Complexity of lung cancer modifiers: mapping of thirty genes and twenty-five interactions in half of the mouse genome

BACKGROUND

Numerous low-penetrance genes control susceptibility to cancer in experimental animals, but the overall genetic information on this group of genes (i.e., number of loci and their mutual interactions) is missing. We performed a systematic search, scanning roughly half of the mouse genome for lung cancer susceptibility (Sluc) genes affecting tumor size or number by using mouse recombinant congenic (RC) strains. In each RC strain (OcB), approximately 12.5% of the genome is derived from the lung cancer-resistant strain B10.O20, whereas the rest is derived from the lung cancer-susceptible strain O20.

METHODS

A total of 730 F2 hybrids from five (OcB x O20) crosses were tested. Pregnant mice were treated on day 18 of gestation with a single dose of N-ethyl-N-nitrosourea. When offspring were 16 weeks old, whole lungs were removed and sectioned semiserially, and the size of all lung tumors (n = 2658) was determined. Analysis of variance was used for detection of linkage, and models (including main effect and two-way interactions) were tested with a statistical program.

RESULTS

We detected a total of 30 Sluc loci (16 new plus 14 previously reported) and 25 two-way interactions. Some of these interactions are counteracting (e.g., Sluc17 and Sluc20), resulting in the partial or total masking of the individual independent effect (main effect) of each involved locus. Seven loci (Sluc1, Sluc5, Sluc12, Sluc16, Sluc18, Sluc20, and Sluc26) and two interactions (Sluc5 x Sluc12 and Sluc5 x Sluc26) were detected in more than one RC strain.

CONCLUSIONS

The extrapolation of our results to the whole genome suggests approximately 60 Sluc loci (90% confidence intervals = 42 to 78). Despite the genetic complexity of lung cancer, use of appropriate mapping strategies can identify a large number of responsible loci and can reveal their interactions. This study provides an insight into the genetic control of lung tumorigenesis and may serve as a paradigm for investigating the genetics of other cancer types.

Low molecular weight group IIA and group V phospholipase A(2) enzymes have different intracellular locations in mouse bone marrow-derived mast cells

The subcellular location of the enzymes of eicosanoid biosynthesis is critical for their co-ordinate action in the generation of leukotrienes and prostaglandins. This activity is thought to occur predominantly at a perinuclear location. Whereas the subcellular locations of cytosolic phospholipase (PL) A(2) and each of the pathway enzymes of eicosanoid generation have been defined, the distribution of the low molecular weight species of PLA(2) has remained elusive because of the lack of antibodies that distinguish among homologous family members. We have prepared affinity-purified rabbit antipeptide IgG antibodies that distinguish mouse group IIA PLA(2) and group V PLA(2). Immunofluorescence staining and immunogold electron microscopy reveal different subcellular locations for the enzymes. Group IIA(2) PLA(2) is present in the secretory granules of mouse bone marrow-derived mast cells, consistent with its putative role in facilitating secretory granule exocytosis and its consequent extracellular action. In contrast, group V PLA(2) is associated with various membranous organelles including the Golgi apparatus, nuclear envelope, and plasma membrane. The perinuclear location of group V PLA(2) is consistent with a putative interaction with translocated cytosolic PLA(2) in supplying arachidonic acid for generation of eicosanoid products, while the location in Golgi cisternae may also reflect its action as a secreted enzyme. The spatial segregation of group IIA PLA(2) and group V PLA(2) implies that these enzymes are not functionally redundant.

High frequency of interactions between lung cancer susceptibility genes in the mouse: mapping of Sluc5 to Sluc14

Although several genes that cause monogenic familial cancer syndromes have been identified, susceptibility to sporadic cancer remains unresolved. Animal experiments have demonstrated multigenic control of tumor susceptibility. Recently, we described four mouse lung cancer susceptibility (Sluc) loci, the main effects of which are masked by their mutual interactions. Because such interactions can considerably affect the strategies for identification of cancer susceptibility genes in humans, it is necessary to establish whether they are common or rare. Here, we report the mapping of 10 additional Sluc loci and show that 13 of the 14 Sluc loci are involved in one or more interactions, demonstrating that interactions of tumor susceptibility genes are frequent and that they probably form complex networks.

Genetics of quantitative and qualitative aspects of lung tumorigenesis in the mouse: multiple interacting Susceptibility to lung cancer (Sluc) genes with large effects

Inbred strains of mice exhibit large differences in their susceptibility to various complex quantitative genetic traits, among which is the susceptibility to lung cancer. These differences are caused by the combined effects of multiple quantitative trait loci (QTLs). Due to their multiplicity, it is relatively difficult and laborious to study the effects of individual QTLs. To dissect complex genetic traits the authors make use of recombinant congenic strains (RCS), a system of mouse inbred strains in which the genetic complexity is reduced. The susceptibility to lung cancer is studied by using the series of O20-congenic-B10.O20 (OcB) RC strains. They are derived from the parental background strain O20 and the parental donor strain B10.O20, two mouse inbred strains that differ from each other in both quantitative and qualitative aspects of lung tumorigenesis. This study describes the segregation of lung tumor number, size, and histology among the OcB RC strains, and indicates that these traits are influenced by multiple interacting QTLs with considerable individual effects. The results suggest that some of the susceptibility loci to lung cancer affect the susceptibility to other types of cancer as well, possibly by functioning systematically.

Complex interactions of new quantitative trait loci, Sluc1, Sluc2, Sluc3, and Sluc4, that influence the susceptibility to lung cancer in the mouse

Many complex traits, including susceptibility to lung cancer, are controlled by multiple genes--quantitative trait loci (QTLs). We facilitated the mapping of QTLs by making use of recombinant congenic strains (RCS), a system of mouse inbred strains in which the genetic complexity is reduced, and by applying MQM-mapping (multiple-QTL models or marker-QTL-marker), a multilocus method with an increased power of detecting of individual QTLs and interacting QTLs (epistasis). The mouse strain O20 develops significantly larger N-ethyl-N-nitrosourea induced lung tumours than mice of the RC strain OcB-9 (ref. 5); the latter share approximately 87.5% of their genes with strain O20 and 12.5% with strain B10.O20 (refs 6,7). QTL analysis of 222 (OcB-9 x O20) F2 mice revealed four new loci that influence susceptibility to lung cancer (Sluc genes). They are involved in two significant, partly counteracting interactions which mask their individual main effects: Sluc1 (on chromosome 19) interacts with Sluc2 (chromosome 2), and Sluc3 (chromosome 6) interacts with Sluc4 (chromosome 11). Together with the data of van Wezel et al. in the accompanying report, our results indicate that interactions between tumour susceptibility genes are a common phenomenon which complicates their mapping.

A gene for susceptibility to small intestinal cancer, ssic1, maps to the distal part of mouse chromosome 4

Mouse inbred strains B10.O20 and O20 were used to study the genetics of susceptibility to small intestinal cancer. Strain B10.O20 is susceptible to tumors of the small intestine induced by a transplacental treatment with the carcinogen N-ethyl-N-nitrosourea on day 18 of gestation, whereas strain O20 is completely resistant. F1 hybrid mice are also completely resistant, indicating that homozygosity of the B10.O20 allele of at least one susceptibility gene is required for the development of tumors of the small intestine. To map this gene, we used backcross 2 (N3) mice. We tested 283 [((B10.O20 x O20)F1 x B10.O20) x B10.020] N3 mice, 67 of which developed tumors of the small intestine. Their genotype was determined with the use of more than 85 simple sequence length polymorphisms (SSLPs) spread over the genome. If a SSLP is linked closely to a susceptibility gene, it is expected that almost none of the N3 mice with small intestinal tumors are heterozygous for this marker. We found that very few tumor-bearing N3 mice were heterozygous for SSLP markers on the distal part of chromosome 4 compared with the N3 mice without small intestinal tumors. This difference is highly significant (chi 2 = 31 for D4Mit158). Therefore, one of the genes that influence susceptibility to small intestinal cancer, ssic1, maps to the distal part of chromosome 4.

A susceptibility gene for alveolar lung tumors in the mouse maps between Hsp70.3 and G7 within the H2 complex

Lung tumor susceptibility (LTS) in the mouse is influenced by multiple loci within the H2 complex. We compared the LTS of two H2 congenic strains with intra H2 recombinations, B10.A(1R) and B10.A(2R), whose genetic difference has been reduced to a region of approximately 50 kilobases within the C4-H2D interval, between Hsp70.3 and G7. After transplacental induction with N-ethyl-N-nitrosourea the load of alveolar lung tumors in strain B10.A(2R) is significantly higher than in strain B10.A(1R) (P < 0.001). For papillary tumors no significant differences were observed. We conclude that the alveolar lung tumor load is influenced by an LTS gene located within the Hsp70.3-G7 interval.

Kras-2 alleles, mutations, and lung tumor susceptibility in the mouse--an evaluation

Lung Tumor Susceptibility (LTS) in the mouse has been shown to be influenced by loci within the Major Histocompatibility Complex (MHC) and the Kras-2 regions. In crosses between susceptible and resistant strains, one allele of Kras-2 has been linked with a high LTS, whereas the other allele has been linked with a low LTS. Furthermore, these Kras-2 alleles affect differently the occurrence of Kras-2 mutations in lung tumors. In this study we evaluated the relationship between LTS, Kras-2 alleles and Kras-2 mutations, using Recombinant Congenic Strains with identical MHC haplotypes. The Kras-2 region indeed influences the frequency of Kras-2 mutations in N-ethyl-N-nitrosourea-induced lung tumors. These mutations are associated with larger tumors. However, Kras-2 affects LTS only marginally. Several strains with identical Kras-2 alleles and mutation frequency differ widely in LTS, while some strains with different alleles and mutation frequency do not. We conclude that a considerable part of the genetic variability in LTS is caused by LTS-loci other than Kras-2 and MHC.