Genetic relatedness of Cryptococcus neoformans clinical isolates grouped with the repetitive DNA probe CNRE-1

Abstract

Cryptococcus neoformans isolates from eight patients with cryptococcal infection were previously assigned into three groups on the basis of repetitive DNA probe (CNRE-1) restriction fragment length polymorphisms. These groups accounted for a disproportionate number of recent clinical isolates in New York City. To further examine the genetic relatedness of isolates within and across CNRE-1 groups, the DNA sequence of the 779-base URA5 gene from each strain was amplified and sequenced. The number of nucleotide differences occurred in the third codon position or in introns. Pairwise comparisons revealed average nucleotide differences within a CNRE-1 group of 4.8 +/- 2.6 (n = 8) and between CNRE-1 groups of 21.9 +/- 7.0 (n =20) (P <0.001) Analysis of URA5 sequences defined three groups that were congruent with those defined by CNRE-1 restriction fragment length polymorphisms. PCR amplification of an rDNA intergenic spacer revealed conservation of the intergenic spacer length within groups. Electrophoretic karyotyping did not distinguish between two isolates in each of two CNRE-1 groups. DNA from all isolates studied hybridized to an alpha mating type-specific probe. We interpret these results as suggesting a clonal population structure for some pathogenic isolates of C. neoformans in New York City.