What’s up with factor XII?

XIIbHere’s a great coag question from a Path Student reader:

Q. I am wondering why Factor VII deficiency causes significant bleeding problems but Factor XII deficiency does not. One source I found stated that this is because the extrinsic pathway is the primary pathway in vivo and that the intrinsic pathway is of lesser importance.

However, the more I thought about it, I then wondered why Hemophilia A and Hemophilia B are so severe since they are involved in the intrinsic pathway. Why doesn’t the extrinsic pathway just pick up the slack and allow the patient to remain asymptomatic as it seems to with Factor XII deficiency?

A. It turns out that factor XII is pretty important in vitro, but not in vivo. In the test tube, XII is activated by contact factors (like HMWK), and then XIIa catalyzes the conversion of XI to XIa. In the body, though, the main thing that converts XI to XIa is thrombin (not XIIa). I don’t even include XII in my drawing of the cascade for my students, since it’s of no clinical consequence. The intrinsic side is complicated enough!

With regard to the extrinsic and intrinsic pathways: both are critical for fibrin formation in vivo. I wouldn’t say that either one is of lesser importance – they just do different things.

In vivo, the cascade starts on the extrinsic side with TF showing up and binding to VIIa. The TF-VIIa complex converts X to Xa, and things proceed from there. The weird thing is that as soon as a little Xa is made, the extrinsic pathway is turned off (by tissue factor pathway inhibitor, or TFPI)!

So then what? By this point, you already have a little thrombin around – and that thrombin goes and kicks off the intrinsic pathway. Thrombin converts XI to XIa, which converts IX to IXa, which – together with VIIIa – converts X to Xa.

Bottom line

The cascade starts with the extrinsic pathway, but that pathway gets shut off very quickly. Thrombin activates the intrinsic pathway, which proceeds to convert fibrinogen to fibrin until you need to turn it off.

How to remember which genes are tumor suppressors vs. proto-oncogenes

molecular neoplasiaHere’s a question that I got by email yesterday and it’s such a good one that I want to share it with everyone.

Q. I love love LOVE your blog and your daily emails, and your book Clot or Bleed saved my butt for studying for my hematology exam. I was just wondering – do you have a good mnemonic or know of an easy way to remember which cancer gene mutations are proto-oncogenes and which are tumor suppressors? 

A. Thanks for letting me know you found the book and other stuff helpful! I’m so glad to hear that.

I don’t have a mnemonic for these genes (if anyone does, please comment below!). However, I think the best way to remember these is to learn what each gene product does (because then you’ll know whether it’s a proto-oncogene or a tumor suppressor gene).

For example: RAS encodes a signal-transducing protein associated with cell growth. It takes the signal from a growth receptor, and helps get that signal down to the nucleus so something can be done about getting the cell to grow. So it’s a growth-promoting gene (a proto-oncogene). If RAS is going to be a cancer-causing gene (an oncogene), it is going to have to be mutated in such a way that it is always turned on.

Here’s another example: the retinoblastoma (RB) gene. The RB gene product inhibits the cell cycle, turning off normal cell growth when necessary. So it is a tumor suppressor gene (this is a dumb name, but we can save that tirade for another time). If you’re going to cause cancer by mutating the RB gene, you’d have to mutate it in such a way that it doesn’t work well (and cells can just whiz through the G1/S checkpoint no problem). And actually, kind of like the brakes on your car, you typically need to mutate BOTH alleles of a tumor suppressor gene in order to cause tumors (if you just mutate one allele, you’ll still have some “brakes” left in the other allele).

Here is my favorite diagram (above) relating to this topic. It’s from Robbins, and it’s got all the important cancer-related genes (or at least the most important ones for you to learn) listed according to what their products do in the cell. Yay! You can see RAS up there just beneath the cell membrane, doing its job as a signal transducer. RB is down in the nucleus, acting as a cell cycle inhibitor.

A nice touch in this diagram is the color coding: all the red things are growth-promoting (so their genes would belong to the proto-oncogene category). Blue things are growth-inhibiting (so they would be encoded by tumor suppressor genes). The green things function as DNA repair mechanisms (the nice little scissors and hammer). If you look at this diagram long enough, you’ll start remembering which color things are – and if you freak out on a test, remembering the color just might get you grounded again.

I am sure that someone does have a snazzy mnemonic. But I figure that since you’re going to have to learn what these gene products do, you might as well just reason out which ones are proto-oncogenes vs. tumor suppressor genes, rather than try to memorize that list separately using a mnemonic.

It’s always best when you can get material to make sense! Cuts waaayyy down on the brute memorization – and also helps get the info into long-term memory. Maybe save the mnemonic strategy for stuff that you can’t reason out, like cranial nerve numbers, or clinical syndromes that don’t make sense, or well, pretty much all of micro.

Microcytosis and hypochromasia

IDA1

Q. What is the pathophysiology of microcytes in iron-deficiency anemia (IDA)? I mean I understand that hypochromasia is due to low hemoglobin content, but what makes the cells smaller? Is it something like first there is hypochromasia and then the cells shrink? Aren’t hypochromatic cells normocytic? Why don’t red cells keep shrinking as they become hypochromatic? Please help. The question is bothering me a lot. 🙂

A. First of all, you’re right: in IDA, the red cells do get smaller. Since the bulk of the red cell is composed of hemoglobin, the less hemoglobin there is in the cell, the smaller the cell volume, and the smaller the cell overall. So in iron-deficiency anemia, there is less iron around, and therefore less hemoglobin – which results in the cells being smaller than normal. Same thing happens in thalassemia: less hemoglobin around (though not because of iron, but because of a genetic defect in a hemoglobin chain), so the red cells are smaller.

Just to clarify: chromasia just refers to the amount of hemoglobin in the cell. Cells can be normochromic (as they are in normal blood), or hypochromic (as they are in IDA). The size of the red cell is measured separately from the chromasia. Normally-sized red cells are called normocytic, small ones are called microcytic, and large ones are called macrocytic.

You asked if hypochromic cells are normocytic – and for the reason stated above, the answer is no, they usually aren’t. They are usually microcytic, because there’s less hemoglobin in the cell, so the cell gets smaller.

Finally, to answer your last question, in iron-deficiency anemia, the red cells do keep shrinking as they become more and more hypochromic! Assuming the iron deficiency is a continuing problem, as each new wave of red cells is produced, there will be less and less iron around – and therefore cells will get smaller and smaller.

So when you look at a blood smear from a patient with IDA (like the one above), you’ll see some cells that are a little bigger (these are older red cells that were made when there was still a fair amount of iron around), and some that are a little smaller (these are newer red cells, made when the iron level had dropped). Check out the two cells in the center of the image: both are hypochromic, but the one in the center is about twice as big as the one to its left.

This is why you can use the RDW to help differentiate between IDA and mild-moderate thalassemia!

What does phospholipid do in the PT and PTT?

phospholipid

Q. I have a quick question about coag lab tests. In the tests that you are adding phospholipid (like the PTT), what exactly is the phospholipid doing?

A. It’s just providing a surface for the coagulation factors to sit on! Many of the coagulation factors need a phospholipid surface to sit on in order to work.

Normally, the platelets provide that surface (they have phospholipids in their membranes) – but you’ve taken the platelets out of the test tube before you do the coagulation lab tests – so you need to add them back in if you want the whole cascade to run.

You also add phospholipid in the PT (INR)! It’s part of the thromboplastin molecule. Thromboplastin is just a tissue-factor-like substance plus phospholipid, all wrapped up in one reagent.

A few of the coag tests don’t require a phospholipid surface. The TT, for example, doesn’t need phospholipid; you’re just adding thrombin and seeing how fast it can convert fibrinogen to fibrin – and that single reaction doesn’t need phospholipid to work. Also, the fibrinogen assay doesn’t require phospholipid because it just measures the amount of fibrinogen.