DNA Damage Response and Cellular Senescence in a Zebrafish Model of Costello Syndrome

Cristina Santoriello1, Gianluca Deflorian1, Federica Pezzimenti1, Koichi Kawakami2, Luisa Lanfrancone1 , Fabrizio d’Adda di Fagagna1, Marina Mione11

1) IFOM-IEO Campus, Milan, Italy, 2)Division of Molecular and Developmental Biology National Institute of Genetics, Mishima Shizuoka, Japan

  Costello syndrome (OMIN #218040) is a rare human genetic disease due to germline activating mutations in the gene H-RAS. We generated a transgenic zebrafish model of the disease and used it to study how activating H-RAS mutations cause the Costello syndrome. Transgenic fish that constitutively express low levels of oncogenic H-RAS (HRASV12) show several hallmarks of the Costello syndrome, but do not display overt signs of canonical Ras target activation. We investigated the occurrence of oncogene-induced senescence (OIS) in tissues of juvenile homozygous transgenic fish. We found reduced proliferation and increased senescence associated (SA) βgalactosidase (βgal) activity in the brain, heart and other tissues, in association with an increased expression of the tp53 target cdkn1a/p21. Analysis of markers of an activated DNA damage response showed that, in tissues with reduced proliferation, a number of cells belonging to pools of local proliferating progenitors cells/stem cells, displayed γH2AX and pATM nuclear staining .
  To modulate the levels of expression of H-RASV12, we generated an additional, inducible transgenic line. Five days after induction of H-RASV12 expression, transgenic larvae developed a phenotype resembling Costello syndrome and, following initial hyperproliferation, showed proliferation arrest and increased DNA damage responses in the brain and other organs. The arrest of proliferation was completely prevented by tp53 knock down.
  Our findings suggest that constitutive expression of low levels of oncogenic H-RAS during vertebrate development leads to DNA damage followed by the activation of a senescence program in adult proliferating cells/stem cells of the brain and other tissues, which, besides explaining some of the pathological traits of Costello syndrome, may provide new hints for controlling oncogenic responses in stem cell populations.