Suppression of high-p{T} neutral pion production in central Pb+Pb collisions at sqrt[S{NN}]=17.3 GeV relative to p+C and p+Pb collisions.

Neutral pion transverse momentum spectra were measured in p+C and p+Pb collisions at sqrt[S{NN}]=17.4 GeV at midrapidity (2.3 less than or approximately equal eta{lab} less than or approximately equal 3.0) over the range 0.7 less than or approximately equal p{T} less than or approximately equal 3.5 GeV/c. The spectra are compared to pi{0} spectra measured in Pb+Pb collisions at sqrt[S{NN}]=17.3 GeV in the same experiment. For a wide range of Pb+Pb centralities (N{part} less than or approximately equal 300), the yield of pi{0}'s with p{T} greater than or approximately equal 2 GeV/c is larger than or consistent with the p+C or p+Pb yields scaled with the number of nucleon-nucleon collisions (N{coll}), while for central Pb+Pb collisions with N{part}greater than or approximately equal 350, the pi{0} yield is suppressed.

Interferometry of direct photons in central 208Pb + 208Pb collisions at 158A GeV.

Two-particle correlations of direct photons were measured in central 208Pb+208Pb collisions at 158A GeV. The invariant interferometric radii were extracted for 100

Ligand-independent dimerization and activation of the oncogenic Xmrk receptor by two mutations in the extracellular domain.

Overexpression of the oncogenic receptor tyrosine kinase ONC-Xmrk is the first step in the development of hereditary malignant melanoma in the fish Xiphophorus. However, overexpression of its proto-oncogene counterpart (INV-Xmrk) is not sufficient for the oncogenic function of the receptor. Compared with INV-Xmrk, the ONC-Xmrk receptor displays 14 amino acid changes, suggesting the presence of activating mutations. To identify such activating mutations, a series of chimeric and mutant receptors were studied. None of the mutations present in the intracellular domain was found to be involved in receptor activation. In the extracellular domain, we found two mutations responsible for activation of the receptor. One is the substitution of a conserved cysteine (C578S) involved in intramolecular disulfide bonding. The other is a glycine to arginine exchange (G359R) in subdomain III. Either mutation leads to constitutive dimer formation and thereby to activation of the ONC-Xmrk receptor. Besides, the presence of these mutations slows down the processing of the Xmrk receptor in the endoplasmic reticulum, which is apparent as an incomplete glycosylation.

Primary structure and expression of the xiphophorus DNA-(cytosine-5)-methyltransferase XDNMT-1.

Small aquarium fishes become increasingly important in the study of normal vertebrate development and disease. Differential DNA methylation might play a role in these processes. In the teleost Xiphophorus, a well-established animal model for melanoma formation, tumour-specific hypomethylation of the melanoma-inducing gene ONC-Xmrk has been observed. We have isolated a cDNA for the DNA-(cytosine-5)-methyltransferase XDNMT-1 from this organism, which encodes the first full-length protein from a fish species. Linkage analysis showed that Xdnmt-1 is different from the Xiphophorus tumour suppressor R, which is involved in the transcriptional repression of the ONC-Xmrk melanoma oncogene in healthy fish. As methylation has been implicated in the regulation of ONC-Xmrk expression, XDNMT-1 might play a role by acting up- or downstream of R. Expression analysis demonstrated that the Xdnmt-1 transcript is present in all adult tissues and cell lines tested. However, developing embryos show a spatially and temporally regulated expression pattern suggesting that the enzyme might play a role during development in fish.

Melanoma loss-of-function mutants in Xiphophorus caused by Xmrk-oncogene deletion and gene disruption by a transposable element.

The overexpression of the Xmrk oncogene (ONC-Xmrk) in pigment cells of certain Xiphophorus hybrids has been found to be the primary change that results in the formation of malignant melanoma. Spontaneous mutant stocks have been isolated that have lost the ability to induce tumor formation when crossed with Xiphophorus helleri. Two of these loss-of-function mutants were analyzed for genetic defects in ONC-Xmrk's. In the lof-1 mutant a novel transposable element, TX-1, has jumped into ONC-Xmrk, leading to a disruption of the gene and a truncated protein product lacking the carboxyterminal domain of the receptor tyrosine kinase. TX-1 is obviously an active LTR-containing retrotransposon in Xiphophorus that was not found in other fish species outside the family Poeciliidae. Surprisingly, it does not encode any protein, suggesting the existence of a helper function for this retroelement. In the lof-2 mutant the entire ONC-Xmrk gene was found to be deleted. These data show that ONC-Xmrk is indeed the tumor-inducing gene of Xiphophorus and thus the critical constituent of the tumor (Tu) locus.

Intragenic sex-chromosomal crossovers of Xmrk oncogene alleles affect pigment pattern formation and the severity of melanoma in Xiphophorus.

The X and Y chromosomes of the platyfish (Xiphophorus maculatus) contain a region that encodes several important traits, including the determination of sex, pigment pattern formation, and predisposition to develop malignant melanoma. Several sex-chromosomal crossovers were identified in this region. As the melanoma-inducing oncogene Xmrk is the only molecularly identified constituent, its genomic organization on both sex chromosomes was analyzed in detail. Using X and Y allele-specific sequence differences a high proportion of the crossovers was found to be intragenic in the oncogene Xmrk, concentrating in the extracellular domain-encoding region. The genetic and molecular data allowed establishment of an order of loci over approximately 0.6 cM. It further revealed a sequence located within several kilobases of the extracellular domain-encoding region of Xmrk that regulates overexpression of the oncogene.

Localization of a CDKN2 gene in linkage group V of Xiphophorus fishes defines it as a candidate for the DIFF tumor suppressor.

The Xiphophorus hybrid melanoma model represents one of the earliest reported cases of genetically regulated tumor susceptibility. Melanoma formation in Xiphophorus hybrids may be explained by the inheritance of two genes: a sex-linked oncogene, Xmrk, and a putative tumor suppressor locus, termed DIFF, located in Linkage Group V (LG V). Several genetic mapping procedures were used to produce a new Xiphophorus LG V map with 20 loci. All markers, particularly a recently cloned Xiphophorus CDKN2 gene family member, called CDKN2X, were tested for associations of genotype with degree of macromelanophore pigment pattern modification and susceptibility to melanoma formation in backcross hybrids of seven genetic types, involving 1,110 fish and three pigment patterns. Highly significant associations of CDKN2X genotypes with such phenotypic effects suggests that this gene is a strong candidate for the classically defined DIFF tumor suppressor gene. Because published results have documented the involvement of the CDKN2A (p16, MTS1, and INK4A) tumor suppressor gene in human melanoma formation, the possibility of CDKN2 genes acting as tumor suppressors in both man and Xiphophorus is likely.