After I was made aware of this finding & commented on it, Dr. Chada & Dr. D’Armiento graciously gave of their time to break down the science for me as well as answer a few questions I had posed with a series of emails & a few calls.
This is the first half of the questions I posed to them after reading through the research paper, picking it apart, talking with a few friends & leaders to connect a few dots, & then comparing my prior learnings in research to date to connect the dots to this finding.
I hope that the introduction I posted yesterday was helpful in simplifying some of the terminology.
For ease, I tried to group similar questions & responses together. I had at least 16 questions that came out of this study. So the first half of those are below. The rest will follow tomorrow.
Q: Could I please ask for a one paragraph summary from you on what this protein does in the body & how it relates to tumor formation in LAM & TSC?
A: HMGA2 is a protein that activates other genes (a transcription factor). The protein is expressed normally in undifferentiated, mesenchymal cells and when HMGA2 is no longer expressed, the mesenchymal cells become differentiated. Our earlier studies demonstrated that HMGA2 causes mesenchymal tumor formation (lipomas, uterine fibroids, for example) when HMGA2 is expressed in differentiated, mesenchymal cells. LAM results from the proliferation of mesenchymal cells that are similar to other mesenchymal tumors where HMGA2 is found to be expressed.
Q: For those of us who don’t have a science background what are transcription factors? How do they work in the body? Are they in genes or chromosomes or how do they originate?
A: Transcription factors are genes (which are the primary template that encode the sequence for their specific protein). The HMGA2 protein (encoded by the HMGA2 gene) functions by activating on other genes & so it is better to think of the HMGA2 pathway rather than HMGA2 specifically.
Q: For those who may not understand can you briefly describe what tuberin is? For those with sporadic LAM without TSC is tuberin a factor? If so, how does it express itself in sporadic cases versus those of us with TSC?
A: Tuberin is the protein encoded by the TSC2 gene & functions to prevent cell proliferation. From our studies, in both sporadic and TSC LAM, tuberin is present in 100% of all LAM samples and so the two hit hypothesis as stated does not apply to LAM.
Q: Can you explain a little bit more about this HGMA protein & where it originates? Does it express first in the kidneys or lungs & then show up in other cells and organs later? Or is this something you’ll still need to research & hope to discover in researching this new pathway?
A: HMGA2 is a transcription factor, which activates other genes. Only a couple of its downstream genes are known. It is of the utmost importance to identify other components of the pathway since they are also putative drug targets. HMGA2 is expressed in undifferentiated, mesenchymal cells of most tissues.
Q: For people who may not understand what haploinsufficiency is, could you please just provide a very basic definition of it? Explain how it works in LAM & TSC & then tie that back to the study findings?
A: Haploinsufficiency is when only one of the two alleles of a gene is mutated. Our findings state that haploinsufficiency at TSC2 increases the incidence of tumor INITIATION but it is NOT necessary for mTOR activation to occur for tumor FORMATION to happen. HMGA2 expression is required for tumor formation & we have to still investigate whether HMGA2 is required for tumor initiation.
Q: Also from those of us who have a basic familiarity with the mTOR pathway & how it works, where is this protein in relation to it? I know it is completely not dependent & separate from mTOR. But is it upstream or downstream from the pathway? Or is it completely outside of it?
A: With respect to our studies, there appears to be no direct relationship or interaction between the mTOR pathway and the HMGA2 pathway in LAM. We do show that in all of the tumors formed in the TSC mouse and in TSC patients (angiomyolipomas & LAM) HMGA2 is expressed.
Q: For those of us who may or may not be familiar with it can you briefly & simply describe the two hit hypothesis in LAM & TSC? How does this study contradict those findings?
A: The two hit hypothesis describes the process by which the activity of tumor suppressor genes (those genes which when expressed, suppress tumor formation) are lost. Each gene has two copies (or alleles) and a single copy is present on each of one of a pair of chromosomes. The two hit hypothesis in LAM states that a mutation in one of the two TSC alleles will arise from an unknown mechanism. This mutation on the DNA will often result in a short or no protein being produced from this mutated allele. This is the first hit. However, we still have the second TSC allele. The second hit of the second hit hypothesis states that the other TSC allele is deleted and so there will be no TSC protein from this allele. Since both alleles are not expressing TSC protein, the cell no longer is making TSC tumor suppressor protein and therefore tumor formation will occur. A prediction of this hypothesis would be that the TSC protein should be absent in LAM cells. In this study we found that in 29/29 LAM cases, the TSC protein (tuberin) was PRESENT. So LAM tumorigenesis occurs despite the presence of the TSC tumor suppressor protein and based on this study the two hit hypothesis is not functioning in LAM.
Q: This study also seems to contradict the widely held theory that LAM is metastatic, meaning it originates in one part of the body and then may break off & spread to another part, similar to what we see in cancer. Did you have a hunch about this before that it wasn’t metastatic or did the study simply yield a result that led to this conclusion.
We have always believed that LAM was not a metastatic disease. Clinically there is no evidence that the cells behave in a metastatic fashion. The groups of mesenchymal tumors, which LAM belongs to, are BENIGN. The evidence for metastasis in LAM is paltry & the analysis implies that tuberin should be absent in LAM cells. In 29/29 cases, tuberin was found to be present.
*Please stay tuned for more tomorrow.*