Selective analysis of cell-free DNA in maternal blood for evaluation of fetal trisomy.

OBJECTIVE: To develop a novel prenatal assay based on selective analysis of cell-free DNA in maternal blood for evaluation of fetal Trisomy 21 (T21) and Trisomy 18 (T18). METHODS: Two hundred ninety-eight pregnancies, including 39 T21 and seven T18 confirmed fetal aneuploidies, were analyzed using a novel, highly multiplexed assay, termed digital analysis of selected regions (DANSR™). Cell-free DNA from maternal blood samples was analyzed using DANSR assays for loci on chromosomes 21 and 18. Products from 96 separate patients were pooled and sequenced together. A standard Z-test of chromosomal proportions was used to distinguish aneuploid samples from average-risk pregnancy samples. DANSR aneuploidy discrimination was evaluated at various sequence depths. RESULTS: At the lowest sequencing depth, corresponding to 204,000 sequencing counts per sample, average-risk cases where distinguished from T21 and T18 cases, with Z statistics for all cases exceeding 3.6. Increasing the sequencing depth to 410,000 counts per sample substantially improved separation of aneuploid and average-risk cases. A further increase to 620,000 counts per sample resulted in only marginal improvement. This depth of sequencing represents less than 5% of that required by massively parallel shotgun sequencing approaches. CONCLUSION: Digital analysis of selected regions enables highly accurate, cost efficient, and scalable noninvasive fetal aneuploidy assessment.

Assessing DNA sequence variations in human ESTs in a phylogenetic context using high-density oligonucleotide arrays.

We have analyzed human genomic diversity in 32 individuals representing four continental populations of Homo sapiens in the context of four ape species. We used DNA resequencing chips covering 898 expressed sequence tags (ESTs), corresponding to 109 kb of sequence. Based on the intra-species data, the neutral hypothesis could not be rejected. However, the mutation rate was two times lower than typically observed in functionally unconstrained genomic segments, suggesting a certain level of selection. The worldwide diversity (297 segregating sites and nucleotide diversity of 0.054%) was partitioned among continents, with the greatest amount of variation observed in the African sample. The long-term effective population size of the human population was estimated at 13,000; a similar figure was obtained for the African sample and a 20% lower estimate was obtained for the other continents. Africans also differed in having a higher number of continental-specific polymorphisms contributing to the higher average nucleotide diversity. These results are consistent with the existence of two distinct lineages of modern humans: amalgamation of these lineages in Africa led to the higher present-day diversity on that continent, whereas colonization of other continents by one of them gave the effect of a population bottleneck.