The fatal anemia is induced by an Apc-haploinsufficient BM microenvironment (A) Kaplan-Meier survival curve of Apc^fl/+ (WT) or Apc^del/+ recipients transplanted with WT, Egr1^+/; Tp53^+/−; Apc^del/+; or Egr1^+/−, Apc^del/+BM cells. Median survival for Apc^del/+recipients was 130, 118, 139, 130, and 139 days, respectively. All Apc^fl/+and Apc^del/+mice were treated with pIpC at 2 months, and recipients were lethally irradiated and transplanted 4 weeks postinduction. (B) Kaplan-Meier survival curve of Apc^fl/+ (WT) or Apc^del/+ recipients transplanted with Apc-WT (CD45.1/CD45.2) BM cells. A representative dot plot shows that Apc^del/+ BM are CD45.2⁺, whereas Apc^del/+ mice are reconstituted with CD45.1⁺ CD45.2⁺ BM cells. (C) PCR analysis of splenocytes isolated from Apc^del/+ recipients transplanted with WT or Egr1^+/− BM cells. Analysis of Egr1 and Apc clearly indicates that splenocytes are donor-derived. A PCR control on the right indicates expected sizes of deleted and WT bands. (D) Analysis of deletion of Apc, as determined by PCR analysis of DNA from stromal cells: Apc^fl/fl (Control), Apc^del/del(Apc-null), Apc^del/+(Apc-het), and WT. In the sample shown, a PCR product from the floxed Apc allele was still visible in Apc^del/del and Apc^del/+ samples, in addition to the anticipated deleted and WT bands, suggesting that deletion in stromal cells may not always occur in every cell. This is in contrast to hematopoietic cells, where the deletion occurs in all cells.¹⁰  (E) Western blot analysis of extracts from spleen cells and stromal cell cultures using β-catenin antibody and actin as a loading control. Apc deletion had occurred in virtually all cells in the stromal cells used for this analysis (not shown).