Genetic evolution of alpha fetoprotein producing gastric cancer

Abstract

Background: Alpha fetoprotein (AFP) producing gastric cancer is an unusual form of aggressive adenocarcinoma with a complex histological picture, including enteroblastic and hepatoid differentiation.

Aims: To investigate the genetic events underlying the phenotypic diversity in AFP producing gastric cancer and the ability of these tumours to produce AFP ectopically.

Methods: Multiple foci from 19 AFP producing gastric adenocarcinomas were microdissected and loss of heterozygosity (LOH) analysis was performed with a panel of microsatellite markers on nine chromosomal arms.

Results: For informative cases, LOH was most frequently detected on 17p (100%), followed by 13q (88%), 3p (87%), 5q and 9p (80%), 11q (70%), 18q (58%), 16q (53%), and 8p (50%). The average fractional allelic loss was 0.72. LOH was detected either homogeneously throughout the microdissected foci, or only in some parts of the neoplastic foci for each case. Heterogeneous patterns of LOH indicated genetic progression and/or divergence in clonal evolution. Furthermore, in six cases with heterogeneous LOH of 13q, 13q LOH was restricted to immunohistochemically AFP positive neoplastic foci.

Conclusion: AFP-GC arises as an aggressive clone with extensive LOH and high fractional allelic loss. The presence of heterogeneous patterns of LOH suggested that the AFP producing carcinoma foci might evolve through genetic progression and/or genetic divergence. Silencing of the crucial gene on 13q may be involved in the acquisition of the AFP producing phenotype.

Figure 1

Frequency of homogeneous and heterogeneous loss of heterozygosity (LOH). Solid bars indicate number of cases with homogenous LOH, hatched bars indicate number of cases with heterogeneous LOH, open bars indicate cases with retention, and shaded bars indicate heterogeneity index.

Figure 2

Representative histology for case HP1. (A) Low power view of the section showing the microdissected foci (T1–T4). T1, microdissected mucosal focus; T2–T4, microdissected invasive foci in the muscularis propria. (B, C) Histology of the microdissected foci (haematoxylin and eosin stain). (B) T4, invasive foci with tubulopapillary carcinoma. T4 is negative for the α fetoprotein (AFP) stain. (C) T2, invasive foci with enteroblastic histology. T2 is immunoreactive for the AFP stain. (D, E) Representative AFP immunohistochemical stains. (D) Negative AFP stain for mucosal focus T1. (E) Positive AFP stain for invasive enteroblastic focus T2.

Figure 3

Representative gels and proposed genetic pathway for case HP1. Gels for D18S474 and D5S644 show homogeneous loss of heterozygosity (LOH) for all of the foci microdissected. Arrowheads in the gels indicate LOH at all microdissected foci (T1–T4). Gels for D13S171 and D13S154 show 13q LOH only in invasive enteroblastic and α fetoprotein (AFP) positive foci, T2 and T3, but retention of mucosal (T1) and invasive tubulopapillary carcinoma foci (T4). Arrows indicate LOH of lower alleles for T2 and T3. Dot, normal alleles; N, normal control DNA; T, microdissected foci. The diagram summarises early LOH of 3p, 5q, 9p, 17p, and 18q, followed by the progressive LOH of 13q in α fetoprotein positive enteroblastic foci.

Figure 4

Representative gels and proposed genetic pathway for case HP15. Arrowhead in the gel for D9S1752 indicates homogeneous loss of heterozygosity (LOH) of all T1–T4 foci. Arrows in the gel for D13S171 indicate LOH of T1, T2, and T4, but not T3. Dot, normal alleles; N, normal control DNA; T1,T2, and T4, microdissected submucosal invasive foci with enteroblastic and hepatoid features; T3, intramucosal tumour focus microdissected. α Fetoprotein (AFP) is strongly positive in invasive foci but faint/negative in the mucosal T3 focus. The diagram shows early and homogeneous LOH of 3p, 5q, 8p, 9p, and 11q, including the mucosal focus. Subsequent LOH of 13q is identified in invasive enteroblastic and hepatoid foci (T1, T2, and T4).

Figure 5

Representative histology for case HP40. (A) Ordinary adenocarcinoma focus in the submucosa, T1 (haematoxylin and eosin (H&E) stain). T1 and T2 foci are negative for α fetoprotein (AFP) immunostaining. (B) Hepatoid carcinoma foci in the muscularis propria, T3 (H&E stain). Hepatoid foci, T3 and T4, are positive for AFP immunostaining.

Figure 6

Gels and deduced genetic pathways for HP40. Arrowheads in gels for D3S1293 and D5S647 indicate loss of heterozygosity (LOH) of all foci T1–T4. Arrows in the gels for Int 2 (11q) and D11S29 indicate 11q LOH of only T4. Arrows in the gels for D13S171 and D13S154 indicate 13q LOH of only T3. Dot, normal alleles; N, normal control DNA; T, microdissected foci. T1 is a microdissected submucosal focus, and T2–T4 are microdissected foci in the muscularis propria. The diagram summarises early and homogeneous LOH of 3p, 5q, 8p, 17p, followed by divergent LOH of 13q in α fetoprotein (AFP) positive hepatoid foci (T3), and 11q LOH in another hepatoid focus (T4).