We Study Structural Maintenance of Chromosomes (SMC) family of protein complexes
Genetics, Electron, Light and Super-Resolution Microscopy, Biochemistry, Cell & Molecular Biology Techniques
Protein complexes of the Structural Maintenance of Chromosomes (SMC) family mediate changes in the higher-order chromatin structure by forming inter- or intra-chromatin fiber connections at distinct regions. Yet, neither the nature of the higher-order chromatin structure nor the molecular basis of its maintenance by SMC complexes is well understood. Eukaryotic cells contain three distinct SMC complexes: cohesin, condensin and Smc5/6. Our laboratory is exploring the function and molecular mechanism of these complexes in budding yeast, Saccharomyces cerevisiae. The elegant simplicity of the yeast model system combined with the advanced genetic, biochemistry, cellular and molecular biology techniques in this organism has been well-established in many studies. The yeast model's translational value in research lies in the high conservation between yeast and humans in processes governing genome stability, cell proliferation, gene expression, and DNA damage responses.
The basic architecture of SMC complex
Cohesin is best known for its role in sister chromatid cohesion, and has been proven essential for their equal segregation during mitosis and meiosis. In addition, cohesin functions in chromosome condensation, meiotic chromosome structuring, post-replicative DNA repair, and in the regulation of gene expression. Cohesin's wide range of activities spans fundamental cell processes essential for cell growth and development. The pivotal roles of cohesin, together with its medical relevance, place this complex at the forefront of current biological research. The core complex of cohesin contains four subunits: Smc1, Smc3, Mcd1/Scc1/Rad21 and Scc3/Irr1/Sa/Stag that form a ring structure. The auxiliary factors Pds5 and Wpl1 associate with cohesin but these factors are not considered part of the core complex. Chromosome condensation is an essential prerequisite for the faithful segregation of genetic information, and is therefore crucial in maintaining genome integrity during mitosis and meiosis. The SMC complex condensin plays an important role in this process. Similar to cohesin, condensin is composed of five subunits: Smc2, Smc4, Brn1/CAP-H, Ycg1/CAP-G1 and YCS4/CAP-D2, which are organized into a ring structure.
We are studying how this sophisticated molecular machines mediate higher-order chromatin structures and how their activity is regulated at different biological contexts.
Mitotic chromosomes from uninfected and HCV infected human liver cells. HCV infection induces chromosome hypercondensation.
SMC coplexes and human diseases
Chromosomal instability (CIN) is a classical hallmark and driving force of tumorigenesis. In normal tissues, cellular mechanisms are firmly regulated, through coordinated efforts of many cellular factors, to prevent neoplastic transformation and tumorigenesis. In contrast, accumulation of genetic alterations can drive a normal cell into a pre-cancerous state. Misregulation of oncogenes, leading to altered cell proliferation and development, is another key factor in the progression of tumorigenesis. The molecular basis of CIN in cancer cells is still elusive. Aiming to identify the molecular basis of CIN massive parallel sequencing has been used to identify mutations in tumors. The cohesin protein complex has been emerged from these experiments as a key factor in tumorigenesis. Furthermore, cohesin was recognized as an important factor in cancer treatment and patients' survival rate. However, the role and mechanisms of cohesin in cancer etiology and its impact on treatment are poorly understood. Our research is aimed to dissect the molecular mechanisms in which mutations in cohesin affect tumorigenesis.
Cornelia de Lang Syndrome (CdLS)
Cornelia de Lange syndrome (CdLS) is an autosomal dominant or X-linked genetic disorder with an estimated occurrence of 1 in 10,000 live births. Classic CdLS is characterized by intellectual disability and developmental retardation. CdLS is associated with mutations in the cohesin protein complex. Current pre-and post-natal diagnosis is based on clinical features. Mutations in cohesin-related genes are associated with the disorders but the molecular basis of the disorder is still unknown in 35% of CdLS patients. The molecular diagnosis is challenging due to the sporadic nature of the mutations and the mild pre-natal abnormalities. We are using advanced genomics and proteomics technologies to identify new mutations in CdLS patients and explore the affected molecular pathways that lead to the development of the disorder.
Hepatitis C virus
Hepatitis C virus (HCV) infection is the leading cause of chronic hepatitis, which often results in liver fibrosis, cirrhosis and hepatocellular carcinoma (HCC). HCV possesses an RNA genome and its replication is confined to the cytoplasm. Yet, infection with HCV leads to global changes in gene expression, and chromosomal instability (CIN) in the host cell. The mechanisms by which the cytoplasmic virus affects these nuclear processes are elusive. Our lab e showed that HCV modulates the function of the Structural Maintenance of Chromosome (SMC) protein complex, cohesin. We demonstrate that infection of hepatoma cells with HCV leads to up regulation of the expression of the RAD21 cohesin subunit and changes cohesin residency on the chromatin. These changes regulate the expression of genes associated with virus-induced pathways. Furthermore, siRNA downregulation of viral-induced RAD21 reduces HCV infection. During mitosis, HCV infection induces hypercondensation of chromosomes and the appearance of multi-centrosomes. We provide evidence that the underlying mechanism involves the viral NS3/4 protease and the cohesin regulator, WAPL. Altogether, our results provide the first evidence that HCV induces changes in gene expression and chromosome structure of infected cells by modulating cohesin.