8. Radiobiology Division

The main project currently underway in the Radiobiology Division is physical mapping of the human genome and positional cloning of disease genes, especially of leukemia-related genes, and tumor suppressor genes. Utilizing the genome-analysis based approach, we have succeeded in disclosing the fusion genes AML1-MTG8 and FUS-ERG in acute myeloid leukemias, with t(8;21) and t(16;21), respectively. Molecular studies of the involvement of these fusion gene products in leukemogenesis are also important undertakings of the Radiobiology Division.

Genome Analysis

A complete NotI restriction map of the entire long arm of human chromosome 11 has been contructed by linking clone mapping. Comparison with other maps disclosed that the marker density in the previous maps is extremely low in the q13.1 to q13.4 region where many disease genes such as MEN1, SCA5, IDDM4, and PGL2 are mapped. This deficit is largely attributable to the lack of representative yeast artificial chromosomes (YACs), since all the YACs from this region were severely disordered. This map which spans >84 Mb provides the most reliable ordering and distance estimation in the entire region from a pericentromeric locus to the terminus of the long arm.
Human chromosome 11q22-q23 is a pathologically important region in which a high level of loss of heterozygosity has been reported for many tumors. This strongly indicates that one or more tumor suppressor genes reside within the deleted region. A contig map that covers most of the deleted regions found in these malignancies was developed to analyze the putative tumor suppressor genes. The map comprises a contig of 66 overlapping YACs and spans a region of 17 Mb from the PGR gene at 11q22.2 to the MLL gene at q23.3.⁽¹⁰⁵⁾
The Down syndrome (DS) region has been defined by analyses of partial trisomy 21. The 2.5 Mb region between D21S17 and ERG is reportedly responsible for the main features of DS. Within this 2.5 Mb region, we focused on a distal 1.6 Mb region from an analysis of Japanese DS patients with partial trisomy 21 and constructed a 1.6 Mb P1 based contig map.⁽¹⁰⁶⁾ We also performed exon-trapping and direct cDNA library screening using this contig and identified five genes in the region, a novel gene TPRD,⁽¹⁰⁷⁾ holocarboxylase synthetase (HCS), G protein-coupled inward rectifier potassium channel 2 (GIRK2), a human homolog of Drosophila minibrain (MNB) and a coding sequence of a novel inward rectifier potassium channel-like gene (IRKK). These genes, except IRKK, are expressed ubiquitously and are relatively large, extending from 100 kb to 300 kb on the genome. This information should facilitate understanding of the detailed genome structure of the DS region and help to elucidate its role in the etiology of DS.

Molecular Mechanism of Tumorigenesis

In the t(8;21) translocation associated with acute myeloid leukemia (AML), the AML1 gene is disrupted and fused to the MTG8 (ETO) gene. The ectopic expression of the AML1-MTG8 fusion protein, in L-G and 32Dc13 murine myeloid progenitor cells induces expression of the receptor for granulocyte colony-stimulating factor (G-CSFR) and stimulates cell proliferation without terminal differentiation to mature granulocytes in response to G-CSF. In contrast, the ectopic expression of AML1 down-regulates the expression of G-CSFR. Highly expressed G-CSFR supports cell proliferation in response to G-CSF. High levels of expression of G-CSFR are also found in leukemic cells from AML patients with t(8;21). These results suggest that the overproduction of G-CSFR induced by AML1-MTG8 fusion protein stimulates G-CSF dependent cell proliferation of myeloid precursor cells and is implicated in leukemogenesis.
The EWS gene is found at the chromosome breakpoints in EwingÕs sarcoma and the FUS/TLS gene at the breakpoints of myxoid liposarcoma and acute myeloid leukemia. These genes encode proteins which carry a highly homologous RNA binding domain. By PCR amplification of human Namalwa cell cDNA using degenerate primers made from the conserved amino acid sequences in the RNA binding domain of EWS and FUS/TLS, we obtained a cDNA fragment, named RBP56, the predicted amino acid sequences of which were similar but not identical to those of EWS and FUS/TLS.⁽¹⁰⁸⁾ The expression of RBP56 mRNA was observed in all of the human fetal and adult tissues examined, as was the expression of EWS and FUS/TLS mRNAs. The RBP56 gene was mapped to chromosome 17q11.2 to 12.

Environmental UV Radiation

Biodosimetry to monitor environmental UV radiation using UV-sensitive Bacillus subtilis spores has been developed, and comparison with Brewer spectrophotometry validated the spectral integration of biological-effectiveness spectra.⁽¹⁰⁹⁾

List of papers from this division

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