RESUMO
SMADs are evolutionarily conserved transducers of the differentiation and growth arrest signals from the transforming growth factor/BMP (TGF/BMP) family of ligands. Upon receptor activation, the ligand-restricted SMADs(1-35) are phosphorylated in the C-terminal MH2 domain and recruit the common subunit SMAD4/DPC-4 gene to the nucleus to mediate target gene expression. Frequent inactivating mutations of SMAD4, or less common somatic mutations of SMAD2 seen in solid tumors, suggest that these genes have a suppressor function. However, there have been no identified mutations of SMAD5, although the gene localizes to the critical region of loss in chromosome 5q31.1 (chromosome 5, long arm, region 3, band 1, subband 1) in myelodysplasia (MDS) and acute myelogenous leukemia (AML). A ubiquitously expressed novel isoform, SMAD5beta, encodes a 351 amino acid protein with a truncated MH2 domain and a unique C-terminal tail of 18 amino acids, which may be the functional equivalent of inactivating mutations. The levels of SMAD5beta transcripts are higher in the undifferentiated CD34(+) hematopoietic stem cells than in the terminally differentiated peripheral blood leukocytes, thereby implicating the beta form in stem cell homeostasis. Yeast 2-hybrid interaction assays reveal the lack of physical interactions between SMAD5beta and SMAD5 or SMAD4. The expression of SMAD5beta may represent a novel mechanism to protect pluripotent stem cells and malignant cells from the growth inhibitory and differentiation signals of BMPs. (Blood. 2000;95:3945-3950)
Assuntos
Processamento Alternativo , Proteínas de Ligação a DNA/genética , Regulação da Expressão Gênica , Células-Tronco Hematopoéticas/fisiologia , Leucemia Mieloide Aguda/genética , Fosfoproteínas/genética , Transativadores/genética , Sequência de Aminoácidos , Sequência de Bases , Células da Medula Óssea/citologia , Aberrações Cromossômicas , Mapeamento Cromossômico , Cromossomos Humanos Par 5 , Regulação Neoplásica da Expressão Gênica , Humanos , Dados de Sequência Molecular , Reação em Cadeia da Polimerase , Reação em Cadeia da Polimerase Via Transcriptase Reversa , Transdução de Sinais , Proteína Smad5RESUMO
SMAD5, a transducer of TGF-beta/BMP inhibitory signals and a tumor suppressor candidate, localizes to the region of invariant loss in human myeloid neoplasms, on chromosome 5q31.1. Recent evidence indicates a gene-dosage effect along the TGF-beta/BMP signaling pathways. We have identified a novel transcript designated DAMS, whose 3' exonic sequences contain in part an alternate 5' exon of SMAD5, in the antisense orientation. Expressed sequenced tags (ESTs) for DAMS are found in fetal tissues (heart, adrenal glands, and total fetus) and pancreatic tumor cDNA libraries. In contrast to SMAD5, DAMS expression is not readily detectable in adult and fetal tissues. Semiquantitative PCR suggests that the stoichiometry between SMAD5 and DAMS transcripts ranges between 15 and 120 in normal and malignant hematopoietic cells. The findings raise the possibility that DAMS may be a fail-safe mechanism for precise regulation of SMAD5 transcript levels that may be critical in maintaining normal homeostasis.
Assuntos
Proteínas de Ligação a DNA/genética , Fosfoproteínas/genética , RNA Antissenso/genética , RNA Neoplásico/genética , Transativadores/genética , Fatores de Transcrição/genética , Transcrição Gênica/genética , Regiões 5' não Traduzidas/genética , Cromossomos Humanos Par 5/genética , Regulação Neoplásica da Expressão Gênica/genética , Hematopoese/genética , Humanos , Leucemia Mieloide/genética , Dados de Sequência Molecular , Reação em Cadeia da Polimerase , RNA Longo não Codificante , RNA Mensageiro/metabolismo , Proteína Smad5 , Células Tumorais CultivadasRESUMO
The high mobility group HMGI chromosomal proteins are an important component of chromatin. The HMGI-C protein consists of three amino terminal DNA binding domains ("AT hooks"), a linker region and an acidic carboxy domain. In mesenchymal tumors, chromosomal translocations of 12q13-15 result in fusion proteins containing the AT hooks and novel carboxy terminals. We have investigated the status of the HMGI-C gene in two cases of leukemia with anomalies of chromosome 12q and identified three novel isoforms (designated alpha, beta and gamma) derived from alternate splicing. One of the patients expressed all three isoforms, whereas the second patient expressed only the gamma isoform; preferential expression of the HMGI-C gamma isoform was also detected in the leukemic cell lines ML3 and BV173. The results are consistent with a crucial role for truncation of the acidic carboxy domain of HMGI-C in abnormal growth.
Assuntos
Proteínas de Ligação a DNA/genética , Regulação Neoplásica da Expressão Gênica/genética , Proteínas de Grupo de Alta Mobilidade/genética , Leucemia/metabolismo , Processamento Alternativo/genética , Sequência de Aminoácidos , Northern Blotting , Cromossomos Humanos Par 12/genética , Clonagem Molecular , Eletroforese em Gel de Ágar , Proteínas de Grupo de Alta Mobilidade/química , Humanos , Íntrons/genética , Dados de Sequência Molecular , Fases de Leitura Aberta/genética , RNA Mensageiro/análise , Análise de Sequência de DNA , Homologia de Sequência de Aminoácidos , Translocação Genética/genética , Células Tumorais CultivadasRESUMO
Acquired interstitial or complete losses of chromosome 5 are recurring anomalies associated with preleukemic myelodysplasia and acute myelogenous leukemia with a poor prognosis. Previous studies have delineated a potential myeloid tumor suppressor locus to a <2.4-Mb interval between the genes for IL9 and EGR1 on 5q31. In this report, we have localized the SMAD5 gene, a homologue of the tumor suppressor genes SMAD4/DPC-4 and SMAD2/JV18.1, to the minimal myeloid tumor suppressor locus and characterized its open reading frame and genomic organization. SMAD5 transcripts are readily detectable in hematolymphoid tissues and leukemic blasts. Absence of intragenic mutations in the remaining SMAD5 allele of leukemic patients and multiple solid tumor cell lines prescreened for loss of heterozygosity suggests that SMAD5 may not be a common target of somatic inactivation in malignancy.