mispairs generated by activation-induced cytidine deaminase-mediated deamination of dC residues, thereby promoting S-S region synapses and introduction of mismatches (mutations). The MutL homolog Mlh3 is the last complement of the mammalian set of MMR proteins. It is highly conserved in evolution and is essential to meiosis and microsatellite stability. We used the recently generated knockout mlh3(-/-) mice to address the role of Mlh3 in CSR and SHM. We found that Mlh3 deficiency alters both CSR and SHM. mlh3(-/-) B cells switched in vitro to IgG and IgA but displayed preferential targeting of the RGYW/WRCY (R = A or G, Y = C or T, W = A or T) motif by Sgamma1 and Sgamma3 breakpoints and introduced more insertions and fewer donor/acceptor microhomologies in Smu-Sgamma1 and Smu-Sgamma3 DNA junctions, as compared with mlh3(+/+) B cells. mlh3(-/-) mice showed only a slight decrease in the frequency of mutations in the intronic DNA downstream of the rearranged J(H)4 gene. However, the residual mutations were altered in spectrum. They comprised a decreased proportion of mutations at dA/dT and showed preferential RGYW/WRCY targeting by mutations at dC/dG. Thus, the MMR Mlh3 protein plays a role in both CSR and SHM.

Animals↗
PubMed2006

Prognostic analysis of clinicopathologic factors in 49 patients with diffuse malignant peritoneal mesothelioma treated with cytoreductive surgery and intraperitoneal hyperthermic perfusion.

BACKGROUND: Diffuse malignant peritoneal mesothelioma (DMPM) is a subset of peritoneal mesothelioma with a poor clinical outcome. We performed a prognostic analysis in a cohort of DMPM patients treated homogeneously by cytoreductive surgery and intraperitoneal hyperthermic perfusion (IPHP). METHODS: Forty-nine DMPM patients who underwent 52 consecutive procedures were enrolled onto the study. Cytoreductive surgery was performed according to the peritonectomy technique, and the IPHP was performed with cisplatin plus doxorubicin or cisplatin plus mitomycin C. We assessed the correlation of the clinicopathologic variables (previous surgical score, age, sex, performance status, previous systemic chemotherapy, carcinomatosis extension, completeness of cytoreduction, IPHP drug schedule, mitotic count [MC], nuclear grade, and biological markers [epidermal growth factor receptor, p16, matrix metalloproteinase 2 and matrix metalloproteinase 9]) with overall and progression-free survival. RESULTS: The mean age was 52 years (range, 22-74 years). The mean follow-up was 20.3 months (range, 1-89 months). Regarding the biological markers, the rates of immunoreactivity of epidermal growth factor receptor, p16, matrix metalloproteinase 2, and matrix metalloproteinase 9 were 94%, 60%, 100%, and 85%, respectively. The strongest factors influencing overall survival were completeness of cytoreduction and MC, whereas those for progression-free survival were performance status and MC. No biological markers were shown to be of prognostic value. CONCLUSIONS: Completeness of cytoreduction, performance status, and MC seem to be the best determinants of outcome. These data warrant confirmation by a further prospective formal trial. No biological markers presented a significant correlation with the outcome. The overexpression of epidermal growth factor receptor, matrix metalloproteinase 2, and matrix metalloproteinase 9 and absent or reduced expression of p16 might be related to the underlining tumor kinetics of DMPM and warrant further investigation with other methods.

Adult↗
PubMed2006

Biased dA/dT somatic hypermutation as regulated by the heavy chain intronic iEmu enhancer and 3'Ealpha enhancers in human lymphoblastoid B cells.

Somatic hypermutation (SHM) in immunoglobulin gene (Ig) variable (V) regions is critical for the maturation of the antibody response. It is dependent on the expression of activation-induced cytidine deaminase (AID) and translesion DNA polymerases in germinal center B cells as well as Ig V transcription, as regulated by the Ig heavy chain (H) intronic enhancer (iEmu) and the 3' enhancer (3'Ealpha) region. We analyzed the role of these cis elements in SHM by stably transfecting Ramos human lymphoblastoid B cells with a rearranged human IgH chain VD (diversity) J (joining) DNA construct containing a V(H) promoter at the 5' end and C(H)1 and C(H)2 exons of Cgamma1 at the 3' end. In this construct, mutations preferentially targeted dA/dT basepairs in the RGYW/WRCY hotspot. Most of the dA/dT mutations and accompanying dC/dG mutations were transitions. Deletion of iEmu resulted in decreased SHM which could be partially restored by insertion of the IgH hs1,2 enhancer. Other two 3'Ealpha enhancers, hs3-hs4, did not significantly increase the mutation frequency, but further strengthened the dA/dT bias. The frequency and spectrum of the mutations were independent of the genomic integration of the transgene or V gene transcription level. Thus, we have established a novel in vitro system to analyze SHM and identify the role of multiple cis-regulatory elements in regulating dA/dT biased SHM. This model system will be useful to further address the role of other cis-regulating elements and recruited trans-acting factors in expressing the modalities of SHM.

B-Lymphocytes↗
PubMed2006

Reduced tetanus antibody titers in overweight children.

Under-nutrition impairs immune responses, but far less is known about the impact of over-nutrition, such as obesity, on the response to vaccines. We measured the effect of childhood overweight status on inflammatory mediators, circulating immunoglobulins and tetanus antibodies in fifteen overweight children (BMI > 85 age-adjusted percentile) and 15 age-matched normal weight controls. Fitness was measured by a progressive ramp type exercise test. Lean body mass (LBM) and fat mass were determined by DXA. Tumor necrosis factor-a (TNF-a), interleukin-6 (IL-6), interleukin-1 beta (IL-1beta) and interleukin-1 receptor antagonist (IL-1ra) were used to assess the inflammatory status; and circulating immunoglobulins (IgM, IgA, IgG and IgG subclasses) and specific IgG titer to tetanus were used to assess humoral immunity. Overweight children had higher LBM and percent fat mass, and lower peak VO2 normalized to body weight. IL-6 was significantly higher in the obese children (2.6 +/- 0.3 vs. 1.3 +/- 0.3 pg/ml, in overweight and normal weight children, respectively; p < 0.05). No significant differences were found in TNF-a, IL-1beta and IL-1ra between the groups. No significant differences were found in immunoglobulin levels (IgM, IgA, IgG and IgG subclasses) between the groups. Anti-tetanus IgG antibodies were significantly lower in the overweight children compared to normal weight controls (2.4 +/- 0.6 vs. 4.2 +/- 0.5 IU/ml, in overweight and normal weight children, respectively; p < 0.05). The reduced specific antibody response to tetanus in obese children and adolescent might be due to mechanical factors such as lower relative vaccination dose, or reduced absorption from the injection site due to increased adipose tissue, or related to reduce immune response due to the chronic low grade inflammation expressed by the higher levels of IL-6.

Adolescent↗
PubMed2005

Diffuse malignant mesothelioma of the peritoneum: a clinicopathological study of 35 patients treated locoregionally at a single institution.

BACKGROUND: In the current study, the authors report the clinicopathologic features of patients with peritoneal diffuse malignant mesothelioma (DMM) who were treated in a uniform fashion at a single institution to assess prognostic factors. METHODS: Thirty-five patients were treated with cytoreductive surgery (CRS) and intraperitoneal hyperthermic perfusion (IPHP). The tumors were classified into epithelial, sarcomatoid, and biphasic types. Immunohistochemistry stains were performed for calretinin, WT-1, pCEA, Ber-EP4, epidermal growth factor receptor (EGFR), p16, matrix metalloprotease-2 (MMP-2), and MMP-9. Statistical correlation was evaluated for age, gender, completeness of cytoreduction (CC), tumor histotype, mitotic count (MC), necrosis, nuclear grade (NG), and biologic markers with regard to overall survival (OS) and progression-free survival (PFS). RESULTS: The patient group was comprised of 15 men and 20 women with a median age of 52 years (range, 24-73 yrs). Twenty-five patients underwent optimal cytoreduction. There were 32 epithelial tumors and 3 biphasic tumors, and 3 patients had an NG of 1, 19 had an NG of 2, and 13 had an NG of 3. The mean MC was 14.1 (range, 0-160 per 50 high-power fields). Necrosis was present in 11 cases. All the tumors were found to be positive for calretinin and WT-1 and were negative for pCEA and Ber-EP4. The NG and MC were found to be significantly associated with OS (P = 0.02 and P = 0.01, respectively) whereas CC was found to be associated with both OS (P = 0.05) and PFS (P = 0.03). No biologic markers were found to be of prognostic significance. CONCLUSIONS: The results of the current study indicate that NG, MC, and CC may be useful prognostic factors in patients treated with CRS and IPHP. The expression of EGFR, MMP-2, and MMP-9 and absent and/or reduced expression of p16 in DMMs confirms the results of previous studies suggesting their role in tumor pathogenesis and kinetics.

Adult↗
PubMed2005

The translesion DNA polymerase theta plays a dominant role in immunoglobulin gene somatic hypermutation.

Immunoglobulin (Ig) somatic hypermutation (SHM) critically underlies the generation of high-affinity antibodies. Mutations can be introduced by error-prone polymerases such as polymerase zeta (Rev3), a mispair extender, and polymerase eta, a mispair inserter with a preference for dA/dT, while repairing DNA lesions initiated by AID-mediated deamination of dC to yield dU:dG mismatches. The partial impairment of SHM observed in the absence of these polymerases led us to hypothesize a main role for another translesion DNA polymerase. Here, we show that deletion in C57BL/6J mice of the translesion polymerase theta, which possesses a dual nucleotide mispair inserter-extender function, results in greater than 60% decrease of mutations in antigen-selected V186.2DJ(H) transcripts and greater than 80% decrease in mutations in the Ig H chain intronic J(H)4-iEmu sequence, together with significant alterations in the spectrum of the residual mutations. Thus, polymerase theta plays a dominant role in SHM, possibly by introducing mismatches while bypassing abasic sites generated by UDG-mediated deglycosylation of AID-effected dU, by extending DNA past such abasic sites and by synthesizing DNA during dU:dG mismatch repair.

Animals↗
PubMed2005

Carney triad: case report and molecular analysis of gastric tumor.

Gastrointestinal stromal tumors (GISTs) are the most common mesenchymal tumors of the GI tract. Most of them are thought to be sporadic, but some arise in the settings of neurofibromatosis type I (NF-1) and the Carney triad. The Carney triad is a syndrome of unknown etiology, occurring predominantly in young females, comprising gastrointestinal stromal tumors, pulmonary chondromas, and extra-adrenal paragangliomas. GISTs of the Carney triad involve predominantly the body and the antrum of the stomach, are generally multifocal, and have a better prognosis than sporadic GISTs. We describe the clinical and pathological features of a case of Carney triad that featured multiple gastric GISTs, mediastinal paraganglioma, and esophageal leiomyoma. Ten years after gastric resection, the patient developed liver and peritoneal metastasis and was treated with Imatinib mesylate for 6 months with no change in the lesions. The molecular analysis of the GIST, the first reported in a gastric tumor from the triad, showed a wild-type KIT and PDGFRA genes.

Adult↗
PubMed2005

DNA lesions and repair in immunoglobulin class switch recombination and somatic hypermutation.

Immunoglobulin (Ig) gene somatic hypermutation (SHM) and class switch DNA recombination (CSR) are critical for the maturation of the antibody response. These processes endow antibodies with increased antigen-binding affinity and acquisition of new biological effector functions, thereby underlying the generation of memory B cells and plasma cells. They are dependent on the generation of specific DNA lesions and the intervention of activation-induced cytidine deaminase as well as newly identified translesion DNA polymerases, which are expressed in germinal center B cells. DNA lesions include mismatches, abasic sites, nicks, single-strand breaks, and double-strand breaks (DSBs). DSBs in the switch (S) region DNA are critical for CSR, but they also occur in V(D)J regions and possibly contribute to the events that lead to SHM. The nature of the DSBs in the Ig locus, their generation, and the repair processes that they trigger and that are responsible for their regulation remain poorly understood. Aberrant regulation of these events can result in chromosomal breaks and translocations, which are significant steps in B-cell neoplastic transformation.

Animals↗
PubMed2005

Etoposide, doxorubicin and cisplatin plus mitotane in the treatment of advanced adrenocortical carcinoma: a large prospective phase II trial.

To investigate the activity of etoposide, doxorubicin, and cisplatin plus mitotane in the management of advanced adrenocortical carcinoma (ACC) patients, 72 patients with measurable disease not amenable to radical surgery were enrolled in a prospective, multicenter phase II trial. EDP schedule (etoposide 100 mg/m(2) on days 5-7, doxorubicin 20 mg/m(2) on days 1 and 8, and cisplatin 40 mg/m(2) on days 1 and 9) was administered intravenously every 4 weeks. Concomitantly, patients were given up to 4 g/day of oral mitotane. Five patients achieved a complete response and 30 a partial response, for an overall response rate of 48.6% (95% CI: 37.1-60.3). Median time to progression in responding patients was 18 months. The EDP regimen was well tolerated, leukopenia being the dose limiting toxicity. One toxic related death due to septic shock, however, was registered. Radical surgical resection of residual disease after chemotherapy was performed in 10 patients. The overall survival of patients attaining a disease free status (clinical complete responders+radically resected) was significantly higher than that of patients with partial response or no response (P<0.002). Androgen secretion was associated with long survival, while glucocorticoid secretion was associated with poor prognosis both in univariate and multivariate analysis. In conclusion, EDP plus mitotane is an active and manageable combination scheme for ACC patients. Surgical resection of residual disease subsequent to chemotherapy leads to a more favourable outcome. The natural history of the disease is significantly influenced by the secretory status of the tumor.

Adolescent↗
PubMed2004

The HoxC4 homeodomain protein mediates activation of the immunoglobulin heavy chain 3' hs1,2 enhancer in human B cells. Relevance to class switch DNA recombination.

The immunoglobulin heavy chain (IgH) 3' regulatory region modulates IgH locus transcription, upon induction by specific trans-acting factors, and plays a significant role in class switch DNA recombination (CSR) and, perhaps, somatic hypermutation (SHM). CSR and SHM are central to the maturation of the antibody response. In contrast to the single 5'-hs3a-hs1,2-hs3b-hs4-3 ' mouse IgH 3 ' regulatory region, the human IgH 3 ' regulatory region exists as a 5'-hs3-hs1,2-hs4-3' cluster duplicated 3 ' of Calpha1 and Calpha2. We show here that the human hs1,2 element is the strongest enhancer of transcription, as directed by a V(H)1 or the ECS-Igamma3 promoter, thereby suggesting a dominant role for hs1,2 over hs3 and hs4 in the overall activity of the 3 ' regulatory region. Within hs1,2, we identified three regions (1, 2, and 3) that are all necessary, but individually not sufficient, for enhancement of transcription. In region 2, a HoxC4 site and a HoxC4/embedded octamer (HoxC4/Oct) site are conserved across human, mouse, rat, and rabbit. These two sites recruit HoxC4 and Oct-1/Oct-2, which act synergistically with the Oca-B coactivator to effect the full hs1,2-enhancing activity. HoxC4, Oct-1/Oct-2, and Oca-B recruitment is negligible in pro-B cells, moderate in pre-B cells, and maximal in germinal center B cells and plasma cells, where HoxC4, Oct-2, and Oca-B expression correlates with hs1,2 activation and ongoing CSR. The hs1,2mediated enhancement of V(H) and C(H) promoter-driven transcription as induced by HoxC4 and Oct-1/Oct-2 suggests an important role of these homeodomain proteins in the overall regulation of the IgH locus expression.

3' Flanking Region↗
PubMed2003

EBV-encoded latent membrane protein 1 cooperates with BAFF/BLyS and APRIL to induce T cell-independent Ig heavy chain class switching.

By substituting the H chain C region of IgM with that of IgG, IgA, or IgE, class switching enables Abs to acquire new effector functions that are crucial for the neutralization of invading pathogens. Class switching occurs through class switch DNA recombination (CSR) and usually requires engagement of CD40 on B cells by CD40 ligand on Ag-activated CD4(+) T cells. CSR must be tightly regulated because abnormal IgG and IgA production favors the onset of autoimmunity, whereas increased switching to IgE leads to atopy. These inflammatory disorders can be triggered or exacerbated by EBV infection. In this study, we show that EBV induces CD40-independent CSR from C( micro ) to multiple downstream C(gamma), C(alpha), and C(epsilon) genes through latent membrane protein 1 (LMP1), a CD40-like viral protein that signals in a ligand-independent fashion. LMP1-induced CSR is associated with transcriptional activation of germline C(gamma), C(alpha), and C(epsilon) genes and triggers the up-regulation of activation-induced cytidine deaminase, a crucial component of the CSR machinery. In addition, LMP1 induces B cells to express B cell-activating factor of the TNF family and a proliferation-inducing ligand, two molecules that mediate B cell survival and T cell-independent Ab production. B cell-activating factor of the TNF family and a proliferation-inducing ligand cooperate with LMP1 to induce Ig class switching because their neutralization by appropriate soluble decoy receptors attenuates CSR in LMP1-expressing B cells. By showing that LMP1 triggers T cell-independent CSR, our findings suggest that EBV could play an important role in the pathogenesis of disorders with aberrant IgG, IgA, and/or IgE production.

Adjuvants, Immunologic↗
PubMed2003

Selective inhibition of class switching to IgG and IgE by recruitment of the HoxC4 and Oct-1 homeodomain proteins and Ku70/Ku86 to newly identified ATTT cis-elements.

Immunoglobulin (Ig) class switching is central to the maturation of the antibody response as IgG, IgA, and IgE are endowed with more diverse biological effector functions than IgM. It is induced upon engagement of CD40 on B lymphocytes by CD40L expressed by activated CD4+ T cells and exposure of B cells to T cell-secreted cytokines including interleukin-4 and transforming growth factor-beta. It begins with germ line IH-CH transcription and unfolds through class switch DNA recombination (CSR). We show here that the HoxC4 and Oct-1 homeodomain proteins together with the Ku70/Ku86 heterodimer bind as a complex to newly identified switch (S) regulatory ATTT elements (SREs) in the Igamma and Iepsilon promoters and downstream regions to dampen basal germ line Igamma-Cgamma and Iepsilon-Cepsilon transcriptions and repress CSR to Cgamma and Cepsilon. This mechanism is inactive in the Calpha1/Calpha2 loci because of the lack of SREs in the Ialpha1/Ialpha2 promoters. Accordingly, in resting human IgM+IgD+ B cells, HoxC4, Oct-1, and Ku70/Ku86 can be readily identified as bound to the Igamma and Iepsilon promoters but not the Ialpha1/Ialpha2 promoters. CD40 signaling dissociates the HoxC4.Oct-1. Ku complex from the Igamma and Iepsilon promoter SREs, thereby relieving the IH-CH transcriptional repression and allowing CSR to unfold. Dissociation of HoxC4.Oct-1. Ku from DNA is hampered by CD153 engagement, a CD40-signaling inhibitor. Thus, these findings outline a HoxC4.Oct-1. Ku-dependent mechanism of selective regulation of class switching to IgG and IgE and further suggest distinct co-evolution and shared CSR activation pathways in the Cgamma and Cepsilon as opposed to the Calpha1/Calpha2 loci.

Antigens, Nuclear↗
PubMed2003

Peritoneal mesothelioma treated by induction chemotherapy, cytoreductive surgery, and intraperitoneal hyperthermic perfusion.

BACKGROUND AND OBJECTIVES: Peritoneal mesothelioma (PM) is a rare disease, with a poor prognosis. We decided to prospectively evaluate the prognostic impact of aggressive surgery followed by intraperitoneal chemotherapy with local hyperthermia. PATIENTS AND METHODS: In this prospective study, 19 patients with PM were treated by cytoreductive surgery (CRS) and intraperitoneal hyperthermic perfusion (IPHP). Mean follow-up was 27 months (range: 1-65). Fifteen (68%) patients had malignant disease, two had well-differentiated papillary mesothelioma, and two had multicystic PM. Thirteen (65%) patients received preoperative chemotherapy. Fifteen cases (75%) underwent optimal cytoreduction (residual disease <2.5 mm). One patient underwent the procedure twice due to locoregional progression. IPHP was performed with closed abdomen technique, using a preheated polysaline perfusate (42.5 degrees C) containing cisplatin + mitomycin C or cisplatin + doxorubicin administered through a heart-lung pump for 60 or 90 min. RESULTS: Three-year overall and progression-free survival was 69 and 66%, respectively. The operative morbidity (grade II/III), mortality, and overall toxicity (grade I-IV) rates were 25, 0, and 30%, respectively. Seventeen (94%) out of 18 patients had resolution of ascites. CONCLUSIONS: This therapeutic strategy proved feasible and was well tolerated. Early results seem promising and consistent with a potentially major impact on survival in selected patients with PM.

Adult↗
PubMed2003

Diversity and somatic hypermutation of the Ig VHDJH, V kappa J kappa, and V lambda J lambda gene segments in lymphoma B cells: relevance to the origin of the neoplastic B cell clone.

Burkitt's lymphoma (BL) is a malignancy of B cells characterized by chromosomal translocations involving the immunoglobulin (Ig) and c-MYC gene loci. To address the putative role of antigen in the clonal expansion of these neoplastic B cells, we analyzed the VHDJH and VLJL gene segments expressed by the established cell lines derived from six endemic BL and six sporadic BL. Eight BL cell lines used genes of the VH3 family, two of the VH4, and two of the VH1. Eight VL chains were kappa (four members of the V kappa3, two of the V kappa1, and two of the V kappa2 subgroups) and four lambda (three members of the V lambda1 and one of the Vl ambda3 subgroup). The VH gene utilization was stochastic (i.e., it reflected the relative representation of the different VH gene family members in the human haploid genome). In contrast, the VL gene utilization was skewed toward the overutilization of the V kappa3 and V lambda1 gene subgroups. When compared with those of the respective germline genes, the sequences of the expressed Ig VDJ genes displayed nucleotide differences that resulted from somatic hypermutation. In three endemic and three sporadic BL cells, nucleotide changes yielding amino acid substitutions segregated within the complementarity determining region, indicating the application of a positive pressure for replacement mutations and suggesting that these neoplastic lymphocytes underwent a process of clonal selection driven by antigen, perhaps emerging from or transitioning through germinal centers.

Antibody Diversity↗
PubMed2003

AID-dependent generation of resected double-strand DNA breaks and recruitment of Rad52/Rad51 in somatic hypermutation.

Somatic hypermutation (SHM) of immunoglobulin (Ig) genes appears to involve the generation of double-strand DNA breaks (DSBs) and their error-prone repair. Here we show that DSBs occur at a high frequency in unrearranged (germline) Ig variable (V) genes, BCL6 and c-MYC. These DSBs are blunt, target the mutational RGYW/RGY hotspot, and would be resolved through nonhomologous end-joining, as indicated by the presence of Ku70/Ku86 on these DNA ends. Upon CD40-induced expression of activation-induced cytidine deaminase (AID), DSBs increase in frequency and are resected to yield 5'- and 3'-protruding ends in hypermutating rearranged V genes, BCL6 and translocated c-MYC. 3'-protruding ends would direct DSB repair through homologous recombination, as indicated by their exclusive presence in S/G2 and recruitment of Rad52/Rad51, leading to SHM, upon mispair by error-prone DNA polymerases modulated by crosslinking of the B cell receptor for antigen.

B-Lymphocytes↗
PubMed2003

Immunoglobulin somatic hypermutation: double-strand DNA breaks, AID and error-prone DNA repair.

Somatic hypermutation (SHM) is critical for antibody affinity maturation and the generation of memory B cells. Somatic mutations consist mainly of single nucleotide changes with rare insertions and deletions. Such changes would be introduced during error-prone repair of lesions involving single-strand DNA breaks (SSBs) or, more likely, double-strand DNA breaks (DSBs), as DSBs occur exclusively in genes that have the potentials to undergo SHM. In the human, such genes include Ig V, BCL6, and c-MYC. In these germline genes, DSBs are blunt. In rearranged Ig V, BCL6, and translocated c-MYC genes, blunt DSBs are processed to yield resected DNA ends. This process is dependent on the expression of activation-induced cytidine deaminase (AID), which is selectively expressed upon CD40-signaling in hypermutating B cells. CD40-induced and AID-dependent free 5'- and 3'-staggered DNA ends critically channel the repair of DSBs through the homologous recombination (HR) repair pathway. During HR, the modulation of critical translesion DNA polymerases, as signaled by cross-linking of the B cell receptor (BCR) for antigen, leads to the insertions of mismatches, i.e., mutations. The nature of DSBs, the possible roles of AID in the modification of DSBs and that of the translesion DNA polymerases zeta and iota in the subsequent repair process that lead to the insertions of mutations are discussed here within the context of an integrated model of SHM.

Animals↗