Inherited variants of MYH associated with somatic G:C-->T:A mutations in colorectal tumors

Inherited defects of base excision repair have not been associated with any human genetic disorder, although mutations of the genes mutM and mutY, which function in Escherichia coli base excision repair, lead to increased transversions of G:C to T:A. We have studied family N, which is affected with multiple colorectal adenomas and carcinoma but lacks an inherited mutation of the adenomatous polyposis coli gene (APC) that is associated with familial adenomatous polyposis. Here we show that 11 tumors from 3 affected siblings contain 18 somatic inactivating mutations of APC and that 15 of these mutations are G:C-->A transversions--a significantly greater proportion than is found in sporadic tumors or in tumors associated with familial adenomatous polyposis. Analysis of the human homolog of mutY, MYH, showed that the siblings were compound heterozygotes for the nonconservative missense variants Tyr165Cys and Gly382Asp. These mutations affect residues that are conserved in mutY of E. coli (Tyr82 and Gly253). Tyrosine 82 is located in the pseudo-helix-hairpin-helix (HhH) motif and is predicted to function in mismatch specificity. Assays of adenine glycosylase activity of the Tyr82Cys and Gly253Asp mutant proteins with 8-oxoG:A and G:A substrates show that their activity is reduced significantly. Our findings link the inherited variants in MYH to the pattern of somatic APC mutation in family N and implicate defective base excision repair in predisposition to tumors in humans.     

Structural determinants for GoLoco-induced inhibition of nucleotide release by Galpha subunits

Heterotrimeric G-proteins bind to cell-surface receptors and are integral in transmission of signals from outside the cell. Upon activation of the Galpha subunit by binding of GTP, the Galpha and Gbetagamma subunits dissociate and interact with effector proteins for signal transduction. Regulatory proteins with the 19-amino-acid GoLoco motif can bind to Galpha subunits and maintain G-protein subunit dissociation in the absence of Galpha activation. Here we describe the structural determinants of GoLoco activity as revealed by the crystal structure of Galpha(i1) GDP bound to the GoLoco region of the 'regulator of G-protein signalling' protein RGS14. Key contacts are described between the GoLoco motif and Galpha protein, including the extension of GoLoco's highly conserved Asp/Glu-Gln-Arg triad into the nucleotide-binding pocket of Galpha to make direct contact with the GDP alpha- and beta-phosphates. The structural organization of the GoLoco Galpha(i1) complex, when combined with supporting data from domain-swapping experiments, suggests that the Galpha all-helical domain and GoLoco-region carboxy-terminal residues control the specificity of GoLoco Galpha interactions.     

A natural allele of Nxf1 suppresses retrovirus insertional mutations

Endogenous retroviruses have shaped the evolution of mammalian genomes. Host genes that control the effects of retrovirus insertions are therefore of great interest. The modifier-of-vibrator-1 locus (Mvb1) controls levels of correctly processed mRNA from genes mutated by endogenous retrovirus insertions into introns, including the Pitpn(vb) tremor mutation and the Eya1(BOR) model of human branchiootorenal syndrome. Positional complementation cloning identifies Mvb1 as the nuclear export factor Nxf1, providing an unexpected link between the mRNA export receptor and pre-mRNA processing. Population structure of the suppressive allele in wild Mus musculus castaneus suggests selective advantage. A congenic Mvb1(CAST) allele is a useful tool for modifying gene expression from existing mutations and could be used to manipulate engineered mutations containing retroviral elements.     

The Widespread Colonization Island of Actinobacillus actinomycetemcomitans

Genomic islands, such as pathogenicity islands, contribute to the evolution and diversification of microbial life. Here we report on the Widespread Colonization Island, which encompasses the tad (tight adherence) locus for colonization of surfaces and biofilm formation by the human pathogen Actinobacillus actinomycetemcomitans. At least 12 of the 14 genes at the tad locus are required for tenacious biofilm formation and synthesis of bundled Flp pili (fibrils) that mediate adherence. The pilin subunit, Flp1, remains inside the cell in tad-locus mutants, indicating that these genes encode a secretion system for export and assembly of fibrils. We found tad-related regions in a wide variety of Bacterial and Archaeal species, and their sequence characteristics indicate possible horizontal transfer. To test the hypothesis of horizontal transfer, we compared the phylogeny of the tad locus to a robust organismal phylogeny using statistical tests of congruence and tree reconciliation techniques. Our analysis strongly supports a complex history of gene shuffling by recombination and multiple horizontal transfers, duplications and losses. We present evidence for a specific horizontal transfer event leading to the establishment of this region as a determinant of disease.     

Cannibalism in the Madagascan dinosaur Majungatholus atopus

Many lines of evidence have been brought to bear on the question of theropod feeding ecology, including functional and physiological considerations, morphological constraints, taphonomic associations, and telling--although rare--indications of direct ingestion. Tooth marks of theropods, although rarely described and generally left unassigned to a particular taxon, can provide unique clues into predator-prey interaction, and can also yield insights into the extent of carcass utilization. Here we describe a sample of tooth-marked dinosaur bone recovered from three well-documented localities in the Upper Cretaceous Maevarano Formation of Madagascar that provides insights into the feeding ecology of the abelisaurid theropod Majungatholus atopus. Intensely tooth-marked elements from multiple individuals show that Majungatholus defleshed dinosaur carcasses. Furthermore, Majungatholus clearly fed upon the remains of not only sauropods, but also conspecifics, and thus was a cannibal. Cannibalism is a common ecological strategy among extant carnivores, but until now the evidence in relation to carnivorous dinosaurs has been sparse and anecdotal.