The LRP gene encoding a major vault protein associated with drug resistance maps proximal to MRP on chromosome 16: evidence that chromosome breakage plays a key role in MRP or LRP gene amplification.
A cDNA encoding the novel drug resistance gene, LRP (originally termed lung resistance-related protein), was isolated from HT1080/DR4, a 220-fold doxorubicin-resistant human fibrosarcoma cell line which displays a multidrug resistance phenotype and overexpresses the multidrug resistance protein (MRP) but does not overexpress P-glycoprotein encoded by the MDR1 gene. Using the full-length 2.8-kb cDNA probe, the gene for LRP was regionally localized to the 16p13.1-16p11.2 chromosomal segment in human metaphases. Dual color fluorescence in situ hybridization studies refined the localization of LRP to 16p11.2, a location approximately 27 cM proximal to MRP (16p13.1). Two color hybridization studies indicated that HT1080/DR4 fibrosarcoma cells contain amplification of both the MRP and LRP genes in a striking striped pattern in the homogeneously staining region, hsr(7)(p12p15). In contrast, only amplified MRP gene sequences were contained within the homogeneously staining region, hsr(18q). Amplification of LRP was not identified in any of seven other drug-resistant tumor cell lines characterized by 20-300-fold levels of doxorubicin resistance, including two cell lines known to overexpress LRP (SW1573/2R120 and GLC4/ADR). Amplified MRP gene sequences were identified in H69AR, GLC4/ADR, and HL-60/AR whereas only MDR1 gene amplification was observed in the S1B120 colon carcinoma cell line. These data indicate that although both the MRP and LRP genes map to the short arm of chromosome 16, they are rarely coamplified and are not normally located within the same amplicon. A key role for chromosome breakage in gene amplification is supported by the presence of non-random karyotypic anomalies near the MRP and LRP normal cellular loci.