Somatic mosaicism has been observed previously in the lymphocyte population of patients with Fanconi anemia (FA). To identify the cellular origin of the genotypic reversion, we examined each lymphohematopoietic and stromal cell lineage in an FA patient with a 2815-2816ins19 mutation in FANCA and known lymphocyte somatic mosaicism. DNA extracted from individually plucked peripheral blood T cell colonies and marrow colony-forming unit granulocyte-macrophage and burst-forming unit erythroid cells revealed absence of the maternal FANCA exon 29 mutation in 74.0%, 80.3%, and 86.2% of colonies, respectively. These data, together with the absence of the FANCA exon 29 mutation in Epstein-Barr virus-transformed B cells and its presence in fibroblasts, indicate that genotypic reversion, most likely because of back mutation, originated in a lymphohematopoietic stem cell and not solely in a lymphocyte population. Contrary to a predicted increase in marrow cellularity resulting from reversion in a hematopoietic stem cell, pancytopenia was progressive. Additional evaluations revealed a partial deletion of 11q in 3 of 20 bone marrow metaphase cells. By using interphase fluorescence in situ hybridization with an MLL gene probe mapped to band 11q23 to identify colonyforming unit granulocyte-macrophage and burst-forming unit erythroid cells with the 11q deletion, the abnormal clone was exclusive to colonies with the FANCA exon 29 mutation. Thus, we demonstrate the spontaneous genotypic reversion in a lymphohematopoietic stem cell. The subsequent development of a clonal cytogenetic abnormality in nonrevertant cells suggests that ex vivo correction of hematopoietic stem cells by gene transfer may not be sufficient for providing life-long stable hematopoiesis in patients with FA. F anconi anemia (FA) is an autosomal recessive disorder characterized by congenital abnormalities, bone marrow failure, and a predisposition to malignancies, including myelodysplastic syndrome and acute myelogenous leukemia (1). Seven FA complementation groups (FA-A through FA-G) have been reported (2), with FA-A (Online Mendelian Inheritance in Man, OMIM no. 227650) constituting approximately two-thirds of the patients. The FANCA, FANCC, FANCE, FANCF, and FANCG genes have been cloned and mapped to chromosomes 16q24. 3, 9q22.3, 6p21.2-21.3, 11p15, and 9p13, respectively (3-8). However, the specific function of these genes remains unclear.FA cells are uniquely hypersensitive to DNA cross-linking agents such as diepoxybutane (DEB) and mitomycin C (MMC) (9). This cellular phenotype can be demonstrated in cultured T cells, B cells, fibroblasts, and fetal cells cultured from amniotic fluid or chorionic villi (10). Hypersensitivity of phytohemagglutinin (PHA)-stimulated peripheral blood lymphocytes (PBLs) to DEB is used routinely as a diagnostic test for the syndrome (11). At a DEB concentration that has a minimal effect on normal PBLs (0.1 g͞ml), 80-100% of PBLs analyzed will have chromatid breaks and exchanges in the typical FA patient. However, in a small grou...
Fanconi anemia (FA) is a genetically heterogeneous autosomal recessive syndrome associated with chromosomal instability, hypersensitivity to DNA crosslinking agents, and predisposition to malignancy. The gene for FA complementation group A (FAA) recently has been cloned. The cDNA is predicted to encode a polypeptide of 1,455 amino acids, with no homologies to any known protein that might suggest a function for FAA. We have used single-strand conformational polymorphism analysis to screen genomic DNA from a panel of 97 racially and ethnically diverse FA patients from the International Fanconi Anemia Registry for mutations in the FAA gene. A total of 85 variant bands were detected. Forty-five of the variants are probably benign polymorphisms, of which nine are common and can be used for various applications, including mapping studies for other genes in this region of chromosome 16q. Amplification refractory mutation system assays were developed to simplify their detection. Forty variants are likely to be pathogenic mutations. Seventeen of these are microdeletions͞microinsertions associated with short direct repeats or homonucleotide tracts, a type of mutation thought to be generated by a mechanism of slipped-strand mispairing during DNA replication. A screening of 350 FA probands from the International Fanconi Anemia Registry for two of these deletions (1115-1118del and 3788-3790del) revealed that they are carried on about 2% and 5% of the FA alleles, respectively. 3788-3790del appears in a variety of ethnic groups and is found on at least two different haplotypes. We suggest that FAA is hypermutable, and that slipped-strand mispairing, a mutational mechanism recognized as important for the generation of germ-line and somatic mutations in a variety of cancer-related genes, including p53, APC, RB1, WT1, and BRCA1, may be a major mechanism for FAA mutagenesis.Fanconi anemia (FA) is an autosomal recessive disorder characterized by congenital abnormalities, bone marrow failure, and predisposition to acute myelogenous leukemia and other malignancies (1-3). FA cells are hypersensitive to DNA crosslinking agents such as diepoxybutane and mitomycin C (4). Eight complementation groups (FA-A through FA-H) have been described, with FA-A accounting for approximately two-thirds of FA families (5-7). The gene responsible for the defect in FA-C (FAC) was isolated by functional complementation and mapped to chromosome 9q22.3 (5, 8). The FAD gene was mapped to chromosome 3p22-26, but has not yet been isolated (9). Recently, FAA was mapped to 16q24.3 (10, 11), and the cDNA was isolated by two independent approaches: positional and expression cloning (12, 13). Both FAA and FAC encode unique proteins, which do not exhibit any homology to known proteins that might suggest a function.A few mutations in FAA were described in the initial cloning reports (12, 13), including two mutations involving base substitutions that result in utilization of a cryptic splice site leading to insertion of 30 bp and six intragenic deletions (4-87...
We analyzed data from 388 subjects with Fanconi anemia reported to the International Fanconi Anemia Registry (IFAR). Of those, 332 developed hematologic abnormalities at a median age of 7 years (range, birth to 31 years). Actuarial risk of developing hematopoietic abnormalities was 98% (95% confidence interval, 93% to 99%) by 40 years of age. Common hematologic abnormalities were thrombocytopenia and pancytopenia. These were often associated with decreased bone marrow (BM) cellularity (75% of cases studied). Clonal cytogenetic abnormalities developed in 23 of 68 persons with BM failure who had adequate studies. Actuarial risk of clonal cytogenetic abnormalities during BM failure was 67% (47% to 87%) by 30 years of age. Fifty-nine subjects developed myelodysplastic syndrome (MDS) or acute myelogenous leukemia (AML). Actuarial risk of MDS or AML was 52% (37% to 67%) by 40 years of age. Risk was higher in persons with than in those without a prior clonal cytogenetic abnormality (3% [0% to 9%] v 35% [0% to 79%]; P = .006). One hundred twenty persons died of hematologic causes including BM failure, MDS or AML and treatment related complications. Actuarial risk of death from hematologic causes was 81% (67% to 90%) by 40 years of age.
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