Proficiency and mechanisms of perturbation of mature T-cell homeostasis by the TCL1 family of oncogenes

In the first part of this thesis, the oncogenic potential of TCL1A family genes was comparatively evaluated by using gamma-retroviral vectors to introduce human TCL1A, MTCP1, and TML1 into hematopoietic stem cells/hematopoietic progenitor cells (HSC/HPC) of wild type mice that were transplanted into wild type recipients. TCL1A and MTCP1 recipient mice predominantly developed B-cell malignancies after a median survival of 388 days and 394 days, respectively. The presented data indicates that TCL1A and MTCP1 are oncogenes with comparable oncogenic potential and shows for the first time that MTCP1 is not only a T-cell oncogene, but is able to transform B cells as well. The third family member TML1 induced the development of immature T-cell malignancies in only a few mice. This study provides first evidence for its oncogenic function. Additionally, the transforming potential of compartment-targeted TCL1A variants was evaluated by retroviral expression of a membrane localizing myristoylated (myr-TCL1A) and a nuclear localizing (nls-TCL1A) variant. Recipients of HSC/HPC transduced with myr-TCL1A and nls-TCL1A predominantly developed B-cell malignancies after a median survival of 360 days and 349 days, respectively. There was a significantly shorter latency period for nls-TCL1A compared to the previously described generic TCL1A. Gene expression analysis revealed higher similarities between expression profiles of tumors induced by TCL1A and nls-TCL1A. Together these data implicate that TCL1A’s predominant oncogenic function might rely on its nuclear presence. The second part of this thesis aims to understand if and how TCR stimulation affects the transforming potential of TCL1A. Mature OT-1 T cells carrying monoclonal TCR’s that specifically recognize ovalbumin (OVA) were retrovirally transduced with TCL1A and repeatedly stimulated in vivo with OVA-peptides. TCR stimulated recipient mice of TCL1A transduced T cells showed a significantly accelerated leukemic outgrowth and a reduced median survival of 305 days, when compared to unstimulated recipients (417 days). These data strongly implicate a pro-leukemogenic cooperation of TCL1A and TCR signals that might be actionable in upcoming interventional designs.

Neurodegeneration and the m-AAA protease: pathogenic cascades in neurons and myelinating cells

The m-AAA protease is a hexameric complex involved in processing of specific substrates and turnover of misfolded polypeptides in the mitochondrial inner membrane. In humans, the m-AAA protease is composed of AFG3L2 and paraplegin. Mutations in AFG3L2 have been implicated in dominant spinocerebellar ataxia (SCA28) and recessive spastic ataxia-neuropathy syndrome (SPAX5). Mutations of SPG7, encoding paraplegin, are linked to hereditary spastic paraplegia. In the mouse, a third subunit AFG3L1 is expressed. Various mouse models recapitulate the phenotype of these neurodegenerative disorders, however, the pathogenic mechanism of neurodegeneration is not completely understood. Here, we studied several mouse models and focused on cell-autonomous role of the m-AAA protease in neurons and myelinating cells. We show that lack of Afg3l2 triggers mitochondrial fragmentation and swelling, tau hyperphosphorylation and pathology in Afg3l2 full-body and forebrain neuron-specific knockout mice. Moreover, deletion of Afg3l2 in adult myelinating cells causes early-onset mitochondrial abnormalities as in the neurons, but the survival of these cells is not affected, which is a contrast to early neuronal death. Despite the fact that myelinating cells have been previously shown to survive respiratory deficiency by glycolysis, total ablation of the m-AAA protease by deleting Afg3l2 in an Afg3l1 null background (DKO), leads to myelinating cell demise and subsequently progressive axonal demyelination. Interestingly, DKO mice show premature hair greying due to loss of melanoblasts. Together, our data demonstrate cell-autonomous survival thresholds to m-AAA protease deficiency, and an essential role of the m-AAA protease to prevent cell death independent from mitochondrial dynamics and the oxidative capacity of the cell. Thus, our findings provide novel insights to the pathogenesis of diseases linked to m-AAA protease deficiency, and also establish valuable mitochondrial dysfunctional mouse models to study other neurodegenerative diseases, such as tauopathies and demyelinating diseases.