Structure and expression of the human trithorax-like gene 1 involved in acute leukemias.

Abstract

The human trithorax-like gene 1 (Htrxl gene) is disrupted in 11q23 translocations that are associated with acute leukemias. Sequencing ofa partial human cDNA revealed an open reading frame encoding 1012 amino acids with extensive homology to the Drosophila trithorax protein, particularly in the zinc finger-like domains. Htrxl gene appears to be unique in the human genome and has been conserved during evolution. Use of the human cDNA as a probe demonstrates that this gene is interrupted in both infant and adult acute myeloid (AML) and lymphoid (ALL) leukemia patients with 11q23 translocations. The structure of the Htrxl gene around the breakpoints shows that this part of the human gene is interrupted by nine introns. As a result of the rearrangement, zinc finger domains are translocated in both ALL and AML patients. Expression studies reveal that the Htrxl gene differentially expresses three transcripts within the normal lymphocyte cell lineage.The 11q23 region is involved in nonrandom chromosomal translocations with a number of different partner chromosomes (1). These rearrangements are observed in acute leukemias, especially-t(4;11), t(1;11), and t(11;19) translocations (2, 3) in acute lymphoid leukemias (ALL) and t(1;11), t(2;11), t(6;11), t(9;11), t(10;11), t(l1;l17), and t(X;ll) in acute myeloid leukemias (AML) (4-8). The frequency of translocations involving this region is notably higher in infants (estimated to be 75%) (9-11) than in children and adults (=5%) (12,13). It has been shown that this region of the genome contains the human trithorax-like gene 1 (Htrxl gene), which is interrupted in the translocation event in infant leukemic patients and in the cell line RS4;11 (14). To further characterize the Htrxl gene, we isolated a portion of the corresponding cDNA, and we present the sequence of the putative Htrxl protein in the region that is interrupted by the translocations. In Drosophila the trithorax gene (trx) is proposed to be a transcription factor and contains several zinc finger domains (15) that are highly conserved in Htrxl. This fragment of the human cDNA recognizes the translocations by Southern blotting in adult leukemia patients with t(4;11), t(6;11), t(9;11), and t(10;11) translocations as well as patient-derived cell lines carrying t(4;11) (RS4;11) (16) and t(X;11) (Karpas 45) (17). We have studied the expression of Htrxl in normal human tissues and lymphocyte subsets and present evidence that this gene expresses at least three different transcripts within the hematopoetic cell lineage. We also describe the structure of this genet around the breakpoints and present evidence that in a small series of adult ALL and AML patients, the breakpoints are clustered within less than 13 kilobases (kb) spanning four introns.

Functional Reconstruction of trans Regulation of the Ultrabithorax Promoter by the Products of Two Antagonistic Genes, trithorax and Polycomb

“…The central region of trx contains nine cysteine-rich zinc-binding motifs that form three major clusters and a fourth one containing a single motif (51). The cysteine residues and some flanking sequences of clusters II and III are highly conserved in the human homolog of trx (28,62,79). The second conserved region resides in the C terminus (see the introduction).…”

“…Lastly, the observation that highlevel activity of the Ubx promoter supported by trx can be largely repressed by Pc in a dose-dependent manner is consistent with genetic studies that suggest an antagonistic interaction between trx and Pc. Furthermore, the functional role of the highly conserved central zinc-binding domain of trx has been implicated by the following observations: (i) an out-offrame deletion (in the trx B11 allele) before this domain results in loss of function, but an in-frame deletion encompassing the zinc-binding cluster IV (in the trx E3 allele) has no drastic effect on Ubx expression (51); and (ii) disruptions of the human homolog of trx caused by chromosomal translocations are frequently found in this domain (28,62,79). Genetic studies have suggested that complex mechanisms are involved in Ubx regulation by maintenance genes (6,40,59,60).…”

“…Molecular studies have shown that trx encodes a large polypeptide of 3,759 amino acids with several clusters of unusual cysteine-rich zinc-binding motifs in the central region (51). Recently, a human trx homolog, ALL-1/HRX/Htrx1, implicated in ontogeny of acute juvenile leukemias has been identified, and sequence comparison has revealed substantial similarity in the central zinc-binding domain and a striking sequence identity (ϳ60%) in the C-terminal 210 amino acids (28,62,79). The subdomain of this C-terminal region has also been found in several other proteins (37,54,80).…”

Comparison of cytogenetics, Southern blotting, and fluorescence in situ hybridization as methods for detecting MLL gene rearrangements in children with acute leukemia and with 11q23 abnormalities

“…[5][6][7][8][9][10][11][12] To date, 13 genes involved in translocations with MLL have been cloned. [13][14][15][16][17][18][19][20][21][22][23][24][25] It has been suggested that alteration of the MLL protein is the critical event in oncogenesis and that fusion partners may not play an important role in the oncogenic process. 26,27 MLL gene rearrangements are associated with acute lymphoblastic leukemia (ALL), acute myeloid leukemia (AML), and secondary acute leukemias.…”

High frequency of fusion transcripts of exon 11 and exon 4/5 in AF-4 gene is observed in cord blood, as well as leukemic cells from infant leukemia patients with t(4;11)(q21;q23)

“…[6][7][8] The structural features of MLL protein, which is highly homologous to Drosophila trithorax protein, suggest that its normal function may be mediated by a physical interaction with DNA. 9,10 Although several chromosome loci are involved in reciprocal 11q23 translocations, t(4;11)(q21;q23) is the most common translocation in infant ALL. 11 In acute leukemia with t(4;11)(q21;q23), the MLL gene on 11q23 usually fuses to the AF-4 (or FEL) gene localized on chromosome band 4q21.…”