Centrosome amplification can initiate tumorigenesis in flies

Abstract

Centrosome amplification is a common feature of many cancer cells, and it has been previously proposed that centrosome amplification can drive genetic instability and so tumorigenesis. To test this hypothesis, we generated Drosophila lines that have extra centrosomes in approximately 60% of their somatic cells. Many cells with extra centrosomes initially form multipolar spindles, but these spindles ultimately become bipolar. This requires a delay in mitosis that is mediated by the spindle assembly checkpoint (SAC). As a result of this delay, there is no dramatic increase in genetic instability in flies with extra centrosomes, and these flies maintain a stable diploid genome over many generations. The asymmetric division of the larval neural stem cells, however, is compromised in the presence of extra centrosomes, and larval brain cells with extra centrosomes can generate metastatic tumors when transplanted into the abdomens of wild-type hosts. Thus, centrosome amplification can initiate tumorigenesis in flies.

Figure 1. Overexpression of SAK Induces Centrosome Amplification and Delays Development

(A and B) Immunostaining of G2 wild-type (WT) (A) and SAKOE (B) neuroblasts with the centriole marker D-PLP (left, green in merged panel), the PCM marker γ-tubulin (middle, red in merged panel), and DNA (blue in merged panel). In the WT neuroblast the two centrosomes are asymmetric (see text for details) and only one contains high levels of γ-tubulin. In the SAKOE neuroblast eight centrosomes are present and these contain varying amounts of γ-tubulin. (C) Quantification of centrosome number in WT (white bars) and SAKOE (red bars) neuroblasts. Note that we only scored D-PLP dots as centrosomes if they also contained some PCM. (D and E) EM micrographs of selected thin serial sections of WT (D) and SAKOE (E) neuroblast spindle poles. In the WT cell only two centrioles were detected at the spindle pole (red and yellow arrows). In this SAKOE cell three centrioles were detected at the spindle pole (red, yellow, and orange arrows). (F) A graph showing the percentage of WT (white bars), SAKOE (red bars), DSas-4 (light gray), and DSas-4,SAKOE (dark gray) pupae that formed between the 5th and 10th day of development. More than 95% of SAKOE pupae eclosed as adults, which is similar to WT controls. Scale bar (A and B) = 10 µm; (D and E) = 0.5 µm.

Figure 2. Mitosis in Cells with Extra Centrosomes

(A–F) Immunostaining of WT (A, C, and E) and SAKOE (B, D, and F) mitotic neuroblasts with D-PLP (left, green in merged panels), α-tubulin (2nd panel, red in 4th panel), Cnn (3^rd panel, red in 5^th panel), and DNA (blue in merged panels). In the WT prophase cell (A), the arrow highlights the dominant centrosome that contains more PCM and nucleates more microtubules (MTs). In the SAKOE prophase cell (B), seven centrosomes are present, but there is no single dominant centrosome. In the WT metaphase (C) and anaphase (E) cells, a centrosome is located at each pole of the spindle. In the SAKOE metaphase (D) and anaphase (F) cells, several centrosomes are clustered at the poles of each spindle (arrowheads) while others are not (arrows). The centrosomes that are not clustered at the spindle poles appear to contain less Cnn and are not associated with robust astral MTs (compare this to the situation in the prophase cell shown in B). Scale bar = 10 µm.

Figure 3. Mad2 and Ncd Are Required for the Suppression of Spindle Multipolarity

(A–C) Immunostaining of mad2,SAKOE mitotic neuroblasts with D-PLP (left, green in merged panels), α-tubulin (2nd panel, red in 5th panel), Cnn (3rd panel, red in 6th panel), and DNA (blue in merged panels). For comparison with WT controls see Figures 2A and 2E. Tripolar anaphases (A), anaphases with lagging chromatids (arrow in B), and polyploid cells (C) are often seen in mad2,-SAKOE brains but very rarely in SAKOE neuroblasts (see Table 1). Note that mad2,SAKOE neuroblasts can also contain much larger numbers of extra centrosomes than is ever seen in SAKOE cells (~30 in the cell shown here). (D and E) Immunostaining of ncd,SAKOE mitotic neuroblasts with D-PLP (left panel, green in merged panels), α-tubulin (2nd panel, red in 4th panel), Cnn (3rd panel, red in 5th panel), and DNA (blue in merged panels). For comparison with WT control neuroblasts see Figures 2C and 2E. In ncd,SAKOE neuroblasts multipolar metaphases can be detected (D) but almost all anaphases are bipolar (E). Scale bar = 10 µm. (F) A graph showing the percentage of WT (white bars), SAKOE (red bars), mad2 (light blue), mad2,-SAKOE (dark blue), ncd (bright green), and ncd,SAKOE (dark green) pupae that formed between the 5th and 10th days of development. Note that the WT and SAKOE data shown here are the same as that shown in Figure 1A, as these experiments were performed at the same time.

Figure 4. Asymmetric Cell Division Is Perturbed in Neuroblasts with Extra Centrosomes

(A–D) Immunostaining of WT (A) and SAKOE (B–D) neuroblasts with D-PLP and aPKC (left panel, green in merged panels), α-tubulin (2nd panel, red in 4th panel), Cnn (3rd panel, red in 5th panel), and DNA (blue in merged panels). In WT metaphase neuroblasts (A) the spindle is always aligned with the polarity axis (defined by the cortical aPKC crescent), as is the case in ~60% of SAKOE neuroblasts with extra centrosomes (B). In ~40% of SAKOE neuroblasts with extra centrosomes (C and D), however, the spindle fails to align properly with the aPKC crescent; spindle misalignment is also detected in some anaphase cells (D). (E) Quantification of mitotic spindle alignment in WT and SAKOE neuroblasts that have extra centrosomes. In the schematic diagram of cells, the green crescent represent aPKC while the black bars represent the mitotic spindle. Scale bar = 10 µm.

Figure 5. Symmetric Divisions in SAKOE Neuroblasts

(A–C) The dynamics of Jupiter::GFP (Jup::GFP) in living WT (A) and SAKOE (B and C) neuroblasts. WT neuroblasts always divide asymmetrically to produce two cells of different sizes. In neuroblasts with extra centrosomes both asymmetric (B) and symmetric divisions (C) are observed. (D–F) Quantification of the number of neuroblasts in the central brain region. Immunostaining of WT (D) and SAKOE (E) brain lobes with Miranda (red) and Hoechst for labeling DNA (blue). (F) A graph showing the average number of neuroblasts in WT (white bars) and SAKOE (red bars) central brain lobes. Scale bar (A–C) = 5 µm. Error bars represent standard deviation (SD).

Figure 6. Transplantation of SAKOE Brain Tissue Induces Tumor Formation in WT Hosts

(A–C) Pictures of WT hosts transplanted with either Tub-GFP,WT (A), Tub-GFP,brat (B), or Tub-GFP,SAKOE (C) brains at 14 days post-injection. (A) The GFP fluorescence of the WT brain is still detectable in the host, but the brain has not detectably overproliferated (arrow). The GFP fluorescence of the brat (B) and SAKOE transplanted brains (C) has expanded to fill the abdomen of the host. (D) Picture of a GFP-positive metastasis in the eye of a WT host. (E) A table showing the frequency of tumor and metastases formation in WT hosts transplanted with the different types of brain tissue.