Effect of ABO-incompatible listing on infant heart transplant waitlist outcomes: analysis of the United Network for Organ Sharing (UNOS) database.

BACKGROUND: Midterm heart transplant outcomes of ABO-incompatible (ABO-I) organ use in infants are favorable. ABO-I transplantation has resulted in reduced waitlist mortality in some countries. This study assessed the effect of an ABO-I listing strategy on pre-transplant outcomes in the United States. METHODS: The Organ Procurement and Transplantation Network (OPTN)/United Network of Organ Sharing (UNOS) database was used to identify infants aged younger than 1 year listed as status 1 for heart transplantation between January 1, 2001, and May 20, 2008. The cohort was divided into 2 groups: eligible for ABO-compatible (ABO-C) transplant and eligible for ABO-I transplant. Baseline characteristics, waitlist times, and outcomes were compared in univariate analysis. Competing risks analysis evaluated differences in time to transplant in the presence of other outcomes. RESULTS: Of 1,029 infants listed for transplant, 277 (27%) were listed for an ABO-I transplant. Overall, 92% of transplant recipients received an ABO-C organ regardless of listing type. Among recipients eligible for ABO-I, only 27% received an ABO-I organ. The percentage that underwent transplant in each group did not differ. Although infants listed for an ABO-I organ had a shorter wait time for transplant, waitlist mortality was similar. CONCLUSIONS: Despite the intended merits of ABO-I heart transplantation, ABO-I listing and organ acceptance have not yielded lower waitlist mortality in the United States under the current UNOS allocation algorithm. Consideration should be given to altering the allocation system to one that gives less preference toward blood group compatibility in hopes of improving organ use and reducing waitlist mortality.

Tandem repeat regions within the Burkholderia pseudomallei genome and their application for high resolution genotyping.

BACKGROUND: The facultative, intracellular bacterium Burkholderia pseudomallei is the causative agent of melioidosis, a serious infectious disease of humans and animals. We identified and categorized tandem repeat arrays and their distribution throughout the genome of B. pseudomallei strain K96243 in order to develop a genetic typing method for B. pseudomallei. We then screened 104 of the potentially polymorphic loci across a diverse panel of 31 isolates including B. pseudomallei, B. mallei and B. thailandensis in order to identify loci with varying degrees of polymorphism. A subset of these tandem repeat arrays were subsequently developed into a multiple-locus VNTR analysis to examine 66 B. pseudomallei and 21 B. mallei isolates from around the world, as well as 95 lineages from a serial transfer experiment encompassing ~18,000 generations. RESULTS: B. pseudomallei contains a preponderance of tandem repeat loci throughout its genome, many of which are duplicated elsewhere in the genome. The majority of these loci are composed of repeat motif lengths of 6 to 9 bp with 4 to 10 repeat units and are predominately located in intergenic regions of the genome. Across geographically diverse B. pseudomallei and B.mallei isolates, the 32 VNTR loci displayed between 7 and 28 alleles, with Nei's diversity values ranging from 0.47 and 0.94. Mutation rates for these loci are comparable (>10-5 per locus per generation) to that of the most diverse tandemly repeated regions found in other less diverse bacteria. CONCLUSION: The frequency, location and duplicate nature of tandemly repeated regions within the B. pseudomallei genome indicate that these tandem repeat regions may play a role in generating and maintaining adaptive genomic variation. Multiple-locus VNTR analysis revealed extensive diversity within the global isolate set containing B. pseudomallei and B. mallei, and it detected genotypic differences within clonal lineages of both species that were identical using previous typing methods. Given the health threat to humans and livestock and the potential for B. pseudomallei to be released intentionally, MLVA could prove to be an important tool for fine-scale epidemiological or forensic tracking of this increasingly important environmental pathogen.

Phylogenetic discovery bias in Bacillus anthracis using single-nucleotide polymorphisms from whole-genome sequencing.

Phylogenetic reconstruction using molecular data is often subject to homoplasy, leading to inaccurate conclusions about phylogenetic relationships among operational taxonomic units. Compared with other molecular markers, single-nucleotide polymorphisms (SNPs) exhibit extremely low mutation rates, making them rare in recently emerged pathogens, but they are less prone to homoplasy and thus extremely valuable for phylogenetic analyses. Despite their phylogenetic potential, ascertainment bias occurs when SNP characters are discovered through biased taxonomic sampling; by using whole-genome comparisons of five diverse strains of Bacillus anthracis to facilitate SNP discovery, we show that only polymorphisms lying along the evolutionary pathway between reference strains will be observed. We illustrate this in theoretical and simulated data sets in which complex phylogenetic topologies are reduced to linear evolutionary models. Using a set of 990 SNP markers, we also show how divergent branches in our topologies collapse to single points but provide accurate information on internodal distances and points of origin for ancestral clades. These data allowed us to determine the ancestral root of B. anthracis, showing that it lies closer to a newly described "C" branch than to either of two previously described "A" or "B" branches. In addition, subclade rooting of the C branch revealed unequal evolutionary rates that seem to be correlated with ecological parameters and strain attributes. Our use of nonhomoplastic whole-genome SNP characters allows branch points and clade membership to be estimated with great precision, providing greater insight into epidemiological, ecological, and forensic questions.