Cloning and Characterization of the Zebrafish mll Ortholog.

Introduction: Zebrafish enable studies of early embryogenesis and hematopoiesis like no other animal models. Many zebrafish orthologs of hematopoietic genes have been identified, and zebrafish models of leukemia are emerging but the zebrafish ortholog of MLL, a critical oncogene disrupted by leukemogenic translocations, has not yet been studied. We cloned the complete zmll cDNA and characterized its temporal expression as a framework for further studies of MLL where current knowledge is still incomplete.

Methods: Bioinformatic tools were employed to interrogate the existence and relationship of a zmll ortholog and synteny with human MLL. Degenerate RT-PCR was used to determine whether MLL amino acid sequences in domains highly conserved across species could identify the orthologous zebrafish transcript. Cross-species Southern blot analysis was performed to determine if a predicted zmll gene from restriction map simulations of a projected genomic sequence could be detected with a human cDNA probe for the MLL breakpoint cluster region (bcr). The full-length zmll cDNA was obtained using a combination of 5′ RACE PCR, long-range and conventional PCR. The corresponding protein was analyzed in a phylogram tree. The temporal pattern of zmll RNA expression was examined using quantitative (Q) RT-PCR.

Results: Bioinformatic analysis using the human MLL protein as the reference sequence identified two putative “similar to MLL proteins” and predicted two proximal transcript sequences on zebrafish chromosome 15. Gene prediction tools suggested a single genomic structure matching both protein sequences. A conserved block of synteny containing several linked genes surrounded the predicted zmll. Furthermore, zmll and human MLL were in the same map order in an uninterrupted segment with the gene for ubiquitination factor E4A. Degenerate RT-PCR analysis of wild-type adult zebrafish RNA based on cross-species amino acid sequences from highly conserved PHD and SET domains amplified the predicted transcript. Cross-species Southern blot analysis with the human probe detected the projected zmll genomic fragments corresponding to the MLL bcr in adult zebrafish genomic DNA. RT-PCR analysis of wild-type adult zebrafish RNA determined that the two predicted “similar to MLL proteins” were from a single gene. The full length 12657 bp, 35-exon zmll cDNA cloned from wild-type 24 hpf zebrafish embryo RNA predicted a 4218 amino acid protein with CXXC, PHD, Bromodomain, FYRN, FYRC, and SET domains and taspase cleavage sites with 45.8% sequence identity and 57.3% similarity to human MLL. Phylogram tree analysis suggested evolutionary divergence of mammals from teleosts but nonetheless conservation of critical functional domains. QRT-PCR demonstrated maternally supplied zmll mRNA during the earliest embryonic developmental timepoints, and expression of zygotic zmll mRNA during embryogenesis and in the zebrafish adult.

Conclusions: These results indicate that there is a single zmll gene with highly conserved functional similarity to human MLL. The temporal pattern of expression, including maternal supply of transcript to the embryo, indicates that zmll is important from early embryogenesis through the entire lifespan of the fish. The high evolutionary conservation of critical domains creates the framework to use zebrafish for studying MLL in hematopoiesis and leukemia.