MPNforum Magazine

The vocabulary of science

Reversing Fibrosis – the Ploidy Connection

Abstracts of scientific papers often seem baffling. Mainly, however, it’s just a question of vocabulary. A vocabulary list of 100 words and a handful of common prefixes and suffixes would take us through most of the literature relevant to MPNs. For the rest we have Google and Wikipedia. Those of us who are diligent caregivers quickly learn the most arcane terms because, like any foreign language the trick is to use it.

We can let others translate for us… but this MPN world is not a foreign land. It’s the country we inhabit and sometimes we need to read the road signs pointing to danger ahead.

So let’s start with a weird one: PLOIDY. It’s as specific and unusual a word we’re likely to find. And it’s central to a finding that came out of a study published this summer that studied the production and reversal of myelofibrosis in mouse populations,.

Science fiction, science fact

In our investigation of MPN therapeutic developments, it’s not often we come face to face with hard science. We encounter applied science all the time, in reports from fellow patients, talks by hematologists and papers by clinical investigators who have tested a drug in a controlled trial.

And yet the cure for our various myeloproliferative diseases resides in only two places – the oncology suites where stem cell transplant takes place and the labs where chemists, geneticists, and molecular biologists follow ideas through to exploration, animal studies and sometimes discovery. This is science stripped of its commercial endpoint.

As a result, most pure theoretical work escapes our attention until it surfaces in publication of a paper. Even then it’s beyond us until we can convert it to a narrative, a story. What follows is a story that follows publication* of a scientific study, “Control of Megakaryocyte Expansion and Bone Marrow Fibrosis by Lysyl Oxidase,” Katya Ravid, et. al., J. Biol. Chem. 2011 286: 27630–27638, June, 2011.

The study is scientific fact; this story introduces a little science fiction.

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An introduction to ploid and friends

There are one or more proteins, enzymes or enablers, whose presence plays a role in developing greater numbers of those large megakaryocytes (MKs) that spew fragments or platelets into the bloodstream.

One such enzyme is called LOX, lysysl oxidase. And its presence in large numbers signals trouble. When well behaved , LOX plays an important construction role in the body, stabilizing platforms outside the cell, building catwalks and girders. It’s a potent initiator of essential activity among various cell types. But LOX enzymes running wild can be a trouble-making gang rampaging through the blood production system.

LOX boosts creation of megakaryocytes (MKs) by oxidizing a communications cell element called Platelet Derived Growth Factor (PDGF, a tyrosine kinase). PDGF then flashes the downstream green light to ramp up production of megakaryoctyes.

Megakaryocytes give rise to platelets, necessary for clotting and other functions. It’s a blood cell. Most cells have two full sets of chromosomes and are diploid – one chromosome from mom, one from dad. Germ cells, sperm and ova, are haploid, single chromosomes. Then there are multiples of “ploidiness” on the road to to polyploidy, or lots of chromosomes in a single cell. In the case of megakaryocytes, like this guy, to be polyploid has some advantages. Since the platelet fragments it releases – as many as 3000 for a 32-ploid megakaryocfyte – have no need of a nucleus, no need to reproduce, it’s more efficient to skip all the sex apparatus in the cell and just produce one large cell with lots of chromosomes. It’s the normal development path for an MK.

Enter the LOX gang. They steer clear of swollen polyploidal megakaryocytes which are no longer good candidates for proliferation but they’re all over the diploid and quatroid MK’s. They pinch and twist their bodies, twist their arms, suggest how good it would feel to oxidize a cute little PDFG signal tower just down the road… and they better get on it if they know what’s good for them. MK’s do just that and all that turning on of signals spurs production of more and more MK’s and produces a flood of platelets.

For us, oblivious to all this action, one nasty result is the development of ET, essential thrombocythemia, with its attendant risks of thrombosis and mini-strokes. Another result is more deadly.

MK in bone marrow

All this proliferation produces mountains of reticulin fiber outside the cell which, being part of the blood producing system, resides withiin the marrow in the bone capsule. This kind of reticulin fibrosis when discovered in bone marrow biopsy, unless accompanied by blood production outside the marrow (extramedullary hematopoeisis), may not be considered clinically significant. This is the cellular phase of myelofibrosis: increased and clustered megakaryocytes and reticulin fibrosis. But once LOX descends on the fibers, the game changes.

LOX as construction crew

LOX enzymes are part of the biologic construction crew picking up work around the body, building support structures. LOX earns its living by working on the connective tissue crew where its job is to cross -link collagen and elastin to make a mucky substance that can join objects together. One of the ingredients of collagen is hydroxylysine which LOX extracts. These residues reside on the inner walls of bone and quickly form a dense matrix of collagen cross-linked fibers and occupy soft and open spaces in the bone’s inner core. Healthy blood cells are pushed aside and eventually die from lack of food and oxygen supply.

This is the fibrotic phase of myelofibrosis: collagenous fibrosis with the absence of normal marrow elements. This time we are well aware of the action taking place in the dark vault of our bones as we suffer clinical response, myeloid metaplasia, splenomegaly, hepatomegaly. And all the rest.

LOX contributes to proliferation not polyploidy

Katya Ravid’s team in her Boston University lab reasoned if LOX was going to be absent in polyploidy MK’s, it must be abundant in the diploid and tetraploid megakaryoctyes, candidates for proliferation. They used specially bred transgenic mice with low ploidy MK’s, high levels of Platelet Derived Growth Factor and extensive fiber. As suspected, megakaryoctyes from these mice showed very high levels of LOX. Treating the mice with a LOX inhibitor – BAPN, beta-aminopropionitrile — resulted in dramatic (37%) reduction of bone marrow fibrosis and decrease in megakaryocyte numbers.

The success of BAPN in reducing fibrosis implicates LOX in the progression of MF. Ravid concludes: “Our results suggest that LOX is a novel regulator of megakaryopoiesis via its effect on PDGFT-B and a potential treatment target for primary myelofibrosis.”

The final scene, a new actor

There’s one last part to this story, another actor: Thrombopoietin. TPO is the primary growth factor driving development of megakaryocytes – proliferation, polyploidization, maturation, and death. TPO acts through a receptor gene to signal that same PDGF signal tower targeted by LOX to call for more MKs

TPO thus applies a natural brake on LOX since TPO “significantly attenuates LOX expression…” Low doses of TPO administered to the transgenic mice result in low detection of LOX. The pathway is direct. TPO promotes the development of polyploidy MKs. LOX activity results in proliferation of megakaryocytes, not polyploidy . Since the LOX focus is on diploid MKs, simply increasing the number of polyploidy MKs decreases proliferation of megakaryocytes

The vocabulary may be new and strange…and yet, within our own marrow and at this very moment, the battle for polyploidy vs proliferation is being waged. The outcome may well significantly determine our fate.

Take me back.

© Zhenya Senyak and MPNforum.com, 2011. Unauthorized use and/or duplication of this material without express and written permission from this blog’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to Zhenya Senyak and MPNforum.com with appropriate and specific direction to the original content