Martha Brown

Martha Brown received her Ph.D. in Microbiology from Queen’s University. Having spent several years working with insect viruses as a graduate student, she was introduced to human adenoviruses as a postdoctoral fellow at the University of Sherbrooke and brought that expertise when she came to Toronto to continue postdoctoral work at the Hospital for Sick Children.

Since joining U of T in the Department of Microbiology, which later merged with the Department of Medical Genetics to form the current Department of Molecular Genetics, Martha has been very active in undergraduate education, having served as Undergraduate Coordinator for the Microbiology program as well as the Molecular Genetics program. She continues to be active in teaching multiple lecture and laboratory courses at the undergraduate and graduate level.

Her teaching and course development extends to high school students in her role as Academic-in-Charge of the Microbiology module in the Youth Summer Program. In May 2014, she received the Faculty of Medicine Excellence in Undergraduate Life Sciences Laboratory Teaching Award. 

Martha has been Biosafety Coordinator for the Medical Sciences Building, then Acting Biosafety Chair for U of T and currently serves as University Biosafety Coordinator for work involving viruses and viral vectors. She recently completed a three year term as a member of the Adenovirus Study Group, an international group of virologists with responsibility for issues pertaining to the identification and appropriate classification of adenoviruses.

Martha’s research focuses on human enteric adenoviruses and on development of antiviral agents to treat serious adenovirus infections. 

Research Synopsis

My lab studies the human fastidious enteric adenoviruses (HAdV-40 and 41), with a primary focus on their unique structural features and the role of these features as determinants of gut tropism. Related projects include the development of HAdV-40/41 as vaccine vectors and the study of antiviral agents for treatment of adenovirus infection.    

My lab, located in the Medical Sciences Building, is one of the smaller labs in the department, with one or two graduate students and one or two undergraduate project students or summer students. Much of the work involves cell culture and infection with human adenoviruses (Biosafety level 2).  There is access to flow cytometers and fluorescence microscopes (widefield and confocal) as well as both transmission and scanning electron microscopes.

Viruses are intricate structures which have profound effects on their host cells simply by binding to them. The events that follow binding are mediated by the host cell in response to cues from viral proteins. 

The major focus of my research is the human enteric adenoviruses types 40 and 41 (HAdV40/41) which are important agents of pediatric gastroenteritis, characterized by highly restricted tropism for small intestinal epithelium in the natural host. Their overall structure is the same as that of other human adenoviruses but there are key differences which affect binding to host cells and events subsequent to binding.

The growth restriction in non-intestinal tissues in vivo is reflected by fastidious growth characteristics in cell culture, relative to other adenovirus serotypes. Partial blocks at multiple steps in the virus replication cycle have been identified; these affect transcription of the E1 (early) region, virion assembly and release of progeny virions from infected cells. The block in E1 transcription can be complemented in 293 cells which express the E1 proteins of HAdV-5 but, even in these cells, infectivity of HAdV-40/41 is orders of magnitude lower than that of HAdV-5. 

Generation of monoclonal antibodies against HAdV-40/41 and mapping of epitopes drew attention to virion structure and early virus-cell interactions as determinants of the restricted gut tropism of HAdV-40/41. Specifically, we identified an epitope unique to enteric adenoviruses, on protein VI, an internal protein known to interact with both the major capsid protein (hexon) and the DNA core, to stabilize the virion structure. The epitope maps to an “extra” stretch of amino acids, without a counterpart in protein VI of other adenoviruses.

This work suggested that the extra stretch of amino acids might interact with hexon in such a way as to increase stability of the virion (enabling it to survive conditions en route to target cells in the intestine) but compromising its ability to uncoat and deliver its genome within non-target cells. Others have shown, with HAdV-5, that endosomal conditions lead to exposure of protein VI which disrupts the endosomal membrane for release of the virion into the cytoplasm. Our hypothesis predicted that HAdV-40/41 virions would be trapped within endosomes. When we studied early virus-cell interactions (using 293 and lung epithelial cells) by tracking fluorescently labeled virions, it was clear that virions did not accumulate at the nucleus as seen for other adenoviruses. More detailed examination of thin sections of cells by electron microscopy identified an unexpected block, though incomplete, in uptake of HAdV-41 virions from the cell surface. The distribution of virions within pits at the cell surface has also raised questions as to the pathway used by those virions which are taken into the cell. Whereas HAdV-5 and HAdV-35 (used as controls) associated mostly with clathrin coated pits, HAdV-41 associated mostly with caveolar pits and to a lesser extent with large non-clathrin coated pits; very few HAdV-41 virions were found in clathrin coated pits (Leung and Brown, 2011).          

Experiments are in progress to characterize the entry pathways used by HAdV-40 and HAdV-41 in 293 cells and in Caco-2 intestinal epithelial cells (representative of natural target cells of HAdV-40/41).  Host factors needed for successful virion entry/genome delivery will be identified. Other experiments involve exposure of virions to conditions that would be encountered following ingestion, en route to target cells in the small intestine.  A third project involves recombineering for introduction of a reporter gene into the HAdV-40 and HAdV-41 genome and for modification of structural proteins to study the effects of these manipulations on virion uptake and genome delivery. Ultimately, we should be able to determine how structural features of HAdV-40/41, specialized for replication in the intestine, govern binding and subsequent events leading to genome delivery.

A related interest is in development of HAdV-40 and/or HAdV-41 as a vector for intestinal delivery of desired genes. HAdV-40 /41 have the potential to be effective vaccine vectors, if not vectors for gene therapy.

In contrast to HAdV-40/41, which typically do not cause disease outside the intestine, other adenoviruses can be a serious threat, especially to immunocompromised hosts.  HAdV-5, which normally causes upper respiratory tract infections, can cause fatal disseminated infection following bone marrow transplant.  Other serotypes cause epidemic keratoconjunctivitis, a serious condition that can result in blindness, even in hosts without underlying immune problems.  Recently, we have begun a project to evaluate antiviral agents and their mechanism of action for inhibition of adenovirus.

Selected Publications

Yea C, Dembowy J, Pacione L and Brown M. Microtubule-mediated and microtubule-independent transport of adenovirus type 5 in HEK293 cells. Journal of Virology 81 (13), 6899-6908 (2007).

Leung, T.K-H and Brown, M.  Block in entry of enteric adenovirus type 41 in HEK293 cells. (2011)  Virus Research 156, 54–63.