Distinct types of diffuse large B-cell lymphoma identified by gene expression profiling

Diffuse large B-cell lymphoma (DLBCL), the most common subtype of non-Hodgkin's lymphoma, is clinically heterogeneous: 40% of patients respond well to current therapy and have prolonged survival, whereas the remainder succumb to the disease. We proposed that this variability in natural history reflects unrecognized molecular heterogeneity in the tumours. Using DNA microarrays, we have conducted a systematic characterization of gene expression in B-cell malignancies. Here we show that there is diversity in gene expression among the tumours of DLBCL patients, apparently reflecting the variation in tumour proliferation rate, host response and differentiation state of the tumour. We identified two molecularly distinct forms of DLBCL which had gene expression patterns indicative of different stages of B-cell differentiation. One type expressed genes characteristic of germinal centre B cells ('germinal centre B-like DLBCL'); the second type expressed genes normally induced during in vitro activation of peripheral blood B cells ('activated B-like DLBCL'). Patients with germinal centre B-like DLBCL had a significantly better overall survival than those with activated B-like DLBCL. The molecular classification of tumours on the basis of gene expression can thus identify previously undetected and clinically significant subtypes of cancer.

LINE-I element insertion at the t(11;22) translocation breakpoint of a desmoplastic small round cell tumor

A t(11;22)(p13;p12) chromosomal translocation, juxtaposing the Wilms' tumor (WT1) and Ewing's sarcoma (EWS) genes, is the cytogenetic hallmark of desmoplastic small round cell tumor (DSRCT), a primitive multiphenotypic sarcoma arising in serosal tissues. Chimeric transcripts generated by this rearrangement encode an aberrant transcription factor that fuses the 5' region of EWS with a 3' WT1 segment. We describe the insertion of a LINE-I DNA mobile genetic element at the genomic breakpoint of a DSRCT chromosomal translocation. A 480 bp heterologous DNA segment with homology to the LINE-I DNA consensus sequence was located between EWS intron 8 and WT1 exon 8 in the productively rearranged allele. Sequence homology corresponded to the LINE-I ORF-2, which encodes a protein with reverse-transcriptase activity. The heterologous inserted fragment was not evident in the germline of normal tissue from the patient, suggesting that transposition occurred in somatic cells, possibly during the process of chromosomal rearrangement. This case represents the first example of LINE-I DNA transposition at the fusion site of a tumor-associated chromosomal rearrangement.

Methods for precise sizing, automated binning of alleles, and reduction of error rates in large-scale genotyping using fluorescently labeled dinucleotide markers. FUSION (Finland-U.S. Investigation of NIDDM Genetics) Study Group

Large-scale genotyping is required to generate dense identity-by-descent maps to map genes for human complex disease. In some studies the number of genotypes needed can approach or even exceed 1 million. Generally, linkage and linkage disequilibrium analyses depend on clear allele identification and subsequent allele frequency estimation. Accurate grouping or categorization of each allele in the sample (allele calling or binning) is therefore an absolute requirement. Hence, a genotyping system that can reliably achieve this is necessary. In the case of affected sib-pair analysis without parents, the need for accurate allele calling is even more critical. We describe methods that permit precise sizing of alleles across multiple gels using the fluorescence-based, Applied Biosystems (ABI) genotyping technology and discuss ways to reduce genotyping error rates. Using database utilities, we show how to minimize intergel allele size variation, to combine data effectively from different models of ABI sequencing machines, and automatically bin alleles. The final data can then be converted into a format ready for analysis by statistical genetic packages such as MENDEL.

A quantitative comparison of DNA sequence assembly programs

We have compared 11 sequence assembly programs for the accuracy and reproducibility with which they assemble DNA fragments into a completed sequence. To test the assemblers under controlled conditions, the rat multidrug resistance (RATMDRM) gene sequence was randomly divided into overlapping 200- to 400-base fragments. Various degrees of error, in the form of miss-identified bases, missed bases, and duplicated bases, were randomly added to these fragments. The probability of an error, and the type of error, was modified using an error distribution template that was developed by comparing the original fragments used to sequence RATMDRM with the final, edited sequence stored in GenBank. From 0 to 15% error was then added to independent sets of fragments, and assemblage was attempted. The quality of the assemblages was evaluated by comparing the number of differences between the assembled sequence and the original sequence. Tests were also done to determine if the order in which fragments were added to a project affected the final sequence and if the quality of assemblage was sequence dependent. Similar results were also obtained using other, unrelated sequences. The programs could be roughly divided into three groups based on the accuracy and reproducibility of assembly. Three (GCG, FAB, and AutoAssembler) consistently produced consensus sequences of low error and high reproducibility. Intermediate results were obtained with five other programs (Sequencher, AssemblyLIGN, XBAP, SeqMan, and AutoAssembler in a mode that made use of an external special processor). Less satisfactory results were obtained with the remaining three programs (GeneWorks, GENeration, and PC/Gene). The ability of the programs to edit the assembled sequence was also compared. Five of the programs were able to display and edit automatic sequencer trace files. The Sequencher program had a particularly well-designed sequence editor that allowed rapid examination and correction of assembly errors.