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Epidemiology |
1 Cancer Research and Treatment Center/Internal Medicine, University of New Mexico, Albuquerque, New Mexico; 2 Department of Epidemiology and Biostatistics, Memorial Sloan-Kettering Cancer Center; 3 Laboratory for Human Genetics and Hematology, The Rockefeller University, New York, New York; 4 Department of Human Genetics, University of Wurzburg, Wurzburg, Germany; and 5 Department of Pediatric Oncology, Hematology and Immunology, University of Dusseldorf, Dusseldorf, Germany
Requests for reprints: Arleen D. Auerbach, Laboratory of Human Genetics and Hematology, The Rockefeller University, 1230 York Avenue, Box 77, New York, NY 10021-6399. Phone: 212-327-7533; Fax: 212-327-8262; E-mail: auerbac{at}mail.rockefeller.edu.
| Abstract |
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| Introduction |
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10% by age 25 years (1). The syndrome is genetically heterogeneous; there are at least 13 complementation groups and to date 13 genes have been identified. These genes encode components of the FA nuclear "core" complex (FANCA, FANCB, FANCC, FANCE, FANCF, FANCG, FANCL, and FANCM), key components of the FA pathway, FANCD2, FANCI, the familial breast cancer protein BRCA2/FANCD1, its partner and localizer, PALB2/FANCN, and the BRIP1/BACH1/FANCJ helicase. The role of FANC proteins in protecting the genome is clearly important (3). Although it has been proposed that the connections among the complementation groups represent a linear biological pathway (4), they may instead reflect a variety of protein networks affecting DNA stability during both S phase of the cell cycle and DNA damage recognition/repair (5). In addition, there is emerging evidence that some FA complexes function to suppress apoptosis and facilitate survival signaling pathways (6). Although associations of FA heterozygosity with breast and other cancers have been reported (7–15), the question as to whether heterozygotes with mutations in any of the Fanconi genes other than FANCD1/BRCA2 and its interacting partners are at increased risk for developing cancer has been enigmatic. To date, "there has been little convincing evidence that FA heterozygotes are at increased risk for cancer, but larger studies are needed to address the possibility of modest risk effects" (16). Consistent with this suggestion, mutated alleles of FANCJ (12) and FANCN (15) have been identified recently as low-risk breast cancer susceptibility alleles in case-control studies.
There are two motives for assessing cancer risk among heterozygotes: to understand better the genetic basis of cancer and to provide prevention and screening information to families of FA patients. To address these issues, we designed a cohort study to characterize molecularly FA carriers and noncarriers and compare these groups in terms of risk for cancer.
| Materials and Methods |
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Confirmation of Diagnosis
Diagnosis of FA for all probands was first confirmed by chromosomal breakage induced by diepoxybutane (17).
Complementation Studies
Once the diagnosis of FA was confirmed and appropriate cell lines were established, complementation studies with retroviral vectors containing normal FANCA, FANCB, FANCC, FANCD2, FANCE, FANCF, FANCG, and FANCL cDNAs were done as described (18). Immunoblotting for detection of monoubiquitinated FANCD2 was also done as described (19). Probands excluded from these complementation groups and with normal ubiquitination of FA-D2 were tested for group FA-D1 by genomic DNA sequencing of BRCA2 by Myriad Genetic Laboratories (Salt Lake City, UT). Patients in this group who were negative for FA-D1 were screened for groups FA-J and FA-N by direct sequencing for mutation detection and demonstration of abnormal protein by immunoblotting. A small percentage of FA patients could not be assigned to any complementation group, including FA-M as tested by immunoblotting. For these families, carrier status of grandparents was considered as unknown for the analysis.
Mutation Analysis
DNA extraction. Buccal swabs were stored frozen at –20°C until DNA was extracted, using Quick-Extract DNA extraction solution according to the manufacturer's instructions (Epicentre). Briefly, this consisted of several steps of 10 s of vortexing followed by incubations, first at 65°C for 30 min and then twice at 98°C for 8 min each. DNA was then stored at –20°C until analysis. PCR amplification was done using 1 µL in a 30 to 45 µL reaction.
Mutation screening. Probands identified by complementation testing as groups FA-A, FA-C, FA-F, or FA-G were screened for mutations in FANCA, FANCC, FANCF, and FANCG using the Transgenomic Wave dHPLC System. Under denaturing conditions at 50°C, small heterozygous insertions/deletions were detected as an additional peak on the chromatogram. PCR products were then denatured at 95°C for 5 min followed by cooling at 1°C/min to 25°C to allow heteroduplex formation in case of heterozygosity. They were then subjected to denaturing high-performance liquid chromatography analysis. In case of homozygosity, the samples are mixed with known normal controls, denatured as described above, and reanalyzed.
Sequencing. Direct genomic and cDNA sequencing was done for FANCA/B/C/D2/E/F/G/J/N according to standard protocols. PCR products were cleaned using ExoSAP-IT (USB) according to the manufacturer's protocol. Most of the sequencing was done by Genewiz, Inc. with Big Dye Terminator version 3.1 Cycle Sequencing Ready Reaction kits (Applied Biosystems) on a 3730xl DNA analyzer. Sequence data were generated by means of Sequencing Analysis version 5.1 (Applied Biosystems) and analyzed with Sequencher version 4.5 (Gene Codes Corp.).
Large deletions seem to be a frequent mutation subtype in FANCA, accounting for
40% of mutated alleles in FANCA, the most common FA gene (20, 21). Multiplex ligation-dependent probe amplification allows quantitation of multiple sequences (22) and was done according to the MRC-Holland protocol for FANCA and consisted of DNA denaturation, probe hybridization, ligation, and PCR steps. The samples were then analyzed by capillary electrophoresis and analyzed with the Genotyper software.
As mutation assays for IFAR probands had been developed, family members were screened for the specific maternal or paternal mutations as applicable. In some FA-A families, assays based on segregation of informative single nucleotide polymorphisms (SNP) in three generations of the pedigree were used to determine carrier status of grandparents (23).
Statistical Methods
Standardized incidence ratios (SIR) were calculated to compare the cancer incidence observed in our study population with the expected rate of the U.S. population obtained from the Surveillance, Epidemiology, and End Results6 registries, which provide cancer rates for years 1975 and beyond. Rates from the Connecticut Cancer Registry were used for years before 1975. The null hypothesis that the SIR is equal to 1 was tested, and 95% confidence intervals (95% CI) were calculated as described in Breslow and Day (24). Due to the descriptive nature of this work, we did not adjust the P values for multiple comparisons. The end point for SIR calculation was age at onset of the cancer of interest, age at death (without a cancer diagnosis for the cancer of interest), or age at last follow-up.
We report SIRs for cancers having >2% incidence in our study cohort: any cancer, breast cancer among female relatives, and prostate cancer among male relatives. First, SIRs were calculated using all the study subjects, regardless of their carrier status. Next, SIRs were estimated separately for heterozygous carriers of FANCA, FANCC, and FANCG mutations. Finally, we calculated SIRs separately for the grandparents and nongrandparent relatives.
| Results |
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| Discussion |
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In this study, three mutations were found among the eight FANCC carriers who developed breast cancer: p.L554P (n = 1), IVS4 (c.711+4A>T; n = 5), and c.322delG (n = 2). In the recently published Cancer Genetic Markers of Susceptibility,7 three FANCC SNPs have a significantly increased risk, whereas one SNP has a significantly decreased risk for breast cancer among homozygotes (25). The question of how mutations in FANCC might act as susceptibility alleles should be further investigated. IFAR probands who carry the c.711+4A>T or the p.L554P mutation in FANCC have the most severe phenotype of all FA patients with mutations that effect the FA nuclear "core complex" proteins; their survival is poorer (52% at 10 years of age) and they have bone marrow failure sooner than other groups (26). Only probands in groups FA-D1 (BRCA2) and FA-N (PALB2), genes with known increased carrier risk for breast cancer, have a poorer survival, due to the unique predisposition of these patients to embryonal tumors.
The functions of FANCC are not yet totally clear. FANCC does not colocalize with BRCA1, BRCA2, and RAD51 in DNA repair foci to regulate DNA repair (27), and although it is part of the FA nuclear "core complex" necessary for FANCD2 and FANCDI monoubiquitination (28, 29), it seems to have additional functions independent of other FA proteins (30), in particular, regulation of apoptosis (31).
Several limitations to our study are important to note. Pathology was not available for all subjects who reported cancer and there could thus be misclassification present in the cancer outcomes. However, this is negligible for breast and prostate cancers (32, 33). We found one gene with elevated risk out of the many comparisons made; thus, this finding could represent a false-positive result.
Another limitation is that we did not have carrier status for all grandparents because in some cases, the grandparents were dead (and therefore did not provide a DNA sample). Furthermore, families where all the grandparents were deceased at the time of study were less likely to participate in the study. Investigations are continuing to determine the complementation group of the Nx individuals, some of whom have been found to carry mutations in new complementation groups, such as PALB2 /FA-N (15) and FANCI (28, 29). Cancer incidence in the new complementation groups will be investigated in the future after genotyping efforts are completed to identify novel groups.
Finally, there is potential bias due to differences in participation by family history of cancer. Unfortunately, one cannot evaluate this potential bias because information on family history of cancer among nonparticipants is not available.
The 2.4-fold increased SIR was noted among all grandmothers as well as all female relatives heterozygous for FANCC mutations. This point estimate is similar to risk estimates found recently among heterozygotes in a case-control study of 1,212 individuals with breast cancer compared with 2,081 controls without breast cancer in the United Kingdom. Seal et al. (12) found that nine individuals with truncating mutations in the FANCJ gene have a relative risk for breast cancer of 2.0 (95% CI, 1.2–3.2) when compared with only two individuals in the control group with the same mutations. In another study of 923 individuals with familial breast cancer compared with 1084 controls, Rahman et al. (15) reported that carrying a PALB2 mutation (also known as FANCN) increased risk of breast cancer by 2.3-fold (95% CI, 1.3–3.9). They found 10 individuals with mutations from the case group and no individuals with mutations in the control group. Our cohort study found that 6 individuals out of 33 carriers of FANCC mutations had breast cancer.
These data suggest that carrier relatives of complementation group FA-C patients should carefully follow the recommendations for breast cancer screening to detect cancer at the earliest possible stage and may wish to test for FANCC mutations if they know that they are related to a patient with FA-C complementation group. The frequency of FA varies among ethnic groups and is particularly high among those of Ashkenazi Jewish heritage; the c.711+4A>T mutation occurs in approximately 1 out of 89 individuals in this population (34). With a carrier frequency of >1% and simple testing available, the c.711+4A>T mutation is now included in the battery of genetic tests routinely provided to the Ashkenazi Jewish population due to its severe consequences.
The question of whether FANCC mutations that cause FA are more common among individuals with breast cancer than in the general population needs further study. However, the major finding from this molecular study of FA carriers is that there does not seem to be an overall increase in risk for cancer. These results not only are of historical interest, being the first molecular characterization of heterozygotes among a large number of pedigrees of FA, but are also of importance to family members of individuals diagnosed with FA because they now have molecular evidence that they need to worry less about the heightened cancer risk found among probands, with the possible exception of breast cancer, for which screening is available.
| Acknowledgments |
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The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
We thank the many physicians who have registered their FA patients in the IFAR, without whom this research could not have been done, and Drs. John Wagner, David Williams, and Farid Boulad and their colleagues; and all the FA family members for their cooperation.
| Footnotes |
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Received 4/24/07. Revised 7/11/07. Accepted 7/26/07.
| References |
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This article has been cited by other articles:
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