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© 2016 by Philip Jordan

Functions of the SMC5/6 Complex

 

SMC5/6 is the least studied SMC protein complex. Our lab is using conditional knockout alleles of Smc5 to delineate the role of the SMC5/6 complex using four different platforms:

1) Spermatogenesis, 2) Oogenesis, 3) Embryonic stem cells and 4) Neurodevelopment. Using these approaches, we are defining the roles of SMC5/6 during mitotic and meiotic cell cycle progression. We are also determining the protein interaction network of the SMC5/6 complex.

 

 
 

Infertility affects 15% of couples worldwide. One major cause of infertility is chromosome missegregation during gametogenesis (formation of sperm and egg). The frequency of chromosome segregation errors increases as women age, especially after the age of ~35, which results in dramatically increased incidence of miscarriage, birth defects and developmental abnormalities. Despite the correlation between age and chromosome errors, in many cases the genetic causes of infertility are unknown. Our lab has found that the SMC5/6 complex is required for accurate chromosome segregation  during female gametogenesis and is essential for fertility.

Conditional knockout (cKO) of Smc5  during  meiosis I in oocytes results in chromosome missegregation

Smc5  cKO leads to aneuploidy, loss of sister chromatid cohesion (red arrows) and chromosome structures with abnormal morphology (yellow arrow)

Conditional knockout (cKO) of Smc5  during  meiosis I in oocytes results in chromosome missegregation

 

Pluripotent stem cells (PSCs) will pass genetic material to their progeny, and preservation of genomic stability is crucial. Given ongoing clinical trials in vision restoration and proposed future applications of PSC-derived tissue-specific cells, it is of high importance to obtain extensive knowledge of genome preservation in PSCs.

We determined that SMC5/6 localizes to the chromatin and is enriched at the pericentromeric heterochromatin. 

       

Using Smc5 conditional knockout mice, we have established mouse embryonic stem cell (mESC) lines and demonstrated that SMC5/6 complex is imperative for mESC genome maintenance. In the absence of SMC5, mESCs undergo massive apoptosis.

 

Detailed studies revealed that SMC5/6-deficient mESCs cannot proliferate due to abnormal mitosis, characterized by formation of DNA bridges, presence of lagging chromosomes, and, as a result, undergo catastrophic missegregation.

These results establish the crucial roles of SMC5/6 protein complex in mESC survival, maintenance of genome integrity and cell cycle progression.

This work is published here: Pryzhkova and Jordan, 2016, Journal of Cell Science, 129: 1619-1634.

 

SMC5/6 complex functions during mammalian development are poorly investigated. However, recent studies from our  lab have shown that the SMC5/6 complex is required for normal embryonic development. Abrogation of SMC5/6 protein complex functions during development leads to microcephaly and dwarfism.

We depleted expression of Smc5 in neural cells. Analysis of adult (55 days) and newborn mice (0-1 day) revealed significant decrease in brain size, specifically, underdeveloped cerebral cortex. Sagittal sections of adult brain stained with hematoxylin and eosin (H&E) demonstrate thinning of cortex in Smc5 cKO mice compared to control animals. However, immunofluorescence staining of histological sections (IHC) of newborn brains shows cortical layer formation similar to control mice. These data suggest that cKO of Smc5 primarily affects neural stem cell proliferation and causes the depletion of neural progenitors in the developing embryo.

 

DNA Repair and Cancer

SMC mutations are associated with many forms of cancer, including myeloid leukemia and neuroblastoma, as well as colorectal and breast cancers. In somatic cells, the SMC5/6 complex is involved in several processes required to maintain genomic stability. These processes include DNA replication, DNA repair, homologous recombination (HR), chromosome topology and chromosome segregation.

SMC5/6 localizes to sites of DNA damage. In the image above, DNA damage is induced via laser-induced irradiation.

Conditional knockout (cKO) of Smc5 results in increased RAD51 foci indicative of an inability to repair DNA damage via HR. Thus, depletion of the SMC5/6 complex results in genome instability.