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In human neuroblastomas, the distal portion of 1p is frequently deleted, as if one or more tumor suppressor genes from this region were involved in neuroblastoma tumorigenesis. Earlier studies had identified a smallest region of overlapping deletion (SRO) spanning approximately 23 cM between the most distally retained D1S80 and by the proximally retained D1S244. In pursuit of generating a refined delineation of the minimally deleted region, we have analyzed 49 neuroblastomas of different stages for loss of heterozygosity (LOH) from 1pter to 1p35 by employing 26 simple sequence length polymorphisms. Fifteen of the 49 tumors (31%) had LOH; homozygous deletion was not detected. Seven tumors had LOH at all informative loci analyzed, and eight tumors showed a terminal or an interstitial allelic loss of 1p. One small terminal and one interstitial deletion defined a new 1.7 cM SRO, approximately 1 Mbp in physical length, deleted in all tumors between the retained D1S2731 (distal) and D1S2666 (proximal). To determine the genomic complexity of the deleted region shared among tumors, we assembled a physical map of the I Mbp SRO consisting predominantly of bacteriophage P1-derived artificial chromosome (PAC) clones. A total of 55 sequence-tagged site (STS) markers (23 published STSs and short tandem repeats and 32 newly identified STSs from the insert ends of PACs and cosmids) were assembled in a contig, resulting in a sequence-ready physical map with approximately one STS per 20 Kbp. Twelve genes (41BB, CD30, DFFA, DJ1, DR3, FRAP, HKR3, MASP2, MTHFR, RIZ, TNR2, TP73) previously mapped to 1p36 are localized outside this SRO. On the basis of this study, they would be excluded as candidate genes for neuroblastoma tumorigenesis. Ten expressed sequence tags were integrated in the contig, of which five are located outside the SRO. The other five from within the SRO may provide an entrance point for the cloning of candidate genes for neuroblastoma.

There is good evidence now that the secretory type II phospholipase A 2 (Pla2g2a) gene represents the Mom1 locus, a genetic modi®er of tumor resistance in the multiple intestinal neoplasia (Min) mouse. Previously we have mapped the human homolog PLA2G2A to 1p35-36.1 within a region that is the target of frequent deletions in sporadic colorectal tumors. Here we show 64% loss of heterozygosity (LOH) at the PLA2G2A locus in primary tumors. We studied PLA2G2A expression in both colorectal tumor cell lines and normal mucosa. Most of the lines lacked detectable PLA2G2A transcripts by Northern analysis. Large dierences in expression were seen among normal mucosa of dierent patients with sporadic tumors. We analysed the coding region of PLA2G2A in eight colorectal cancer cell lines with hemizygous deletion at 1p35-36/PLA2G2A, in none we did detect a mutation. Biallelic expression of PLA2G2A was observed in a cell line heterozygous for an exon 3 polymorphism, rendering unlikely that imprinting is a pathway participating in the loss of PLA2G2A function. It remains uncertain if PLA2G2A, in particular its apparent lack of expression in tumor cells, might be a factor in human colorectal tumorigenesis.

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