Gene expression analyses in otefin mutant Drosophila ovaries

LEM Domain proteins are key components of the nuclear lamina. Mutations in LEM-D proteins cause dystrophic diseases associated with compromised adult stem cells, yet it remains unclear how LEM-D proteins support stem cell function. Studies described here use the homologue of the LEM-D protein emerin in Drosophila, Otefin (Ote) as a model to understand LEM-D protein function in adult stem cells. Loss of Ote causes female sterility due to a complex germline stem cell (GSC) phenotype that includes both an early block in germline differentiation followed by GSC death. In vivo cell cycle analysis revealed that ote mutant GSCs display a lengthened S phase.We find that loss of the DNA Damage Response (DDR) Chk2 is able to not only rescue the lengthened S phase, but also GSC death and the block in germline differentiation. Activation of detrimental checkpoint in absence of Ote is conserved in both male and female GSCs and surprisingly occurs independent of detectable canonical DDR triggers, including transposon de-repression and DNA damage. Two defects were found to occur upstream of Chk2 activation: nuclear lamina morphological defects and altered heterochromatin organization. Together, our data identify the primary cause for a compromised adult stem cell population in the absence of a LEM-D protein. Loss of otefin protein results in significant morphological defects in Drosophila ovary, with near complete loss of germline cells, making direct comparison of transcriptional profiles in Ote- and Ote+ ovaries challenging. We found that heterozygous loss of Chk2 suppresses early defect in germline differentiation in otefin mutants, thereby allowing generation of ote- egg chambers morphologically indistinguishable from wild type. Therefore, to identify transcriptional effects of ote loss, we generated two recombinant chromosomes carrying otefin mutant Pk allele in combination with one of the two independently generated Chk2 null alleles P6 (Pk-P6) or P30 (Pk-P30). The flies carrying the resulting double mutant chromosome over balancer CyO, containing wild type copies of both Chk2 and ote, were labeled as heterozygous (HET); the flies carrying double mutant chromosome over another otefin mutant allele G are otefin null and were labeled as such (NULL). The experiment compared gene expression in young virgin female ovaries dissected from HET and NULL flies; four biological replicates were analyzed for each group.