טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentYanku Yifat
SubjectThe roles of dHectH9, a Hect Ubiquitin Ligase during
Drosophila Development
DepartmentDepartment of Medicine
Supervisor Professor Amir )Oryan )Orian


Abstract

HectH9 is a member of the HECT family of ubiquitin ligases that mediates ubiquitylation of key proteins such as the p53 and Myc. Yet, its role in development of multi-cellular organisms and specifically with regards to Myc's function during development is less clear.

My broad aim was to characterize the roles of the Drosophila melanogaster ortholog of HectH9, dHectH9, in salivary gland development.

Specifically I:

-       Characterized the roles of dHectH9 during the process of endoreplication.

-       Studied the interactions between dHectH9, dMyc and p53 in the context of salivary gland and larval development.

-       Determined the putative role(s) of dHectH9 in the regulation of developmentally-regulated cell death of the salivary gland.

Toward these aims I developed diverse genetic and molecular tools that enabled me to manipulate the levels of dHectH9.

My major findings are:

1. I find that dHectH9 is essential for salivary gland development and endoreplication. Inactivation of dHectH9 using dHectH9RANi results in failure of endoreplication. The cells within the salivary glands are characterized with small heterogeneous nuclei and they accumulate dense chromatin along the nuclear envelope. These effects are similar but not identical to the phenotype observed upon targeting dMyc using dMycRNAi, which results in small homogenous cells and nuclei. Interestingly, in dHectH9-targeted cells dMyc protein level is unaffected and dMyc is transcriptionally active.

2. dHectH9 is required to inhibit premature cell death during developmentally-regulated tissue remodeling processes. dHectH9 over-expression suppresses experimentally-induced cell death, while its targeting results in ectopic and premature salivary gland death.

3. Phenotypes associated with loss of dHectH9 are suppressed by either over-expression of dMyc or by co-reduction of dmP53. Since both proteins are substrates of HectH9 in vertebrates, these observations point toward possible feedback control mechanisms between dHectH9, dMyc and dmP53. 


Taken together, I suggest that dHectH9 may serve as a survival factor in replicating cells during development and differentiation. Furthermore, dHectH9 enables cells to cope with oncogenic stress such as the high level of dMyc during replication and to evade dMyc-induced cell death. I suggest that targeting dHectH9 under these conditions tilt the equilibrium towards cell death in the presence of functional dmP53.

Since Myc protein level is highly elevated in many cancers, my findings suggest that targeting HectH9 in P53 wild-type tumors will be a strategy to “force” Myc-dependent cancers to undergo cell death. Furthermore, our findings also suggest that HectH9 is a potential inhibitor of cell death.