17q21.31 duplication causes prominent tau-related dementia with increased MAPT expression

To assess the role of rare copy number variations in Alzheimer's disease (AD), we conducted a case-control study using whole-exome sequencing data from 522 early-onset cases and 584 controls. The most recurrent rearrangement was a 17q21.31 microduplication, overlapping the CRHR1, MAPT, STH and KANSL1 genes that was found in four cases, including one de novo rearrangement, and was absent in controls. The increased MAPT gene dosage led to a 1.6-1.9-fold expression of the MAPT messenger RNA. Clinical signs, neuroimaging and cerebrospinal fluid biomarker profiles were consistent with an AD diagnosis in MAPT duplication carriers. However, amyloid positon emission tomography (PET) imaging, performed in three patients, was negative. Analysis of an additional case with neuropathological examination confirmed that the MAPT duplication causes a complex tauopathy, including prominent neurofibrillary tangle pathology in the medial temporal lobe without amyloid-β deposits. 17q21.31 duplication is the genetic basis of a novel entity marked by prominent tauopathy, leading to early-onset dementia with an AD clinical phenotype. This entity could account for a proportion of probable AD cases with negative amyloid PET imaging recently identified in large clinical series.

SORL1 rare variants: a major risk factor for familial early-onset Alzheimer's disease

The SORL1 protein plays a protective role against the secretion of the amyloid β peptide, a key event in the pathogeny of Alzheimer's disease. We assessed the impact of SORL1 rare variants in early-onset Alzheimer's disease (EOAD) in a case-control setting. We conducted a whole exome analysis among 484 French EOAD patients and 498 ethnically matched controls. After collapsing rare variants (minor allele frequency ≤1%), we detected an enrichment of disruptive and predicted damaging missense SORL1 variants in cases (odds radio (OR)=5.03, 95% confidence interval (CI)=(2.02-14.99), P=7.49.10(-5)). This enrichment was even stronger when restricting the analysis to the 205 cases with a positive family history (OR=8.86, 95% CI=(3.35-27.31), P=3.82.10(-7)). We conclude that predicted damaging rare SORL1 variants are a strong risk factor for EOAD and that the association signal is mainly driven by cases with positive family history.

De novo deleterious genetic variations target a biological network centered on Aβ peptide in early-onset Alzheimer disease

We hypothesized that de novo variants (DNV) might participate in the genetic determinism of sporadic early-onset Alzheimer disease (EOAD, onset before 65 years). We investigated 14 sporadic EOAD trios first by array-comparative genomic hybridization. Two patients carried a de novo copy number variation (CNV). We then performed whole-exome sequencing in the 12 remaining trios and identified 12 non-synonymous DNVs in six patients. The two de novo CNVs (an amyloid precursor protein (APP) duplication and a BACE2 intronic deletion) and 3/12 non-synonymous DNVs (in PSEN1, VPS35 and MARK4) targeted genes from a biological network centered on the Amyloid beta (Aβ) peptide. We showed that this a priori-defined genetic network was significantly enriched in amino acid-altering DNV, compared with the rest of the exome. The causality of the APP de novo duplication (which is the first reported one) was obvious. In addition, we provided evidence of the functional impact of the following three non-synonymous DNVs targeting this network: the novel PSEN1 variant resulted in exon 9 skipping in patient's RNA, leading to a pathogenic missense at exons 8-10 junction; the VPS35 missense variant led to partial loss of retromer function, which may impact neuronal APP trafficking and Aβ secretion; and the MARK4 multiple nucleotide variant resulted into increased Tau phosphorylation, which may trigger enhanced Aβ-induced toxicity. Despite the difficulty to recruit Alzheimer disease (AD) trios owing to age structures of the pedigrees and the genetic heterogeneity of the disease, this strategy allowed us to highlight the role of de novo pathogenic events, the putative involvement of new genes in AD genetics and the key role of Aβ network alteration in AD.

Lack of referral for genetic counseling and testing in BRCA1/2 and Lynch syndromes: a nationwide study based on 240,134 consultations and 134,652 genetic tests

Based on nationwide data from the French national cancer institute (INCa), we analyzed the evolution of cancer genetics consultations and testing over time, and the uptake of targeted tests in relatives of families with BRCA1/2 or MMR genes mutation. Genetic testing and consultations for familial high-risk individuals are exclusively funded and monitored by the INCa in France. All nationwide cancer genetics centers reported annually standardized parameters of activity from 2003 to 2011. The analysis included a total of 240,134 consultations and 134,652 genetic tests enabling to identify 32,494 mutation carriers. Referral for hereditary breast and ovarian cancer (HBOC) or colorectal cancer predisposition syndromes represented 59 % (141,639) and 23.2 % (55,698) consultations, respectively. From 2003 to 2011, we found a dramatic and steady increase of tests performed for BRCA1/2 (from 2,095 to 7,393 tests/year, P < 0.0001) but not for MMR genes (from 1,144 to 1,635/year, P = NS). The overall percentage of deleterious mutations identified in the probands tested was 13.8 and 20.9 % in HBOC and Lynch syndromes, respectively. Pooled analysis for BRCA1/2 and Lynch syndrome tests showed an inverse relationship between the percentage of mutation detected and the absolute number of tests performed over the time (overall Cochran-Armitage test for trend: P < 0.001). In families with BRCA1/2 or MMR identified mutations, there was an average number of 2.94 and 3.28 relatives performing targeted tests, respectively. This nationwide study shows a lack of referral and genetic testing in Lynch as compared to HBOC syndromes. Only a third of relatives of a proband with a predisposing mutation performed a targeted test. Enhanced information about benefit of genetic testing should be given to clinicians and patients for Lynch syndrome and relatives of a proband carrying an identified predisposing mutation.

High frequency of potentially pathogenic SORL1 mutations in autosomal dominant early-onset Alzheimer disease

Performing exome sequencing in 14 autosomal dominant early-onset Alzheimer disease (ADEOAD) index cases without mutation on known genes (amyloid precursor protein (APP), presenilin1 (PSEN1) and presenilin2 (PSEN2)), we found that in five patients, the SORL1 gene harbored unknown nonsense (n=1) or missense (n=4) mutations. These mutations were not retrieved in 1500 controls of same ethnic origin. In a replication sample, including 15 ADEOAD cases, 2 unknown non-synonymous mutations (1 missense, 1 nonsense) were retrieved, thus yielding to a total of 7/29 unknown mutations in the combined sample. Using in silico predictions, we conclude that these seven private mutations are likely to have a pathogenic effect. SORL1 encodes the Sortilin-related receptor LR11/SorLA, a protein involved in the control of amyloid beta peptide production. Our results suggest that besides the involvement of the APP and PSEN genes, further genetic heterogeneity, involving another gene of the same pathway is present in ADEOAD.

The first mutations in the MYH gene reported in Moroccan colon cancer patients

BACKGROUND

Biallelic germline mutations in the MYH gene cause MYH-associated polyposis (MAP) disease, an autosomal recessive form of inherited colorectal cancer. People with MAP tend to develop attenuated multiple adenomatous colon polyps during their lifetime and will have an increased risk of colorectal cancer. Contrary to familial adenomatous polyposis, the number of adenomas is often lower in MAP (from 5 to 100), and even some patients have recently been reported with no identified adenomas. There have been many investigations into MAP that have been conducted in many different countries. Currently there is limited data on MAP in Morocco, and it is reasonable to think, that the prevalence of this form of genetic predisposition is as high as other autosomal recessive genetic diseases found in countries with high rates of consanguinity. The aim of this study is to examine the frequency of MYH mutations in colorectal cancer and/or attenuated polyposis in Moroccan patients.

PATIENTS AND METHODS

The study population consisted of 62 patients; 52 with colorectal cancer, three of them had attenuated polyposis (2 to 99 adenomatous polyps). 10 other patients were referred to our department for polyposis without colorectal cancer. We carried out DNA analysis in 62 patients to screen for the three recurrent mutations c.494A>G (p.Tyr165Cys), c.1145G>A (p.Gly382Asp) and c.1185_1186dup, p.Glu396GlyfsX43, whereas 40 subjects were screened for germline MYH mutations in the whole coding sequence of the MYH gene by direct DNA sequencing. All these 40 patients, except two, had colorectal cancer without polyposis.

RESULTS

Three patients with colorectal cancer and attenuated polyposis carried biallelic mutations in the MUTYH gene one with the c.494 A>G mutation, one with the c.1105delC mutation, one with the c.1145G>A mutation. One patient with 25 adenomas without colorectal cancer carried the c.1145G>A mutation at a homozygote state and one patient with 3 polyps was heterozygote for the mutation c.1145G>A. No biallelic mutations of MYH gene were detected in colorectal cancer patients and in patients with small number (<5) of polyps without colorectal cancer.

CONCLUSION

We report the first biallelic MYH mutations in four Moroccan patients with clinical criteria of MAP; three of them had colorectal cancer with attenuated polyposis. No MYH mutations were found in colorectal patients without polyposis. Despite the relatively small sample size of the current study, our findings suggest that the MAP is not a frequent cause of colon cancer in Morocco as we had expected, and the molecular analysis of MYH gene should be restricted to patients displaying the classical phenotype of MAP.

P2-13-06: Feasibility of Fast-Track Germline Genetic Analyses for Localized Breast Cancer Patients and Its Impact on Local Management

Background

Patients with BRCA1-BRCA2 germline mutation who developed a first breast cancer (BC) have a high risk to develop a second BC, especially if aged less than 40 at first diagnosis. Thus, a positive BRCA test may influence the therapeutic management and orients towards non conservative surgery to reduce new BC occurrence. We set up a fast-track process to get BRCA1/2 and sometimes p53 mutation results within the shortest possible interval, in recently diagnosed BC patients whose probability to be mutation carriers was high (&gt; 20%).

Methods: Between 05/02/2009 and 03/01/2011, 61 patients diagnosed with localized BC requesting neo-adjuvant or adjuvant chemotherapy and whose personal or familial history fulfilled BRCA1/2 or P53 genetic testing criteria, were proposed genetic counselling. After a first multisciplinary team meeting (MT), genetic testing was proposed and performed according to usual genetic counselling rules. Psychological support was offered at every step of the process. Patients were informed that their test would be analysed as part of a fast-track process and were aware of the potential consequences of a positive test. Results were communicated and discussed during a second MT, when initial therapeutic plan was eventually changed.

Results: All women who were referred to the genetic clinic for fasttrack testing accepted the analysis. The mean age at diagnosis was 38 (range 26–55). The whole process (from 1st genetic consultation till disclosure consultation) was completed in a median interval of 99 days (range 19–245). All pts received appointment within 5 days, when results were available. 50% pts postponed for more than 15 days their disclosure appointment (range 0–113 days). One patient (pt) declined result disclosure. 18 pts (30%) were found to be carrier of a deleterious mutation (13 BRCA1, 4 BRCA2, 1 p53). Variants of unknown significance (UV) in BRCA1 or BRCA2 genes were identified in another 4 patients. In 18 cases, surgery type initially proposed was changed because of the presence of the germline mutation: all pts were proposed ipsilateral mastectomy eventually associated to immediate reconstruction as often possible, as well as controlateral mastectomy. Final surgery turned to bilateral mastectomy in 3 patients with deleterious BRCA mutation and in one suspect UV 14 patients with mutation accepted the change, but four BRCA2 and one BRCA1 mutation carriers refused mastectomy or bilateral mastectomy. Radiotherapy was cancelled in a p53 mutation carrier.

Conclusion: This data show that genetic testing might be considered as a part of therapeutic decision. Moreover, 22% (4/18) opted for a simultaneous controlateral prophylactic mastectomy. On the other hand, some women did not change their mind with mutation knowledge (4/18, 22%). Psychological impact of this procedure is currently retrospectively assessed by a self-questionnaire, fulfilled by mail. Analyses will be available for the meeting.

Molecular determinants of anti-EGFR sensitivity and resistance in metastatic colorectal cancer

Since 2004, the clinical impact of monoclonal antibodies (mAbs) targeting the epidermal growth factor receptor (EGFR) on patients with metastatic colorectal cancer (MCRC) has been clearly established. The combination of these biological agents with conventional chemotherapy has led to a significant improvement in response rate, progression-free survival and overall survival in first-line as well as in second- or third-line treatment of MCRC. However, the high variability of response and outcome in MCRC patients treated with these anti-EGFR mAbs has highlighted the need of identifying clinical and/or molecular predictive markers to ensure appropriate use of targeted therapies. The presence of somatic KRAS mutations has been clearly identified as a predictive marker of resistance to anti-EGFR in MCRC, and the use of anti-EGFR mAbs is now restricted to patients with no detectable KRAS mutation. Several studies have indicated that amplification of EGFR, overexpression of the EGFR ligands and inactivation of the anti-oncogene TP53 are associated with sensitivity to anti-EGFR mAbs, whereas mutations of BRAF and PIK3CA and loss of PTEN expression are associated with resistance. Besides these somatic variations, germline polymorphisms such as those affecting genes involved in the EGFR pathway or within the immunoglobulin receptors may also modulate response to anti-EGFR mAbs. Until now, all these markers are not completely validated and only KRAS genotyping is mandatory in routine practice for use of the anti-EGFR mAbs in MCRC.

Frequent mutation in North African patients with MUTYH-associated polyposis

MUTYH-associated polyposis (MAP) has been characterized as an autosomal recessive disease predisposing to a variable number of colorectal adenomas with a high risk of cancer. Numerous studies have indicated that two missense mutations (Y179C and G396D) account for about 80% of MUTYH allelic variants in Europeans. Ethnic and geographic differences in the mutation spectrum have been observed. The aim of this study was to report mutations in patients from North Africa, determine the incidence of the c.1227_1228dup mutation in our cohort of MUTYH patients and to evaluate the existence of a founder effect. Within a group of 36 families with MAP, 11 were shown to have a homozygous c.1227_1228dup mutation. These families came from Algeria (n = 5), Tunisia (n = 4), Morocco (n = 1) and Portugal (n = 1). Probands belonging to families of North African origin showed a significantly higher frequency of c.1227_1228dup (78.6% vs 4.5%, p < 0.0001). Haplotype analyses were performed using 10 microsatellite markers surrounding the MUTYH gene spanning a region of 4.4 cM. We identified a common haplotype of at least 1.3 cM in all families suggesting a founder effect for this mutation.

[Intra- and interfamilial phenotype variation in Birt-Hogg-Dubé syndrome: Consequences for therapy]

BACKGROUND

Birt-Hogg-Dubé syndrome (BHDS) is an autosomal-dominantly inherited genodermatosis that predisposes to the development of benign hair follicle tumours, lung cysts, kidney tumours, and possibly colonic cancers, due to mutations in the FLCN gene. We report cases involving a new mutation in three unrelated families.

MATERIALS AND METHODS

Blood samples of three probands were submitted for a molecular diagnosis of BHDS. Following DNA extraction, FLCN gene sequencing was performed. The identified mutations were confirmed on a second sample. A cancer genetics consultation was organized and specific tests (dermatological examination, CT scan of chest and abdomen and colonoscopy) were proposed for each BHDS patient.

RESULTS

FLCN gene-sequencing analysis revealed an identical complex harmful mutation in all three families. The first proband showed fibrofolliculomas (FF), a history of pneumothorax and colonic adenoma. The mutation was found in a brother and two sisters, who were asymptomatic, and in a niece with FF. The second proband showed FF. The mutation was found in her mother, who had FF. The third proband presented diffuse emphysema and very rare FF.

DISCUSSION

This case report shows extremely wide intra- and interfamilial phenotype variation within individuals having a similar FLCN gene mutation. In large cohorts of BHDS patients, no genotype-phenotype correlation has been shown. This case emphasises the vital importance of presymptomatic diagnosis for each member of a BHDS family by means of a cancer genetics consultation, followed by a CT scan of the chest and abdomen, colonoscopy and annual kidney imaging.

Influence of loco-regional radiation therapy on subsequent cancer risk among BC pts with p53 germline mutations

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Background: The management of breast cancer (BC) patients (pts) with germline p53 mutations, a rare genetic condition, is not the object of specific recommendations. Preclinical data and preliminary clinical observations suggest a theoretically major radio-sensitivity and high risk of secondary radio-induced malignancies. It remains discussed whether the knowledge of a germline p53 mutation may influence treatment (trt) choices and have prognostic importance. We reviewed our cohort of BC diagnosed as first tumor in pts with germline p53 mutations within the past 11 years, with an attempt to describe secondary malignancies occurring after trt of primary tumor. Patients and Methods: 9 pts have been diagnosed and treated in our institution within the past 11 years for BC as first tumor in the context of a documented germline p53 mutation. Their tumor characteristics, treatment and follow-up data were extracted from prospectively-registered medical records. Results: Median age at diagnosis of primary BC was 32 (22–48). 7/9 pts had a family history compatible with Li Fraumeni syndrome, while 2 had no family history. No pt had previous knowledge of her p53 mutation. Primary tumors were ductal carcinoma in situ (4), infiltrating ductal carcinoma (4), phyllode tumor (1). 3 pts had conservative surgery and 6/9 underwent mastectomy (M). 6 received loco-regional radiation therapy (RT). None had prophylactic contra lateral mastectomy. Loco-regional (LRR) and contra-lateral relapses are listed in the Table , as well as incidence of second primaries within or outside radiation field. Second primaries were sarcoma in 3/4 cases and 1 was papillary thyroid carcinoma. Conclusions: Because of high risk of second breast primary and probably very high risk of radio-induced breast cancer, BC pts with germline p53 mutations should be advised bilateral prophylactic mastectomy and avoidance of radiation therapy. In this context, the knowledge of the mutation might be of great importance. [Table: see text]

No significant financial relationships to disclose.

KRAS mutation is highly predictive of cetuximab resistance in metastatic colorectal cancer

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Background: In metastatic colorectal cancer (MCRC), no molecular predictive markers to cetuximab response have been yet established. The aim was to evaluate whether KRAS gene mutations, EGFR immunochemistery (IHC) and EGFR gene copy number correlate with response to cetuximab. Methods: 59 patients with MCRC treated by cetuximab between July 2004 and December 2005 were retrospectively included. Clinical data were collected and tumour response was evaluated according to RECIST criteria. EGFR IHC was performed using the Dako kit. The EGFR gene copy number was determined by FISH (Fluorescence in-Situ Hybridization). Detection of KRAS gene mutations on exon 2 was performed by sequencing of extracted paraffin-embedded DNA and then by 2 methods, SNaPshot and PCR-LCR, specifically developed to detect small fractions of mutated tumor cells. Response to cetuximab was studied according to clinical data, IHC, FISH and KRAS mutation analysis using the Fischer exact test. Predictive factors of response were determined by logistic regression. Skin reactions were collected but not considered for this analysis as regards the lack of accurate grading in a retrospective study. Times to progression (TTP) were calculated using the Kaplan-Meier method and compared with log-rank test. Results: 12 patients (20.3%) responded to cetuximab (2 patients with complete response and 10 patients with partial response), 19 (32.2%) had stable disease and 28 (47.5%) were in disease progression. A KRAS mutation was detected in 22/59 tumours and, in 6 cases, was missed by sequencing analysis but detected using the SNaPshot and PCR-LCR assays. No KRAS mutation was found in responders patients. KRAS mutation was associated with disease progression (p = 0.0005) and TTP was significantly decreased in mutated KRAS patients (3 vs 5.5 months, p = 0.015). There was no correlation between EGFR IHC and cetuximab response. No EGFR gene copy number increase was detected in responders patients. Predictive factors of cetuximab resistance were KRAS mutation (p=0.003; OR:0.10; 95IC:0.22–0.40) and age<60 (p=0.024; OR:0.13; 95IC:0.02–0.77). Conclusions: KRAS mutation is highly predictive of cetuximab resistance in MCRC. Our study also highlights the need of sensitive methods to ensure an efficient mutation detection.

No significant financial relationships to disclose.

Comparison of a quantitative PCR method with FISH for the assessment of the four aneuploidies commonly evaluated in CLL patients

Four chromosomal defects associated with outcome are commonly evaluated by fluorescent in situ hybridization (FISH) in chronic lymphocytic leukemia (CLL), namely deletions of the 13q13-q14, 11q22 and 17p13 regions and trisomy 12. In this study, we compared a quantitative PCR method--quantitative multiplex PCR of short fluorescent fragment (QMPSF)--with FISH for the detection of these acquired aneuploidies in a series of 110 patients with Binet stage A CLL. Genes located in the deleted or gained regions were selected as target genes and amplified using a method based on the simultaneous amplification of short fluorescent genomic fragments under quantitative conditions. A chromosomal imbalance involving one or several of the four loci was detected by either method in 72 patients (65%). A chromosome 13 deletion was present in 61 patients (54%), a 11q22 deletion in nine (8%), a trisomy 12 in nine and a 17p deletion in one. FISH and QMPSF results were identical for 103 out of 110 patients and discrepancies could be explained in most cases. This study demonstrates that a quantitative multiplex PCR represents a cost-effective method that could replace FISH in CLL patients. However, although QMPSF is perfectly adapted to the detection of primary defects, care should be taken when searching for clonal evolutions present in a small proportion of tumor cells.

Clinical relevance of KRAS mutation detection in metastatic colorectal cancer treated by Cetuximab plus chemotherapy

The predictive value of KRAS mutation in metastatic colorectal cancer (MCRC) patients treated with cetuximab plus chemotherapy has recently been suggested. In our study, 59 patients with a chemotherapy-refractory MCRC treated with cetuximab plus chemotherapy were included and clinical response was evaluated according to response evaluation criteria in solid tumours (RECIST). Tumours were screened for KRAS mutations using first direct sequencing, then two sensitive methods based on SNaPshot and PCR-ligase chain reaction (LCR) assays. Clinical response was evaluated according to gene mutations using the Fisher exact test. Times to progression (TTP) were calculated using the Kaplan–Meier method and compared with log-rank test. A KRAS mutation was detected in 22 out of 59 tumours and, in six cases, was missed by sequencing analysis but detected using the SNaPshot and PCR-LCR assays. Remarkably, no KRAS mutation was found in the 12 patients with clinical response. KRAS mutation was associated with disease progression (P=0.0005) and TTP was significantly decreased in mutated KRAS patients (3 vs 5.5 months, P=0.015). Our study confirms that KRAS mutation is highly predictive of a non-response to cetuximab plus chemotherapy in MCRC and highlights the need to use sensitive molecular methods, such as SNaPshot or PCR-LCR assays, to ensure an efficient mutation detection.

Variations in the APP gene promoter region and risk of Alzheimer disease

We genotyped five polymorphisms, including two polymorphisms with known effects on transcriptional activity, in a large cohort of 427 Alzheimer disease (AD) cases and 472 control subjects. An association between rs463946 (-3102 G/C) and AD was found and was confirmed in a replication sample of a similar size. By contrast, analysis of three recently described rare mutations influencing APP transcription did not confirm their association with AD risk.

Molecular profiling of bladder tumors based on the detection of FGFR3 and TP53 mutations

PURPOSE

On a routine basis we performed systematic molecular screening for FGFR3 and TP53 mutations in 121 bladder tumors. We then specifically analyzed the predictive value of the recurrence of FGFR3 and TP53 genotypes in superficial lesions.

MATERIALS AND METHODS

The FGFR3 gene was analyzed by direct sequencing of exons 7, 10 and 15, whereas TP53 status was determined using the p53 functional assay in yeast.

RESULTS

We identified a missense FGFR3 mutation in 66% of pTa, 26% of pT1 and 12% of pT2 tumors. Of activating FGFR3 mutations 54% and 85% were found in low G1 and intermediate G2 grade tumors, respectively, but in only 20% of high grade G3 tumors. We detected inactivating TP53 mutations in 10% of pTa, 42% of pT1 and 58% of pT2 tumors. Moreover, TP53 mutations were found only in 23% of grade G1 and 3% of grade G2 tumors but in 44% of high grade G3 tumors. When the 2 genotypes were combined, we observed that 58% of pTa tumors had the (mutant FGFR3, WT TP53) genotype, whereas 58% of invasive lesions harbored the inverse genotype (WT FGFR3, mutant TP53). The (mutant FGFR3, WT TP53) genotype and the (WT FGFR3, mutant TP53) genotype were detected in 23% and 38% of pT1G3 tumors, respectively. In the subgroup of 92 patients with superficial pTa-T1 bladder tumors we did not find that the TP53 or FGFR3 genotype alone or combined had a predictive value for tumor recurrence.

CONCLUSIONS

Our data again represent solid proof for the pivotal role of FGFR3 and TP53 mutations in superficial and invasive bladder tumors, respectively. However, other molecular markers should be identified for borderline pT1G3 bladder tumors, which are probably at the crossroads of these 2 distinct molecular pathways.

Golli-MBP copy number analysis by FISH, QMPSF and MAPH in 195 patients with hypomyelinating leukodystrophies

The inherited disorders of CNS myelin formation represent a heterogeneous group of leukodystrophies. The proteolipoprotein (PLP1) gene has been implicated in two X-linked forms, Pelizaeus-Merzbacher disease (PMD) and spastic paraplegia type 2, and the gap junction protein alpha12 (GJA12) gene in a recessive form of PMD. The myelin basic protein (MBP) gene, which encodes the second most abundant CNS myelin protein after PLP1, presents rearrangements in hypomyelinating murine mutants and is always included in the minimal region deleted in 18q- patients with an abnormal hypomyelination pattern on cerebral MRI. In this study, we looked at the genomic copy number at the Golli-MBP locus in 195 patients with cerebral MRI suggesting a myelin defect, who do not have PLP1 mutation. Although preliminary results obtained by FISH suggested the duplication of Golli-MBP in 3 out of 10 patients, no abnormal gene quantification was found using Quantitative Multiplex PCR of Short Fluorescent fragments (QMPSF), Multiplex Amplifiable Probe Hybridization (MAPH), or another FISH protocol using directly-labelled probes. Pitfalls and interest in these different techniques to detect duplication events are emphasised. Finally, the study of this large cohort of patients suggests that Golli-MBP deletion or duplication is rarely involved in inherited defects of myelin formation.

Detection of genomic imbalances by array based comparative genomic hybridisation in fetuses with multiple malformations

BACKGROUND

Malformations are a major cause of morbidity and mortality in full term infants and genomic imbalances are a significant component of their aetiology. However, the causes of defects in many patients with multiple congenital malformations remain unexplained despite thorough clinical examination and laboratory investigations.

METHODS

We used a commercially available array based comparative genomic hybridisation method (array CGH), able to screen all subtelomeric regions, main microdeletion syndromes, and 201 other regions covering the genome, to detect submicroscopic chromosomal imbalances in 49 fetuses with three or more significant anomalies and normal karyotype.

RESULTS

Array CGH identified eight genomic rearrangements (16.3%), all confirmed by quantitative multiplex PCR of short fluorescent fragments. Subtelomeric and interstitial deletions, submicroscopic duplications, and a complex genomic imbalance were identified. In four de novo cases (15qtel deletion, 16q23.1-q23.3 deletion, 22q11.2 deletion, and mosaicism for a rearranged chromosome 18), the genomic imbalance identified clearly underlay the pathological phenotype. In one case, the relationship between the genotype and phenotype was unclear, since a subtelomeric 6q deletion was detected in a mother and her two fetuses bearing multiple malformations. In three cases, a subtelomeric 10q duplication, probably a genomic polymorphism, was identified.

CONCLUSIONS

The detection of 5/49 causative chromosomal imbalances (or 4/49 if the 6qtel deletion is not considered as causative) suggests wide genome screening when standard chromosome analysis is normal and confirms that array CGH will have a major impact on pre and postnatal diagnosis as well as providing information for more accurate genetic counselling.

Impact of the MDM2 SNP309 and p53 Arg72Pro polymorphism on age of tumour onset in Li-Fraumeni syndrome

Li-Fraumeni syndrome, resulting from p53 (TP53) germline mutations, represents one of the most devastating genetic predispositions to cancer. Recently, the MDM2 SNP309 (T-->G variation) was shown to be associated with accelerated tumour formation in p53 mutation carriers. The impact of the common p53 codon 72 polymorphism on cancer risk remains controversial. We therefore investigated the effect of these two polymorphisms in 61 French carriers of the p53 germline mutation. The mean age of tumour onset in MDMD2 SNP309 G allele carriers (19.6 years) was significantly different from that observed in patients homozygous for the T allele (29.9 years, p<0.05). For the p53 codon 72 polymorphism, the mean age of tumour onset in Arg allele carriers (21.8 years) was also different from that of Pro/Pro patients (34.4 years, p<0.05). We observed a cumulative effect of both polymorphisms because the mean ages of tumour onset in carriers of the MDM2G and p53Arg alleles (16.9 years) and those with the MDM2T/T and p53Pro/Pro genotypes (43 years) were clearly different (p<0.02). Therefore, our results confirm the impact of the MDM2 SNP309 G allele on the age of tumour onset in germline p53 mutation carriers, and suggest that this effect may be amplified by the p53 72Arg allele. Polymorphisms affecting p53 degradation therefore represent one of the rare examples of modifier genetic factors identified to date in mendelian predispositions to cancer.

Molecular diagnosis of autosomal dominant early onset Alzheimer's disease: an update

BACKGROUND

Autosomal dominant early onset Alzheimer's disease (ADEOAD) is genetically heterogeneous. Mutations of the amyloid precursor protein (APP), presenilin 1 (PSEN1), and presenilin 2 (PSEN2) genes have been identified.

OBJECTIVE

To further clarify the respective contribution of these genes to ADEOAD.

METHODS

31 novel families were investigated. They were ascertained using stringent criteria (the occurrence of probable or definite cases of Alzheimer's disease with onset before 60 years of age in three generations). All cases fulfilled the NINCDS-ADRDA criteria for probable or definite Alzheimer's disease. The entire coding regions of PSEN1 and PSEN2 genes and exons 16 and 17 of APP gene were sequenced from genomic DNA RESULTS: PSEN1 mutations, including eight previously unreported mutations, were detected in 24 of the 31 families, and APP mutations were found in five families. In this sample, the mean ages of disease onset in PSEN1 and APP mutation carriers were 41.7 and 51.2 years, respectively.

CONCLUSIONS

Combining these data with previously published data, yielding 65 ADEOAD families, 66% of the cases were attributable to PSEN1 mutations and 16% to APP mutations, while 18% remained unexplained.

BRAF screening as a low-cost effective strategy for simplifying HNPCC genetic testing

BACKGROUND

According to the international criteria for hereditary non-polyposis colorectal cancer (HNPCC) diagnostics, cancer patients with a family history or early onset of colorectal tumours showing high microsatellite instability (MSI-H) should receive genetic counselling and be offered testing for germline mutations in DNA repair genes, mainly MLH1 and MSH2. Recently, an oncogenic V600E hotspot mutation within BRAF, a kinase encoding gene from the RAS/RAF/MAPK pathway, has been found to be associated with sporadic MSI-H colon cancer, but its association with HNPCC remains to be further clarified.

METHODS

BRAF-V600E mutations were analysed by automatic sequencing in colorectal cancers from 206 sporadic cases with MSI-H and 111 HNPCC cases with known germline mutations in MLH1 and MSH2. In addition, 45 HNPCC cases showing abnormal immunostaining for MSH2 were also analysed.

RESULTS

The BRAF-V600E hotspot mutation was found in 40% (82/206) of the sporadic MSI-H tumours analysed but in none of the 111 tested HNPCC tumours or in the 45 cases showing abnormal MSH2 immunostaining.

CONCLUSIONS

Detection of the V600E mutation in a colorectal MSI-H tumour argues against the presence of a germline mutation in either the MLH1 or MSH2 gene. Therefore, screening of these mismatch repair (MMR) genes can be avoided in cases positive for V600E if no other significant evidence, such as fulfilment of the strict Amsterdam criteria, suggests MMR associated HNPCC. In this context, mutation analysis of the BRAF hotspot is a reliable, fast, and low cost strategy which simplifies genetic testing for HNPCC.

Cleft lip/palate and CDH1/E-cadherin mutations in families with hereditary diffuse gastric cancer

We report the association of CDH1/E-cadherin mutations with cleft lip, with or without cleft palate (CLP), in two families with hereditary diffuse gastric cancer (HDGC). In each family, the CDH1 mutation was a splicing mutation generating aberrant transcripts with an in-frame deletion, removing the extracellular cadherin repeat domains involved in cell-cell adhesion. Such transcripts might encode mutant proteins with trans-dominant negative effects. We found that CDH1 is highly expressed at 4 and 5 weeks in the frontonasal prominence, and at 6 weeks in the lateral and medial nasal prominences of human embryos, and is therefore expressed during the critical stages of lip and palate development. These findings suggest that alteration of the E-cadherin pathway can contribute to human clefting.

The promoter -194 C polymorphism of the nicotinic alpha 7 receptor gene has a protective effect against the P50 sensory gating deficit

As suggested by several studies, abnormal sensory gating measured by the P50 paradigm could be an endophenotype predisposing to schizophrenia. In a previous work, we have shown a significant association between the presence of at least one -2 bp deletion located within exon 6 of the CHRNA7-like gene and the P50 abnormality in the general population. A recent study involved polymorphisms located in the core promoter region of the CHRNA7 gene as risk factors for the P50 inhibitory deficit. Screening for promoter variants in a large population of schizophrenic patients (n=111) and control subjects (85), for whom auditory-evoked potentials had been recorded did not allow us to replicate these results. By contrast, we showed a significant association between the -194 C allele and a T/C ratio <0.45, thus demonstrating a protective effect of this variant for the sensory gating deficit. Such conflicting results can be reconciled if we consider that the -194 C polymorphism has no causative effect, but is in linkage disequilibrium with other causal variations for the P50 sensory gating deficit, and that different alleles are in disequilibrium in different populations.

LMO2-associated clonal T cell proliferation in two patients after gene therapy for SCID-X1

We have previously shown correction of X-linked severe combined immunodeficiency [SCID-X1, also known as gamma chain (gamma(c)) deficiency] in 9 out of 10 patients by retrovirus-mediated gamma(c) gene transfer into autologous CD34 bone marrow cells. However, almost 3 years after gene therapy, uncontrolled exponential clonal proliferation of mature T cells (with gammadelta+ or alphabeta+ T cell receptors) has occurred in the two youngest patients. Both patients' clones showed retrovirus vector integration in proximity to the LMO2 proto-oncogene promoter, leading to aberrant transcription and expression of LMO2. Thus, retrovirus vector insertion can trigger deregulated premalignant cell proliferation with unexpected frequency, most likely driven by retrovirus enhancer activity on the LMO2 gene promoter.

The -2 bp deletion in exon 6 of the 'alpha 7-like' nicotinic receptor subunit gene is a risk factor for the P50 sensory gating deficit

Abnormality in the P50 auditory-evoked potential gating is an endophenotype associated with schizophrenia. Biochemical and genetic studies have suggested that the alpha 7 nicotinic acetylcholine receptor (nAChR) is involved in this sensory gating deficit. Two related alpha 7 genes (CHRNA7 and CHRNA7-like gene) resulting from a partial duplication (from exon 5 to exon 10) are present in the human genome. Two types of genetic variation, a large deletion and a -2 base-pair deletion in exon 6 resulting in a truncation of the open reading frame, affect specifically the CHRNA7-like gene. We developed a simple multiplex PCR assay on genomic DNA, allowing the quantification of the number of exons 6 and the distinction of all possible exon 6 genotypes. Genotyping of 70 schizophrenic patients and 77 controls showed that carrying at least one -2 bp deletion of exon 6 did not constitute a risk factor for schizophrenia. In contrast, the distribution of genotypes differed significantly between subjects with normal and abnormal P50 ratios, with an over-representation of genotypes carrying at least one -2 bp deletion of exon 6 among subjects exhibiting an abnormal P50 ratio. We thus conclude that the -2 bp deletion within the CHRNA7-like gene is a risk factor for P50 sensory gating deficit. Interestingly, most of the effect came from the non schizophrenic group, which may suggest that in schizophrenic patients other risk factors account for the large proportion of subjects exhibiting an abnormal P50 ratio.