Proto oncogenes . Dr. Abhinav Golla , Associate Professor , Lab Director & Consultant Pathologist . Aadhya Medicure Pathlabs .
- 1. PROTO-ONCOGENES By Dr.Abhinav Golla MBBS,MD Pathology Medicure Diagnostics & research center, vijayanagar colony , Hyderabad,Telangana
- 2. • Proto-oncogenes : Normal cellular genes whose products promote cell proliferation • Oncogenes : Mutated or over expressed versions of proto-oncogenes that function autonomously having lost dependence on normal growth promoting signals • Onco-proteins : A protein encoded by an oncogene that drives increased cell proliferation
- 3. Growth Factor Receptor Growth factor Proteins in cytoplasm Cell cycle Cell cycle regulators
- 4. • Proto oncogenes have multiple roles but all participate in signalling pathways that drive proliferation. • Proto oncogenes may encode growth factors, growth factor receptors , signal transducers, transcription factors or cell cycle components. • Proto oncogenes oncogenes oncoproteins (constitutively active) mutation
- 5. GROWTH FACTORS • Normal cells require stimulation by growth factors to proliferate. • Most soluble growth factors paracrine • However some cancer cells autocrine • Examples : Glioblastoma (PDGF and PDGFR) Many sarcomas ( TGF – α and EGFR)
- 6. • In tumors in which an autocrine loop is activated, the growth factor gene is usually normal but • The signals transduced by other oncoproteins cause overexpression and increased secretion of growth factors. • This causes initiation and amplification of the autocrine loop.
- 7. GROWTH FACTOR RECEPTORS • A large number of oncogenes encode growth factor receptors of which receptor tyrosine kinase is one of the most important one. • They are transmembrane proteins with an Extracellular ligand binding domain and A cytoplasmic tyrosine kinase domain
- 9. • Receptor tyrosine kinases can be constitutively activated by many mechanisms like Point mutations Gene rearrangements Gene amplifications
- 10. • Point mutations – ERBB1 is a proto oncogene that encodes EGFR. Point mutations in ERBB1 causes constitutive activation of EGFR seen in a few Lung Adeno Ca. • Gene amplifications – ERBB2 encodes HER2. ERBB2 amplification is seen in certain breast carcinomas leading to over expression of HER2 receptor and constitutive tyrosine kinase activity.
- 11. • Gene rearrangements : Activate other receptor tyrosine kinases. • Eg : Deletion of a part of chromosome 5 Fusion of part of ALK gene with part of EML4 gene Chimeric EML4 ALK protein (constitutive tyrosine kinase activity)
- 12. SIGNAL TRANSDUCTION PROTEINS • Receptor tyrosine kinase activation stimulates RAS and two major downstream signalling arms 1. The MAPK cascade 2. PI3K/AKT pathway • These pathways are frequently involved by the gain of function mutations in different types of cancer.
- 14. • RAS MUTATIONS - Point mutations in RAS family of genes constitute the most common abnormality involving proto oncogenes. • 15 – 20% of all human tumours express RAS mutation. • RAS genes are of 3 types – HRAS,KRAS,NRAS • RAS proteins are members of a family of membrane associated small G proteins. • Mutations markedly reduce the GTPase activity of RAS.
- 16. • MUTATIONS IN BRAF : BRAF is a serine/threonine protein kinase. • Activating mutations in BRAF stimulate the downstream kinases and ultimately activate the transcription factors. • These mutations are seen in 100% of Hairy cell leukaemias, >60% of melanomas, 80% of benign nevi and to a smaller extent in a wide variety of neoplasms.
- 17. • MUTATIONS IN PI3K FAMILY OF PROTEINS : • PI3K is a heterodimer with regulatory and catalytic subunits and has several tissue specific isoforms. • It is recruited by activated receptor tyrosine kinase. • It activates a cascade of serine/threonine kinases including AKT which is a key signalling node. • AKT has many substrates like mTOR,BAD,FOXO.
- 18. Pi3k AKT mTOR BAD FOXO PROTEIN & LIPID SYNTHESIS (PRO APOPTOTIC) (PRO APOPTOTIC) ENHANCE CELL SURVIVAL
- 19. • PI3K is negatively regulated by an important braking factor PTEN. • Alterations in virtually all components of the PI3K/AKT pathway have been found in various cancers. • PI3K(gain of function) and PTEN(loss of function) are the most commonly mutated ones. • PI3K mutations generally affect the catalytic subunit and result in increased enzymatic activity.
- 20. • ALTERATIONS IN NON RECEPTOR TYROSINE KINASES : • Mutations that confer oncogenic activity occur in several non receptor tyrosine kinases also. • They can be chromosomal translocations or rearrangements. • They create fusion genes encoding constitutively active tyrosine kinases. • Eg : ABL tyrosine kinase
- 21. • Many different oncogenic tyrosine kinases consist of fusion proteins in which non tyrosine kinase partner drives self association. In this example BCR promotes self association which is sufficient to unleash the tyrosine kinase activity of ABL.
- 22. • Oncogene addiction : The tumour cells are highly dependant on the activity of one or more oncogenes. • For example despite accumulation of mutations in other cancer associated genes in CML cells, signalling through BCR – ABL tyrosine kinase is required for most tumour cells to survive and proliferate. • Hence inhibition of its activity is a highly effective therapy.
- 23. • Non receptor tyrosine kinases can also be activated by point mutations that evade the negative regulatory check of the enzymes. • Eg : JAK2 mutation • Activating point mutations in JAK2 relieve the normal dependence of hematopoietic progenitors on growth factors like erythropoietin. • Disorders associated with JAK2 mutations are several myeloproliferative disorders like polycythemia vera, essential thrombocytosis and primary myelofibrosis.
- 24. TRANSCRIPTION FACTORS • All the signal transduction pathways converge on the nucleus where expression of the target genes advance the cell through mitotic cycle. • The ultimate consequence of deregulated signalling pathways is inappropriate and continuous stimulation of transcription factors. • So , growth autonomy can also occur due to the mutations of transcription factors that regulate the expression of pro growth genes • Transcription factors of this class include the products of MYC, MYB, JUN, FOS, REL proto oncogenes.
- 25. • MYC : It is present in virtually all eukaryotic cells. • It belongs to immediate early response genes that are rapidly and transiently induced by RAS/MAPK pathways. • Several SNPs that are strongly linked to elevated risk of prostate and ovarian cancers fall within a large region devoid of recognizable genes adjacent to MYC on chromosome 8. • These genetic variants alter the function of enhancer elements that regulate the function of MYC expression
- 26. Chromosome 8 MYC SNPS in this location are linked to elevated risks of ovarian and prostate cancer
- 27. MYC WARBURG EFFECT UPREGULATES TELOMERASE CYCLIN D REPROGRAM SOMATIC CELLS UPREGULATING rRNA ENHANCES ASSEMBLY OF RIBOSOMES FOR PROTEIN SYNTHESIS CANCER CELLS STEMNESS CELL CYCLE PROGRESSION BUILDING BLOCKS ENDLESS REPLICATIVE CAPACITY
- 28. • MYC can be deregulated by a variety of mechanisms like Genetic alterations of MYC MYC translocations – Burkitt’s lymphoma and few B & T cell tumours MYC amplifications – breast colon and lung cancers Oncogenic mutations involving components of upstream signalling pathways elevate MYC protein levels. • Thus constitutive RAS/MAPK, NOTCH, WNT, HEDGEHOG signalling can all transform cells in part through upregulation of MYC.
- 29. CELL CYCLE REGULATORS • Orderly progression of the cells through cell cycle is regulated by cyclins and cyclin dependant kinases(CDKs). • The Cyclin-CDK complexes phosphorylate crucial target proteins that drive cells forward through cell cycle. • While cyclins activate CDKs, there are several CDK inhibitors that exert a negative control over cell cycle.
- 30. • There are two main cell cycle checkpoints. 1. At G1/S transition and 2. G2/M transition. • These are tightly regulated by a balance between growth promoting and growth supressing factors as well as by sensors of DNA damage. • When activated, these DNA damage sensors arrest cell cycle progression and if cell damage cannot be repaired initiate apoptosis.
- 31. • Defects in G1/S checkpoint are more important in cancer as they lead to dysregulated growth and mutator phenotype. • Mutations that affect G1/S checkpoint can be broadly divided into 1. Gain of function mutations of cyclin D and CDK4 oncogenes that promote G1/S progression. 2. Loss of function mutations in tumour suppressor genes that inhibit G1/S progression.
- 32. TAKE HOME MESSAGE • Proto-oncogenes may encode growth factors, growth factor receptors, signal transducers, transcription factors, or cell cycle components. • The corresponding oncogenes generally encode oncoproteins that serve functions similar to their normal counterparts, with the important difference that they are usually constitutively active.
- 33. • GROWTH FACTORS :Glioblastomas, sarcomas • GROWTH FACTOR RECEPTORS: 1)Point mutations – A subset of Lung Adenocarcinomas 2)Gene amplification – Breast carcinoma 3)Gene rearrangement – Lung cancer • SIGNAL TRANSDUCTION PROTEINS: 1)RAS mutations - Pancreatic adenocarcinoma, cholangiocarcinoma 2)BRAF mutations – Hairy cell leukaemia, melanomas 3)PI3K mutations – Breast carcinoma 4)Non receptor tyrosine kinase – CML
- 34. • TRANSCRIPTION FACTORS: 1)MYC translocation – Burkitt’s lymphoma 2)MYC amplification – Breast colon and lung cancers • CELL CYCLE REGULATORS: 1) Gain of function mutations in CyclinD and CDK4 – Lymphomas, melanomas and sarcomas 2) Loss of function mutations in tumour suppressor genes – Pancreatic carcinomas, Glioblastoma, ALL