The MLL gene from human chromosome 11q23 is involved in >30 different chromosomal translocations resulting in a plethora of different MLL fusion proteins. Each of these tends to associate with a ...specific leukaemia type, for example, MLL–AF9 is found mainly in acute myeloid leukaemia. We have studied the role of the Mll–AF9 gene fusion made in mouse embryonic stem cells by an homologous recombination knock‐in. Acute leukaemias developed in heterozygous mice carrying this fusion as well as in chimeric mice. As with human chromosomal translocation t(9;11), the majority of cases were acute myeloid leukaemias (AMLs) involving immature myeloblasts, but a minority were acute lymphoblastic leukaemia. The AMLs were preceded by effects on haematopoietic differentiation involving a myeloproliferation resulting in accumulation of Mac‐1/Gr‐1 double‐positive mature myeloid cells in bone marrow as early as 6 days after birth. Therefore, non‐malignant expansion of myeloid precursors is the first stage of Mll–AF9‐mediated leukaemia followed by accumulation of malignant cells in bone marrow and other tissues. Thus, the late onset of overt tumours suggests that secondary tumorigenic mutations are necessary for malignancy associated with MLL–AF9 gene fusion and that myeloproliferation provides the pool of cells in which such events can occur.
The LMO2 and TAL1 genes were first identified via chromosomal translocations and later found to encode proteins that interact during normal erythroid development. Some T cell leukaemia patients have ...chromosomal abnormalities involving both genes, implying that LMO2 and TAL1 act synergistically to promote tumorigenesis after their inappropriate co‐expression. To test this hypothesis, transgenic mice were made which co‐express Lmo2 and Tal1 genes in T cells. Dimers of Lmo2 and Tal1 proteins were formed in thymocytes of double but not single transgenic mice. Furthermore, thymuses of double transgenic mice were almost completely populated by immature T cells from birth, and these mice develop T cell tumours approximately 3 months earlier than those with only the Lmo2 transgene. Thus interaction between these two proteins can alter T cell development and potentiate tumorigenesis. The data also provide formal proof that TAL1 is an oncogene, apparently acting as a tumour promoter in this system.
The Arctic mutation, encoding E693G in the
amyloid precursor protein (APP)
gene E22G in amyloid-β (Aβ), causes dominantly inherited Alzheimer’s disease. Here, we report the high-resolution cryo-EM ...structures of Aβ filaments from the frontal cortex of a previously described case (
A
β
PParc1
) with the Arctic mutation. Most filaments consist of two pairs of non-identical protofilaments that comprise residues V12–V40 (human Arctic fold A) and E11–G37 (human Arctic fold B). They have a substructure (residues F20–G37) in common with the folds of type I and type II Aβ42. When compared to the structures of wild-type Aβ42 filaments, there are subtle conformational changes in the human Arctic folds, because of the lack of a side chain at G22, which may strengthen hydrogen bonding between mutant Aβ molecules and promote filament formation. A minority of Aβ42 filaments of type II was also present, as were tau paired helical filaments. In addition, we report the cryo-EM structures of Aβ filaments with the Arctic mutation from mouse knock-in line
App
NL−G−F
. Most filaments are made of two identical mutant protofilaments that extend from D1 to G37 (
App
NL−G−F
murine Arctic fold). In a minority of filaments, two dimeric folds pack against each other in an anti-parallel fashion. The
App
NL−G−F
murine Arctic fold differs from the human Arctic folds, but shares some substructure.
The HOX11 homeobox gene was identified via the translocation t(10;14) in T cell leukaemia. To determine the function of this gene in mice, null mutations were made using homologous recombination in ...ES cells to incorporate lacZ into the hox11 transcription unit. Production of beta-galactosidase from the recombinant hox11 allele in +/- mutants allowed identification of sites of hox11 expression which included the developing spleen. Newborn hox11 -/- mice exhibit asplenia. Spleen formation commences normally at E11.5 in hox11 -/- mutant embryos but the spleen anlage undergoes rapid and complete resorption between E12.5 and E13.5. Dying spleen cells exhibit molecular features of apoptosis, suggesting that programmed cell death is initiated at this stage of organ development in the absence of hox11 protein. Thus hox11 is not required to initiate spleen development but is essential for the survival of splenic precursors during organogenesis. This function for hox11 suggests that enhanced cell survival may result from the t(10;14) which activates HOX11 in T cell leukaemias, further strengthening the association between oncogene-induced cell survival and tumorigenesis.
RBTN2 is activated by the chromosomal translocation t(11;14) (P13;p11) in some T cell leukaemias. Histologically similar T cell tumours develop with long latency in transgenic mice when either CD2 or ...thy1.1 promoters control rbtn2 expression. During the asymptomatic period, perturbation of T cell differentiation occurs in the thymus. The major anomalies present during this phase are an increase in the percentage of large thymocytes lacking CD4 and CD8 markers and also of small thymocytes express both the T cell marker CD3 and the B cell-specific form of CD45. These abnormal T cell populations can be clonal and thus a primary result of aberrant expression of the LIM-protein Rbtn2 is alteration of T cell differentiation preceding overt malignancy. These data provide a biological explanation for the role of Rbtn2 in tumorigenesis and presumably RBTN2 expression in T cells after the translocation t(11;14) in children has the same effect.
RBTN2 is a LIM domain protein which can be activated by the translocation t(11;14)(p13;q11) in childhood T cell acute leukaemia. Transgenic mice were examined in which rbtn2 protein is expressed in ...the T cell lineage. An average of 72% of these mice developed T cell tumours before 18 months of age, compared with 9% in transgenic mice expressing the related gene Rbtn-1. Rbtn2-induced tumours first appeared at 5 months of age and were clonal. They displayed a range of phenotypes, the most notable being CD3/CD45R double-positive cells. Tumours expressing either T cell receptor alpha/beta or gamma/delta heterodimers were found. Thus rbtn2 can promote tumours within a range of T cell types and maturities. The latency period before tumour development indicates that secondary events must occur before the onset of overt malignancy.
Two members of the RBTN gene family, RBTN1/Ttg-1 and RBTN2/Ttg-2, were found by their association with T-cell tumour-specific chromosomal translocations and are thought to be involved in the ...aetiology of such T-cell tumours. Here a transgenic mouse model is described in which T-cell tumours are induced by the presence of RBTN1 and RBTN2 transgenes that direct expression in thymus-derived cells. The latency period for lymphoid tumour appearance is variable, and tumours occur in a small proportion of transgenic animals that develop T-cell acute lymphoblastic malignancies. No significant increase in the rate of tumour development was observed in RBTN1 transgenic mice infected with Moloney murine leukaemia virus, nor did tumours arise in mice bearing a construct in which RBTN1 was expressed from the insulin transcriptional promoter. These data, which provide formal proof of the oncogenic activity of these genes, suggest that aberrant expression of transcription factor genes, such as RBTN1 and RBTN2, functions in tumour aetiology by disturbing some aspect of T-cell differentiation.
A chromosomal translocation t(11;14) (p15;q11) is described in a human acute T‐cell leukaemia of immature phenotype (CD3‐, CD4‐, CD8‐). The translocation occurs at a T‐cell receptor joining J delta ...segment, 12 kb upstream of the constant C delta gene and 98 kb upstream of the C alpha gene at chromosome band 14q11. Nucleotide sequencing shows that both J delta and C delta are very conserved between mouse and man. The region of chromosome 11 involved in the translocation is transcriptionally active and produces a 4‐kb mRNA. The DNA sequence at the chromosome 11 junction shows a perfect match to a recombinase signal sequence implying that this translocation occurred by recombinase error. The occurrence of the translocation breakpoint at the C delta locus, normally rearranged in immature T cells, and the structure of the translocation junctions suggests that the translocation occurred during an attempt at normal rearrangement of the J delta segment in an early thymocyte.
More than 50 human proteins with a wide range of functions have a 120 residue phosphoinositide binding module known as the PX domain. The 1.7 A X-ray crystal structure of the PX domain from the ...p40(phox) subunit of NADPH oxidase bound to PtdIns(3)P shows that the PX domain embraces the 3-phosphate on one side of a water-filled, positively charged pocket and reveals how 3-phosphoinositide specificity is achieved. A chronic granulomatous disease (CGD)-associated mutation in the p47(phox) PX domain that abrogates PtdIns(3)P binding maps to a conserved Arg that does not directly interact with the phosphoinositide but instead appears to stabilize a critical lipid binding loop. The SH3 domain present in the full-length protein does not affect soluble PtdIns(3)P binding to the p40(phox) PX domain.
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