5079

PAX5

BSAP;paired box 5;PAX5;paired box 5

B cell specific activator protein|B-cell lineage specific activator|B-cell-specific transcription factor|paired box homeotic gene 5|paired box protein Pax-5|transcription factor PAX 5

9p13

protein-coding

High-resolution single nucleotide polymorphism genomic microarray (SNP-chip) is a useful tool to define gene dosage levels over the whole genome, allowing precisedetection of deletions and duplications/amplifications of chromosomes in cancer cells. We found that this new technology can also identify breakpoints of chromosomes involved in unbalanced translocations, leading to identification of fusion genes. Using this technique, we found that the PAX5 gene was rearranged to a variety of partner genes including ETV6, FOXP1, AUTS2, and C20orf112 in pediatric acute lymphoblastic leukemia (ALL). The 3' end of the PAX5 gene was replaced by the partner gene. The PAX5 fusion products bound to PAX5 recognitionsequences as strongly as wild-type PAX5 and suppressed its transcriptional activity in a dominant-negative fashion. In human B cell leukemia cells, bindingof wild-type PAX5 to a regulatory region of BLK, one of the direct downstream target genes of PAX5, was diminished by expression of the PAX5-fusion protein, leading to repression of BLK. expression of PAX5-fusion genes in murine bone marrow cells blocked development of mature B cells. PAX5-fusion proteins may contribute to leukemogenesis by blocking differentiation of hematopoietic cells into mature B cells. SNP-chip is a powerful tool to identify fusion genes in human cancers.

The Pax gene family encodes transcription factors with similar structures but distinctive roles in development and with limited expression in adult tissues. Reexpression of PAX proteins is frequently observed in human cancers, reflectingrecapitulation of embryologic or developmental function. Defining expression of PAX family members is important in the immunohistochemical differential diagnosis of cancer, understanding oncogenesis, and defining targets for therapy. Immunostaining for PAX5 has become a commonly used technique in differential diagnosis of B-lineage hematologic malignancies. In seeking to define the range and degree of expression of PAX5 in nonhematologic pediatric cancers by immunohistochemical analysis with the anti-PAX5 monoclonal antibody routinely used in research and diagnosis, we observed strong immunostaining in a number ofmalignant tissues, including Wilms tumor. The pattern of expression of PAX5 in Wilms tumor was found to be identical to that of PAX2, raising the possibility of antibody cross-reactivity. This was subsequently confirmed by Western blotting and immunostaining of transfected cells and quantitative reverse transcriptase-polymerase chain reaction. Since the same PAX5 monoclonal antibodyhas been used consistently in the literature, these findings indicate a need forreappraisal of published PAX5 immunostaining results.

BACKGROUND: Acute lymphoblastic leukemia (ALL) is a heterogeneous form of hematological cancer consisting of various subtypes. We are interested to study the genetic aberration in precursor B-cell ALL with specific t(12;21) translocation in childhood ALL patients. A high resolution 244K array-based Comparative Genomic Hybridization (array-CGH) was used to study eleven ETV6/RUNX1-positive childhood acute lymphoblastic leukemia (ALL) patients. RESULT: 155 chromosomal aberrations (119 losses, 36 gains) were reported in the array findings, corresponding to 76.8% deletions and 23.2% amplifications. The ETV6 gene deletion occurred in 4 of the patients, corresponding to 45% of the sample. The most common alterations above 1 Mb were deletion 6q (13%), 12p (12%)and 9p (8%), and duplication 4q (6%) and Xq (4%). Other genes important in ALL were also identified in this study including RUNX1, CDKN2A, FHIT, and PAX5. The array-CGH technique was able to detect microdeletion as small as 400 bp. CONCLUSION: The results demonstrate the usefulness of high resolution array-CGH as a complementary tool in the investigation of ALL.

Medulloblatoma is a pediatric brain tumor originating in the human cerebellum. Acollection of 23 medulloblastomas was analyzed for expression of the developmental control genes of the PAX and EN gene families by RNase protection and in situ hybridization. Of ALL nine PAX genes investigated, only PAX5 and PAX6 were consistently expressed in most medulloblastomas (70 and 78% of ALL cases, respectively), as were the genes EN1 (57%) and EN2 (78%). EN1, EN2, and PAX6 genes were also expressed in normal cerebellar tissue, and their expression in medulloblastoma is consistent with the hypothesis that this tumor originates in the external granular layer of the developing cerebellum. PAX5 transcripts were,however, not detected in the neonatal cerebellum, indicating that this gene is deregulated in medulloblastoma. In the desmoplastic variant of medulloblastoma, PAX5 expression was restricted to the reticulin-producing proliferating tumor areas containing undifferentiated cells; PAX5 was not expressed in the reticulin-free nonproliferating islands undergoing neuronal differentiation. These data suggest that deregulated expression of PAX5 correlates positively with cell proliferation and inversely with neuronal differentiation in desmoplastic medulloblastoma.