Dr. Sagar Sengupta

Title: Determining the signalling and repair pathways that are altered during genomic instability in nucleus and mitochondria

Theme of Research:

A. Development of therapeutics against colon cancer detection:
Colon cancer is one of the most common type of cancer in India. A major issue for colon cancer treatment is the recurrence. Recurrence occur due to the development of chemoresistance to the drugs used to treat the primary colon cancer. Using a unbiased screening procedure and multiple mice models we have identified three small molecules (which are FDA) we have identified three molecules which can act to prevent chemoresistance and allow the chemotherapeutic drugs like cisplatin, camptothecin and oxaliplatin to optimally function (Kaur et al., Journal of Clinical Investigation, 2024). Efforts are being made to use these small molecules for adjunct therapy in cancer treatments.

There is a unmet need of colon cancerdetection methods, especially in the early stages. We discovered six microRNAs (which were named DNA damage sensitive microRNAs or DDSMs) which were upregulated only in colon cancer cells. Analysis done with more than 410 patient information in TCGA and the biopsy materials of 54 patients from AIIMS, New Delhi indicated found that the DDSMs were all upregulated even in Stage I coloncancer tissues and not in the surrounding normal tissues(Priya et al., Journal of Cell Science, 2021). This microRNA based signature is being taken forward as a potential biomarker for coloncancer detection.

B. Determine how tumor suppressors recognize the damaged DNA: 
Human cells are exposed to multitude of DNA damage every day. Our work has tried to understand how DNA damage was recognized by the cells so that it could be repaired by either of the two main repair pathways – homologous recombination and non-homologous end joining. We showed that cells have overlapping yet distinct processes to recognize different types of DNA damages. Ubiquitylation of tumour suppressor BLM by chromatin associated E3 ligases, interactions with process specific proteinsand a biphasic mode of recruitment to the site of damage which was dependent on cell cycle – were some of the key determinants of this process (Tikoo et al., The EMBO Journal, 2013; Tripathi et al., Nature Communications, 2018).

C. Understanding the role of tumor suppressors in the repair of damaged DNA:
Our work had provided evidence that a phosphorylation-dependent interaction between several key proteins involved in the repair pathway allows cells to repair the damaged DNA by homologous recombination (Tripathi et al., Journal of Cell Biology, 2007; Carcinogenesis, 2008; Kaur et al, Molecular Cancer Research, 2010). Subsequently we have provided evidence how the chromatinorganization is acted upon by chromatin remodelers in cells to allow better accessibility of the factors which would repair the damaged genome (Srivastava et al., Journal of Cell Science, 2009).

D. Dissecting the role of protein turnover during cancer development:
We have tried to understand the inter-regulation between the tumour suppressors and the E3 ligases during cancer development. We have demonstrated how a well-known E3 ligase in colon cancer development, called Fbw7a, is allowed to optimally recognize its oncogenic substrates like the oncoproteins c-Myc and c-Jun (Chandra et al., Journal of Cell Science, 2012; Priyadarshini et al., Cell Reports, 2018). Further also determined the mechanism degradation of two tumour suppressors by Fbw7a, thereby allowing the potential development of a non-degradable versions which can resistturnover and thereby be active for a longer duration (Kharat et al., Oncogene, 2016; Tripathi et al., Journal of Biological Chemistry, 2019).

E. Identifying and characterizing the functions of a new mitochondrial helicase:
Apart from the nucleus, mitochondria have its own genetic material. We identified a new mitochondrial helicase, RECQL4. We identified how this helicase goes to the mitochondria, interacts with different mitochondrial proteins and affects the mitochondrial replication process (De et al., Journal of Cell Science, 2012). Further we provided mechanistic evidence how RECQL4 enhanced the functions of the sole mitochondrial polymerase, PolgA (Gupta et al., Carcinogenesis, 2014). Lack of such a functional interaction led to more accumulation of DNA damage in mitochondrial DNA which contributed to the initiation of cancer (Kumari et al., Journal of Cell Science, 2016).

F. Understanding the mechanism of the entry of proteins into mitochondria:
For long time mitochondrial protein entry was thought to be dependent only on the presence or absence of mitochondrial localization signal (MLS). We recently provided evidence that apart from MLS, there existed another mechanism for mitochondrial entry. Ubiquitylation of two proteins involved in mitochondrial replication, PolgA and RECQL4, by a specific E3 ligase(called MITOL) on the outer surface of the organelle prevented their entry. This post-translational modification which allowed only the non-ubiquitylated PolA  and RECQL4 to enter the organelle. We demonstrated this phenomenon contributed to the mitochondrial disorders, namely autosomal dominant progressive external ophthalmoplegia (adPEO) and Rothmund Thomson Syndrome (RTS).Manipulation of the ubiquitylation status of PolgA and RECQL4 mutants reverted the process allowing the functional reactivation of these proteins (Hussain et al., PLoS Biology, 2021).

Awards:
1. 1991 - Silver medallist in M.Sc., University of Calcutta, India.
2. 1997 - Best thesis award, Department of Microbiology and Cell Biology, Indian Institute of Science, Bangalore, India.
3. 2011 - National Bioscience award for Career Development by Department of Biotechnology, India
4. 2011 - Elected as a member of Guha Research Conference
5. 2015 - Elected Fellow of the National Academy of Science (NASI)
6. 2017 - Elected Fellow of the Indian Academy of Sciences (IASc)
7. 2017 - Elected Fellow of the Indian National Science Academy (INSA)
8. 2018 - J C Bose Fellowship
9. 2021 - Professor Vishwa Nath Memorial Award Lecture by INSA
10. 2022 - Elected Vice President of Indian Association of Cancer Research
11. 2023 - GK Manna Memorial Award Lecture by All India Congress of Genetics and Genomics
12. 2023 - Lalji Singh Memorial Award Lecture by the Society for Mitochondrial Research & Medicine
13. 2024 - J C Bose Fellowship (2nd term)

Collaborators:
Arnab Mukhopadhyay (National Institute of Immunology, New Delhi, India)
Avinash Bajaj (Regional Centre for Biotechnology, Faridabad, India)
Patrick Legembre (Universite de Rennes, Rennes, France)
SamudralaGourinath (Jawaharlal Nehru University, New Delhi, India)
RituKulshreshtha (Indian Institute of Technology Delhi, New Delhi, India)