GIST with PDGFR Alpha Mutations
GIST Support International posed questions about GISTs caused by mutations in the PDGF receptor alpha (PDGFRα) to Abhijit Ramachandran. Mr. Ramachandran is the Project Manager of AROG Pharmaceuticals, LLC. He began his career as a Research Analyst at AROG's sister company Dava Oncology LP and was involved in accelerating clinical trials. Later he became involved in the in-licensing of crenolanib (CP-868,596) from Pfizer to AROG in April, 2010. Then he moved to AROG as a project manager and has been involved in development of crenolanib, a small molecule PDGFR inhibitor. He manages the pre-clinical and clinical development strategy for crenolanib, which is currently being evaluated in clinical trials for patients with D842V mutated GIST and in patients with glioma. He holds a M.S. in Biomedical Engineering with a focus on de-novo gene sequencing technology and cancer therapeutics from the University of Texas at Arlington.
Below is a summary version of the answers to GSI's questions, shortened by Marina Symcox, PhD. For additional details, please see the longer version prepared by Mr. Ramachandran.
1. What are the roles of PDGFRα and PDGFRβ in normal tissues?
Platelet-Derived-Growth-Factor receptors stimulate the growth of various cell types, particularly in connective tissues. They are part of a larger family of related proteins called “receptor tyrosine kinases.” Two separate forms of the receptor are PDGFRα (PDGFR alpha, sometimes abbreviated as PDGFRA) and PDGFRβ (PDGFR beta, sometimes abbreviated as PDGFRB). These receptors, located at a cell’s outside membrane, become activated after binding to a group of growth factors secreted into the tissue, collectively named “Platelet-Derived-Growth-Factor.” Upon activation, the receptors trigger a cascade of chemical events inside a cell that support its growth.
The growth factor PDGF, which controls the receptor’s function, was originally found in platelets and normally plays a role to facilitate wound healing. The receptor PDGFRα is expressed in certain nervous system cells, and it is required during early embryonic development. The receptor PDGFRβ is expressed in cells which form the walls of blood vessels. PDGFRβ plays an important role in angiogenesis (formation of blood vessels).
2. Are abnormalities in PDGF receptors linked to GIST or other human diseases?
Too many pro-growth signals from the PDGF receptors can cause diseases including several forms of cancer. In some diseases, the PDGF receptors are too abundant on a cell’s surface so that their collective signal is exceedingly strong. Alternatively, excessive amounts of the growth factor PDGF can be released into the tissue, leading to inappropriately high activation of the receptors. These mechanisms are observed in Ewing sarcoma and glioma. PDGFRβ is important for angiogenesis, and it is overexpressed in the supportive tissue of a number of solid tumors, including breast cancer, ovarian cancer and lung cancer leading to tumor progression.
In addition, DNA damage can occur in the genes that encode instructions for making the PDGF receptors. Even if the receptors are not overexpressed, the gene mutations may result in defective receptors with abnormal behaviors. A small percentage of gastrointestinal stromal tumors (5-7%) harbor mutations in the DNA gene for PDGFRα. In GISTs, the PDGFRα protein made from its damaged gene transmits continuous signals for cell growth even when growth factors are absent. The result is uncontrollable signaling for growth that supports tumor development.
3. Please compare PDGFRα-mutant GIST versus KIT-mutant GIST.
PDGFRα and PDGFRβ share extensive physical similarities with KIT, another receptor tyrosine kinase that when abnormal promotes development of GIST. Nevertheless, the PDGF receptors are distinct from KIT in important ways. Their specific roles in the body are different than KIT’s. PDGF receptors and KIT do not respond to the same set of growth factors.
The majority of GIST cases (>80%) express mutated KIT, whereas 5-7% have mutated PDGFRα, and the remainder (15%) of tumors have no mutations in either KIT or PDGFRα. Primary GIST tumors will harbor a mutation in one of the two receptor genes but not both, since each is an alternative path leading to similar consequences of uncontrollable cell growth.
Recent studies have demonstrated that although there are similarities between PDGFRα-mutant and KIT-mutant GISTs at the molecular level, a number of differences exist between these tumors. GISTs with PDGFRα mutations occur almost exclusively in the gastric (stomach) location. Most of these tumors typically show "epithelioid" morphology and lower rates of cell division. Often, PDGFa-mutant GISTs follow more indolent clinical courses. The gene expression profiles of PDGFRα-mutant GIST cluster separately from the KIT-mutant types.
The PDGFRα gene mutations occurring in GIST are usually point mutations and deletions. They are often analogous to the mutations observed for the KIT receptor, though the “exon” numbering system that designates location is slightly different for each receptor.
Figure 1. Mutations of KIT or PDGFRα found in GISTs are localized to certain exons (exons 8, 9, 11, 13 or 17 in KIT; 12, 14, or 18 in PDGFRα).
4. What is the significance of the PDGFRα D842V mutation in GIST? Is there an analogous mutation in KIT linked to human disease?
The D842V mutation in exon 18 accounts for approximately 60% of all PDGFRα mutations known in primary GISTs. This mutation confers primary resistance to commercially available tyrosine kinase inhibitors (TKIs). Furthermore, other GISTs with primary mutations responsive to TKIs may acquire a secondary D842V mutation during the course of therapy, and these too become resistant.
The D842V mutation in PDGFRα is homologous to the D816V mutation in KIT. KIT D816V is not observed in primary GIST tumors but emerges during the course of TKI therapy as an acquired secondary mutation causing resistance to imatinib and sunitinib. Both mutations affect the same aspartic acid residue that is conserved in analogous regions of PDGFa and KIT receptors, suggesting a common structural basis for the resistance.
5. What is the current clinic management protocol for D842V-mutant GIST?
Surgery remains the treatment of choice for localized GISTs. Recurrent or inoperable GISTs require systemic drug therapy with a TKI. However, patients with GISTs harboring the PDGFRα D842V mutation will likely experience early resistance to imatinib and sunitinib.
Patients who have inoperable D842V-mutant GIST may opt to participate in clinical trials, or to consult with an academic expert sarcoma center.
6. Why is the PDGFRα D842V mutation resistant to the standard TKIs?
The D842V mutation may interfere with the movement of the receptor’s “activation loop,” a small structural loop that swings physically in “open” or “closed” positions as a mechanism to control the receptor’s signaling activity. The activation loop lies close to the site where imatinib binds. The D842V mutation may reduce the access of imatinib to its binding site.
7. What are emerging experimental strategies that may benefit patients with PDGFRα D842V-mutant GIST?
The investigational drug crenolanib (CP-868,596) is a highly selective and potent inhibitor of both PDGFRα and PDGFRβ, which is 100-500 times more potent in inhibiting PDGFRα and PDGFRβ than imatinib or sunitinib. Crenolanib has demonstrated preclinical activity against both primary and secondary D842V mutations in experiments conducted by Dr. Michael Heinrich at Oregon Health Sciences University. A phase II clinical trial (NCT01243346) is designed to evaluate the antitumor efficacy of crenolanib in GISTs harboring the PDGFRα D842V mutation.
Investigational monoclonal antibodies such as IMC-3G3 recognize and bind to the extracellular region of PDGF receptors, the receptors parts which project outside of a cell. The actions of antibodies against the PDGFRα should not be affected by the D842V mutation since the antibodies do not bind near D842V mutation, which is located in the region of the receptor that extends to the inside of a cell.
Other strategies include drugs targeting pathways downstream from PDGFRα such as inhibitors of PI3K/mTOR, or drugs that accelerate the physical destruction of PDGFRα such as HSP90 inhibitors.
8. When should a patient suspect that he/she may have a PDGFRα D842V-mutant GIST?
PDGFRα D8423V-mutant GIST can be suspected at multiple levels, including initial resistance to imatinib and sunitinib.
- Anatomically, approximately 90% of PDGFRα-mutant GISTs arise in the stomach, whereas GISTs harboring KIT mutations occur throughout the GI tract.
- PDGFRα-mutant GISTs typically display epithelioid cell morphology, while most KIT-mutant GISTs have a spindle cell shape.
- While KIT (CD117) expression is strong in most KIT-mutant tumors, up to 40% of PDGFRα-mutant tumors are weak or negative for KIT expression.
Direct DNA sequencing is required to confirm the D842V mutation. Several different types of mutations have been observed in exon 18 where the D842V mutation lies. The appropriate treatment varies according to the exact identity of the mutation, since not all mutations within exon 18 are wholly resistant to standard TKIs. Some available mutational methods such as denaturing HPLC provide information about where a mutation is located (i.e. the exon location), but they do not directly test for what the mutation is. DNA sequencing explicitly characterizes the exact nature of the mutation.