Drosophila lethal giant larvae neoplastic mutant as a genetic tool for cancer modeling

Abstract

Drosophila lethal giant larvae (lgl) is a tumour suppressor gene whose function in establishing apical-basal cell polarity as well as in exerting proliferation control in epithelial tissues is conserved between flies and mammals. Individuals bearing lgl null mutations show a gradual loss of tissue architecture and an extended larval life in which cell proliferation never ceases and no differentiation occurs, resulting in prepupal lethality. When tissues from those individuals are transplanted into adult normal recipients, a subset of cells, possibly the cancer stem units, are again able to proliferate and give rise to metastases which migrate to distant sites killing the host. This phenotype closely resembles that of mammalian epithelial cancers, in which loss of cell polarity is one of the hallmarks of a malignant, metastatic behaviour associated with poor prognosis. Lgl protein shares with its human counterpart Human giant larvae-1 (Hugl-1) significant stretches of sequence similarity that we demonstrated to translate into a complete functional conservation, pointing out a role in cell proliferation control and tumorigenesis also for the human homologue. The functional conservation and the power of fly genetics, that allows the researcher to manipulate the fly genome at a level of precision that exceeds that of any other multicellular genetic system, make this Drosophila mutant a very suitable model in which to investigate the mechanisms underlying epithelial tumour formation, progression and metastatisation. In this review, we will summarise the results obtained in these later years using this model for the study of cancer biology. Moreover, we will discuss how recent advances in developmental genetics techniques have succeeded in enhancing the similarities between fly and human tumorigenesis, giving Drosophila a pivotal role in the study of such a complex genetic disease.

Keywords: Drosophila; Hugl-1; Lethal giant larvae; animal model.; epithelial cancers; tumour suppressor.

Fig. (1)

Upper panel: wild-type and l(2)gl⁴ wing imaginal discs. The wild type disc at 5 days after egg laying (AEL) ceases proliferating and starts to differentiate into the adult wing; on the contrary, l(2)gl⁴ disc never exits cell cycle. Lower panel: morphological parallelism between lgl mutant progression and a mammalian epithelial cancer development. The anterior (A) and posterior (P) compartments of the wing discs are independent developmental units and cells belonging to either of the two never intermix in a wild type disc [10].

Fig. (2)

Schematic representation of proteins involved in the establishment of the apical-basal cell polarity in an epithelial cell. The explanation is in the text. AJ: Adherens Junction; SJ: Septate Junction.

Fig. (3)

Drosophila and human Lgl-1 proteins structure.

Fig. (4)

a: Flp-FRT system [38]: the site-specific yeast recombinase Flippase (FLP) mediates recombination of pericentromeric FRT sequences. If these sequences are located at the same position on homologous chromosomes, one carrying a mutant allele and the other carrying the visible cell marker on the same chromosomal arm, the consequence of the recombination event would be the formation of two daughter cells: a cell homozygous for the mutation and a wild type cell. These cells and their progeny are clearly distinguishable as the first completely lacks the cell marker while the second is marked with twice the intensity with respect to the background. b: UAS-Gal4 system [39]: in this binary system a line contains the yeast Gal4 transactivator proteins that is expressed under the control of a given promoter (in this case, tubulin promoter) and the second line contains the transgene of interest under the control of UAS cassettes, yeast enhancers specifically recognized by Gal4. In the progeny of the cross between those two lines, both elements are present in the cells so that the transgene will be transcribed under the control of the chosen promoter. c: MARCM system [40]: it is a combination of the Flp-FRT and UAS-Gal4 systems, with the addition of another yeast element: Gal80 repressor. Through this system, the presence of Gal80 either in one or two copies blocks Gal4 function, so that only in the homozygous mutant cell it will be possible to express any number of UAS-transgenes.

Fig. (5)

Scheme of a Drosophila egg chamber. The oocyte and the nurse cells are of germinal origin, while the follicular cells form an epithelial sheet of somatic origin coating the oocyte. The apical and basal polarity of the follicular cells is also indicated.