My primary interest is to decipher the molecular mechanism of tumor suppression using the C19ORF5-mediated RASSF1A tumor suppression as a model. Epigenetic silencing of RASSF1A by hypermethylation of promoter-specific CpG-island occurs at high frequency in human tumors.  This, induction of cell death and reduction in anchorage-independent growth and tumorigenicity by overexpression in cultured cells, and increasing incidence of tumorigenesis in knockout mice suggest that RASSF1A is a tumor suppressor. RASSF1A acts as a microtubule stabilizing factor and regulates assembly of mitotic spindles. Similar to microtubular stabilizer paclitaxel and docetaxel that are among the most effective cancer chemotherapeutic drugs, RASSF1A causes microtubule hyperstabilization, mitotic disruption and specific recruiting of C19ORF5 on stabilized microtubules when it reaches high levels.

C19ORF5 is a ubiquitously-distributed homologue of the neuron-specific microtubule-associated protein MAP1A and MAP1B, and similarly interacts with LC3, the mammalian homologue of autophagic marker ATG8.  Autophagy, the major pathway for degradation of large bulk of macromolecules and organelles, is a process initiated with the formation of LC3-associated isolation membrane that engulfs substrates to form autophagosomes which finally fuse with lysosomes for degradation.  When the substrates are mitochondria, the process is called mitophagy.  C19ORF5 also interacts with mitochondrion-associated protein LRPPRC whose mutation leads to French Canadian variant of Leigh Syndrome (LSFC), a disorder characterized with neurodegeneration and a cytochrome c oxidase deficiency.  LRPPRC interacts with the Parkinson disease-related PARK2 gene product Parkin and acts as a checkpoint to control autophagic initiation and a signal to direct which mitochondria will be sequestered for autophagic turnover.  The full length and heavy chain of C19ORF5 proteins regulate autophagic initiation and maturation in a noncanonical pathway for removal and recycle of dysfunctional organelles such as endoplasmic reticulum and mitochondria and misfolded proteins under homeostatic condition or in response to nutritive stress.  Meanwhile, a short form of C19ORF5 accumulates on hyperstabilized spindle microtubules during mitotic arrest and blocks mitophagic initiation leading to mitochondrial collapse and aggregation along with disrupted mitotic spindle, which trigger mitotic cell death.

Although cells depend on mitochondria for their energy supply, they fall victim to worn-out mitochondria if they are not eliminated by autophagy.  Defective mitochondria may trigger reactive oxidative stress causing tumor-promoting cell damage and genomic instability.  At the right time in mitosis, blocked mitophagy may contribute to prevention of tumorigenesis at its earliest origin by induction of mitochondria-induced mitotic cell death.  Positive regulation of general autophagy and specific negative regulation of mitophagy prevents inheritance and amplification of aneuploidy which is being given increasing attention as an underlying factor in the continuous evolution of genetic flexibility in tumors.  We have discovered that products of C19OPRF5 gene play roles in the suppression of DNE-induced hepatocarcinomas in mice.  The RASSF1A-C19ORF5-LRPPRC complex may suppress tumorigenesis through autophagic regulation. 

Since autophagy is also important regulatory process in the post-mitotic cells such as cardiomyocytes and neurons.  We found that the swollen mitochondria accumulate between disorganized enlarged myofibrils of C19ORF5 knockout mice.  The cardiac myofibrils of the C19ORF5 knockout mice exhibit impaired sarcomere contraction.  Similar to its interactive protein RASSF1A, C19ORF5 may regulate cardiac hypertrophy. 

The connection to neurodegenerative diseases such as Parkinson’s disease and Alzheimer’s disease is suggested by the LRPPRC-associated LSFC and interaction of LRPPRC with Parkin. The connection to Alzheimer’s disease is reinforced by the interaction of C19ORF5 with NR3A-containing NMDA receptor and GluR2-containing AMPA receptor.  We believe that deficiency in the C19ORF5-regulated autophagy causes accumulation of dysfunctional mitochondria and disruption of learning and memory processes, and finally triggers neurodegeneration and neuronal cell death.

Currently, we are seeking to characterize the mechanisms by which C19ORF5 regulate (1) dynamics of microtubules and mitochondria, (2) control autophagy, and (3) suppress tumorigenesis through autophagic regulation.  The experimental approach will use C19ORF5 knockout mice carrying a transgenic autophagic marker GFP-LC3 and their primary hepatocyte cultures as models for biochemical, cell biological and cancer biological studies. The ultimate goal is to understand the general mechanism of tumor suppression and develop strategies to prevent cancers at their origin and terminate cancer relapse after therapy in general using the hepatoma as a prototype.