The ATPase activity of MLH1 is required to orchestrate DNA double-strand breaks and end processing during class switch recombination

MLH1 ATPase activity is essential for class switch recombination but not for somatic hypermutation.

Antibody diversification through somatic hypermutation (SHM) and class switch recombination (CSR) are similarly initiated in B cells with the generation of U:G mismatches by activation-induced cytidine deaminase but differ in their subsequent mutagenic consequences. Although SHM relies on the generation of nondeleterious point mutations, CSR depends on the production of DNA double-strand breaks (DSBs) and their adequate recombination through nonhomologous end joining (NHEJ). MLH1, an ATPase member of the mismatch repair (MMR) machinery, is emerging as a likely regulator of whether a U:G mismatch progresses toward mutation or DSB formation. We conducted experiments on cancer modeled ATPase-deficient MLH1G67R knockin mice to determine the function that the ATPase domain of MLH1 mediates in SHM and CSR. Mlh1^GR/GR mice displayed a significant decrease in CSR, mainly attributed to a reduction in the generation of DSBs and diminished accumulation of 53BP1 at the immunoglobulin switch regions. However, SHM was normal in these mice, which distinguishes MLH1 from upstream members of the MMR pathway and suggests a very specific role of its ATPase-dependent functions during CSR. In addition, we show that the residual switching events still taking place in Mlh1^GR/GR mice display unique features, suggesting a role for the ATPase activity of MLH1 beyond the activation of the endonuclease functions of its MMR partner PMS2. A preference for switch junctions with longer microhomologies in Mlh1^GR/GR mice suggests that through its ATPase activity, MLH1 also has an impact in DNA end processing, favoring canonical NHEJ downstream of the DSB. Collectively, our study shows that the ATPase domain of MLH1 is important to transmit the CSR signaling cascade both upstream and downstream of the generation of DSBs.

FGFR3 mutations indicate better survival in invasive upper urinary tract and bladder tumours

BACKGROUND

Promoter hypermethylation and microsatellite instability are frequent in tumours of the upper urinary tract (UTT) and infrequent in bladder tumours. FGFR3 mutations are common findings in bladder tumours and are associated with a good prognosis.

OBJECTIVE

To investigate the occurrence of FGFR3 mutations in UTT and determine the prognostic effect of these genetic changes.

DESIGN, SETTING, AND PARTICIPANTS

Tissue from the initial tumour was obtained from 280 patients (117 bladder tumours and 163 UTT). Patients were selected from pathologic archives to represent the disease spectrum of UCC throughout the urinary tract. Following UCC excision, patients underwent surveillance for a median of 56 mo (range 1-216 mo) or until death.

MEASUREMENTS

FGFR3 mutation analysis was successfully performed on 252 of the 280 primary tumours using the SNaPshot method. Two-tailed statistical analyses were done using the chi(2), Fisher exact tests, and log rank tests. Cox proportional hazard ratios were estimated to obtain risks of recurrence, progression, and death, and to find independent prognostic factors in a multivariate model.

RESULTS AND LIMITATIONS

FGFR3 mutations occurred with the same frequency in bladder and upper tract tumours. Mutations were associated with low-stage tumours and a milder disease course in bladder, ureter, and renal pelvis tumours. Strikingly, our data suggest that these mutations indicate a better survival in patients with invasive tumours from the bladder and upper urinary tract.

CONCLUSIONS

FGFR3 mutation status might be used to select patients with invasive UCC who have a lower risk of death.

FGFR3 mutations in seborrheic keratoses are already present in flat lesions and associated with age and localization

Somatic activating fibroblast growth factor 3 (FGFR3) mutations in human skin can cause seborrheic keratoses, one of the most frequent skin tumors in man. However, details of the involved mechanisms remain elusive. We analyzed 65 acanthotic seborrheic keratoses with varying vertical diameters for FGFR3 mutations using a SNaPshot multiplex assay. Immunohistochemistry was performed for Ki-67, bcl-2 and FGFR3 protein in all seborrheic keratoses and 19 normal skin samples. FGFR3 mutations were detected in 37 of 65 seborrheic keratoses (57%). These mutations were found both in flat (initial) and thick seborrheic keratoses. FGFR3 mutations were significantly associated with increased age and localization on the head and neck (P<0.01). Ki-67 expression was significantly higher in seborrheic keratoses than in normal epidermis independent of the FGFR3 status (P<0.001). Furthermore, FGFR3 mutations were associated with an increased expression of bcl-2 and FGFR3 protein (P<0.05). Our results indicate that FGFR3 mutations can occur early in the pathogenesis of at least a subset of seborrheic keratoses. Increased age appears to be a risk factor for these mutations. The preferential occurrence of FGFR3 mutations in seborrheic keratoses of the head and neck suggests a causative role for cumulative lifetime ultraviolet light exposure.

Low frequency of molecular changes and tumor recurrence in inverted papillomas of the urinary tract

AIM

Inverted papilloma (IP) of the urinary tract can be difficult to distinguish from noninvasive urothelial carcinoma with prominent inverted growth pattern (invNIUC). Ancillary markers may help to resolve such cases and clarify the reported malignant potential of some IPs.

METHODS

Eighty-nine urothelial lesions initially diagnosed as IP were reviewed by 4 experienced urologic pathologists and studied immunohistochemically (Ki67, p53, CK20, MSH2, MLH1, and MSH6). Mutations of the FGFR3 gene, deletions (loss of heterozygosity) of 9p, 9q, and 17p, microsatellite instability, and elevated microsatellite instability at selected tetranucleotides were also analyzed.

RESULTS

Considerable interobserver variability in histopathologic diagnoses was noticed. Only 62 (69.7%) initial diagnoses were confirmed by the review pathologists whereas 23 tumors (25.8%) were redefined as invNIUC. Molecular analyses revealed infrequent alterations in IPs, including microsatellite instability (1.8%), elevated microsatellite instability at selected tetranucleotides (13.2%), FGFR3 mutations (9.8%), 9p deletions (3.9%), 9q deletions (13.2%), 17p deletions (5.1%), nuclear p53 accumulation (18.9%), and aberrant immunostaining for MSH2 (5.8%), MLH1 (11.8%), and MSH6 (3.8%). IP and invNIUC differed in FGFR3 mutations and Ki-67 labeling index (P<0.001 each), and 9q loss of heterozygosity (P=0.03). There were fewer recurrences in IP (5.4%) compared with invNIUC (40.9%; P<0.0001).

CONCLUSIONS

IP is a benign lesion that lacks specific genetic alterations found in exophytic noninvasive papillary urothelial tumors. These lesions could be reactive in nature, perhaps secondary to chronic inflammation or a neoplastic process that lack specific genetic alterations. Nevertheless given the clinical and molecular data of this study a conservative clinical approach is appropriate.

FGFR3 mutations and a normal CK20 staining pattern define low-grade noninvasive urothelial bladder tumours

OBJECTIVES

Molecular markers superior to conventional clinicopathologic parameters are needed to predict disease courses in bladder cancer patients. In this study, we investigated four markers (Ki-67, TP53, CK20, FGFR3) in primary urothelial bladder tumours and compared them with traditional pathologic features.

METHODS

Tissue microarrays were used to analyse CK20, TP53, and Ki-67 expression immunohistochemically in 255 unselected patients. FGFR3 mutations were detected by SNaPshot analysis.

RESULTS

Abnormal CK20 expression was strongly associated with higher tumour grades and stages (p < 0.001); however, 65% of pTa tumours revealed an abnormal CK20 pattern. In the group of pTaG1 tumours, 59% presented with an abnormal CK20 pattern, whereas 82% carried the FGFR3 mutation. In the group of bladder tumours with normal CK20 pattern, the FGFR3 gene was mutated in 89%, whereas a mutated FGFR3 gene was found in only 37% of cases with abnormal CK20 expression (p < 0.001). All markers proved to be strong predictors of disease-specific survival in univariate studies. However, in multivariate analyses they were not independent from classical pathologic parameters. None of the molecular markers was significantly associated with tumour recurrence.

CONCLUSIONS

Dysregulation of CK20 expression is an early event in the carcinogenesis of papillary noninvasive bladder cancer, but occurs later than FGFR3 mutations. The group of low-grade noninvasive papillary tumours is defined by the presence of an FGFR3 mutation and a normal CK20 expression pattern.

Mosaicism of activating FGFR3 mutations in human skin causes epidermal nevi

Epidermal nevi are common congenital skin lesions with an incidence of 1 in 1,000 people; however, their genetic basis remains elusive. Germline mutations of the FGF receptor 3 (FGFR3) cause autosomal dominant skeletal disorders such as achondroplasia and thanatophoric dysplasia, which can be associated with acanthosis nigricans of the skin. Acanthosis nigricans and common epidermal nevi of the nonorganoid, nonepidermolytic type share some clinical and histological features. We used a SNaPshot multiplex assay to screen 39 epidermal nevi of this type of 33 patients for 11 activating FGFR3 point mutations. In addition, exon 19 of FGFR3 was directly sequenced. We identified activating FGFR3 mutations, almost exclusively at codon 248 (R248C), in 11 of 33 (33%) patients with nonorganoid, nonepidermolytic epidermal nevi. In 4 of these cases, samples from adjacent histologically normal skin could be analyzed, and FGFR3 mutations were found to be absent. Our results suggest that a large proportion of epidermal nevi are caused by a mosaicism of activating FGFR3 mutations in the human epidermis, secondary to a postzygotic mutation in early embryonic development. The R248C mutation appears to be a hot spot for FGFR3 mutations in epidermal nevi.

Chromosome 9 deletions are more frequent than FGFR3 mutations in flat urothelial hyperplasias of the bladder

Flat urothelial hyperplasias (FUHs) in patients with papillary bladder tumours frequently show deletions of chromosome 9, suggesting that FUH could be the first neoplastic step in the development of papillary bladder cancer. FGFR3 mutations are frequent in non-invasive papillary tumours with low risk of progression. Our aim was to investigate the frequency of FGFR3 mutations and deletions of chromosomes 9p/q and 8p/q in FUH. Thirty FUH and 9 simultaneous or consecutive tumours were detected by 5-ALA-based photodynamic cystoscopy. DNA was isolated from frozen sections and whole genome amplification was done by I-PEP-PCR, followed by LOH analysis on chromosomes 8p/q and 9p/q. FGFR3 mutations were detected by SNaPshot analysis. LOH analysis on FUH revealed deletions at 9p/q (11/30, 37%) and 8p/q (3/30, 10%). FGFR3 mutations were found in 7/30 FUH (23%). Only 2 FUH showed an FGFR3 mutation without deletions of chromosome 9. In contrast, 6 FUH revealed chromosome 9 deletions but wild type FGFR3 (p = 0.03). These results suggest that chromosome 9 deletions are the earliest genetic alterations in bladder cancer. The detection of FGFR3 mutations in FUH further supports the role of this lesion as precursor of papillary bladder cancer.

High frequency of FGFR3 mutations in adenoid seborrheic keratoses

FGFR3 germline mutations cause autosomal dominant skeletal disorders including achondroplasia, thanatophoric dysplasia, severe achondroplasia with developmental delay and acanthosis nigricans, and Crouzon syndrome. Somatic mutations of FGFR3 have been identified in bladder cancer, multiple myeloma, and other neoplasms. FGFR3 mutations have also been detected in 40% of seborrheic keratoses (SKs) of the hyperkeratotic and acanthotic subtype, which are very common benign skin tumors. Using a multiplex SNaPshot assay that covers 11 activating FGFR3 mutations, we investigated a series of 27 SKs of the adenoid subtype. Mutations were detected in 23 of 27 (85%) adenoid SKs. R248C mutations were the most frequent mutation type. In two SKs, the A393E mutation was found, which has not been described in acanthotic and hyperkeratotic SKs so far. Three adenoid SKs displayed two simultaneous FGFR3 mutations. Adenoid SKs seem to be characterized by a higher frequency of FGFR3 mutations than hyperkeratotic and acanthotic SKs. The mechanism for the high rate of somatic FGFR3 mutations in these benign skin tumors remains elusive, but UV light exposure may play a potential role, especially in the R248C mutations.

A simple and fast method for the simultaneous detection of nine fibroblast growth factor receptor 3 mutations in bladder cancer and voided urine

PURPOSE

Mutations in the fibroblast growth factor receptor 3 (FGFR3) occur in 50% of primary bladder tumors. An FGFR3 mutation is associated with good prognosis, illustrated by significantly lower percentage of patients with progression and disease-specific mortality. FGFR3 mutations are especially prevalent in low grade/stage tumors, with pTa tumors harboring mutations in 85% of the cases. These tumors recur in 70% of patients. Efficient FGFR3 mutation detection for prognostic purposes and for detection of recurrences in urine is an important clinical issue. In this paper, we describe a simple assay for the simultaneous detection of nine different FGFR3 mutations.

EXPERIMENTAL DESIGN

The assay consists of one multiplex PCR, followed by extension of primers for each mutation with a labeled dideoxynucleotide. The extended primers are separated by capillary electrophoresis, and the identity of the incorporated nucleotide indicates the presence or absence of a mutation.

RESULTS

The assay was found to be more sensitive than single-strand conformation polymorphism analysis. Mutations could still be detected with an input of only 1 ng of genomic DNA and in a 20-fold excess of wild-type DNA. Moreover, in urine samples from patients with a mutant tumor, the sensitivity of mutation detection was 62%.

CONCLUSIONS

We have developed a fast, easy to use assay for the simultaneous detection of FGFR3 mutations, which can be of assistance in clinical decision-making and as an alternative for the follow-up of patients by invasive cystoscopy for the detection of recurrences in urine.

[Correlation of FGFR3 mutations and chromosomal alterations in bladder cancer]

It has been suggested that mutation of FGFR3 is associated with non-invasive tumors of low malignant potential and low risk of recurrence and progression. The aim of this study was to analyze the distribution of FGFR3 mutations in bladder tumors of different grade and stage and to determine the relation of FGFR3 mutations to chromosomal alterations detected by CGH. Frozen sections of 100 bladder cancer samples served as templates for manual microdissection. DNA was isolated from dissected samples containing at least 80% tumor cells. Mutations in FGFR3 were analyzed by SNaPshot analysis. CGH was carried out according to standard protocols. FGFR3 mutations were detected in 45 out of 92 samples (48.9 %). Concerning T-category, the following mutation frequencies occurred: pTa - 69 %, pT1 - 38 %, pT2/3 - 0 %. The mutation frequency was significantly associated with tumor grade: G1 - 72%, G2 - 56%, G3 - 4%. In pTaG1 tumors, mutations were found in 74 %. A significant lower number of genetic alterations per tumor detected by CGH was associated with FGFR3 mutations (2 vs. 8). This association was also seen in pTaG1 tumors: 2.5 (with mutation) vs. 7.5 (without mutation). Our results confirm that FGFR3 mutations characterize non-invasive low-risk tumors of low malignancy. The low malignant potential of these tumors is underlined by a low number of chromosomal alterations per tumor. Therefore, FGFR3 could represent a prognostic marker of chromosomally stable tumors with low malignant potential.

[Urothelial neoplasms in individuals younger than 20 years show very few genetic alterations and have a favourable clinical outcome]

AIMS

Urothelial neoplasms in patients 19 years or younger are rare, with conflicting data regarding clinical outcome and no molecular data available.

METHODS

Urothelial tumors of 14 patients 4 to 19 years old were identified, reclassified according to the 2004 WHO classification and data on presentation, risk factors and outcome were collected. 14 cases were microdissected and extensive molecular analyses were done, including FGFR3 and TP 53 mutation screening, Comparative Genomic Hybridisation (CGH), Urovysion FISH analysis, PCR for HPV, microsatellite analysis using an extended NIH consensus panel for detection of microsatellite instability (MSI) and 6 LOH markers on chromosome arms 17p, 9p and 9q and immunohistochemistry for TP 53, MIB1, CK20 and the mismatch repair proteins hMSH2, hMLH1 and hMSH6.

RESULTS

Based on the 2004 WHO classification, 1 urothelial papilloma, 7 PUNLMPs, 5 low grade, and 1 high grade papillary urothelial cancers were included. There were no multifocal tumors and only 1 patient had recurrence. All patients were alive with no evidence of disease (4.5 years follow-up). We did not find mutations in FGFR3, deletions of chromosome arms 9p, 9q or 17p, MSI or MRP loss or HPV positivity. Chromosomal alterations in CGH, urothelial dedifferentiation with CK20 over-expression or aneuploidy were rare and only detected in 3 cases. One TP53 mutation was found in the only tumor with overexpression of TP53.

CONCLUSIONS

Urothelial neoplasms in individuals younger than 20 years have predominantly a low grade and favourable clinical outcome. The most frequent genetic alterations found in elderly patients are extremely rare. Urothelial neoplasms in young patients could represent a biologically distinct form of bladder disease with lack of genetic instability in most cases.

Induction of CAMEL/NY-ESO-ORF2-specific CD8+ T cells upon stimulation with dendritic cells infected with a modified Ad5 vector expressing a chimeric Ad5/35 fiber

Delivery of the full-length tumor antigen might be more successful in immunotherapy than single peptides and has the advantage that patients no longer need to be selected for their HLA type. In this study, we tested the in vitro induction of CAMEL/NY-ESO-ORF2-specific T cells by dendritic cells infected with an adenovirus (Ad) type 5 vector containing the fiber shaft and knob of human serotype Ad35 (Ad5F35 vector). Our data show induction of CD8(+) T cells specific for the known HLA-A(*)0201-binding CAMEL/NY-ESO-ORF2(1-11) epitope by DC infected with Ad5F35-CAMEL, but not by DC pulsed with the recombinant CAMEL protein. In one healthy donor, even CD8(+) T cells specific for a new HLA-B7-binding CAMEL/NY-ESO-ORF2(46-54) epitope were raised. In conclusion, the in vitro induction of CAMEL/NY-ESO-ORF2-specific CD8(+) T cells in healthy donors by DC infected with Ad5F35-CAMEL strongly supports further investigation of the Ad5F35 vector as a vehicle for gene transfer into DC for the generation of tumor antigen-specific CD8(+) T cell responses in vivo.