Defining a Cluster of Differentiation, or CD

One of the goals of this blog is to introduce readers to some of the language of medicine. As much as jargon is sometimes unnecessary, sometimes the specificity and detail in medical terms aids precision.

So what is a cluster of differentiation, or CD?

In medical practice, the two-letter acronym specifies a molecule, or antigen, usually on a cell’s surface. In 1982, an international group of immunologists got together for the First International Workshop on Human Leukocyte Differentiation Antigens. The initial focus was on leukocyte (white blood cell) molecules. The goal was to agree on definitions of receptors and other complex proteins to which monoclonal antibodies bind, so that scientists could communicate more effectively.

A few examples of CDs about which you might be curious:

CD1 – the first-named CD; this complex glycoprotein is expressed in immature T cells, some B cells and other, specialized immune cells in the skin; there are several variants (CD1a, -b, -c…) encoded by genes on human chromosome 1.

CD4 – a molecule on a mature “helper” T cell surface; T lymphocytes with CD4 diminish in people with untreated HIV disease.

CD20 – a molecule at the surface of immature B lymphocytes that binds Rituxan, an antibody used to treat some forms of lymphoma, leukemia and immune disorders.

 

In this schematic, an antibody recognizes a specific molecule, or cluster of differentiation, at a cell surface.

The CDs were named (i.e. numbered) not necessarily by the order of discovery, but by the order of their being deemed as bona fide CDs by HLA Workshop participants. There’s a pretty good, albeit technical, definition in FEBS Letters, from 2009:

Cluster of differentiation (CD) antigens are defined when a surface molecule found on some members of a standard panel of human cells reacts with at least one novel antibody, and there is good accompanying molecular data.

Perhaps the best way to think about CDs is that they’re unique structures, usually at a cell’s surface, to which specific antibodies bind. By knowing the CDs, and by examining which antibodies bind to cells in a patient’s tumor specimen, pathologists can distinguish among cancer types. Another use is in the clinic, when oncologists give an antibody, like Campath – which binds CD52, the responsiveness might depend on whether the malignant cells bear the CD target.

Still, I haven’t come across an official (such as NIH), open-source and complete database for all the CDs. Most can be found at the Human Cell Differentiation Molecules website, and information gleaned through PubMed using the MeSH browser or a straight literature search.

Wikipedia is disappointing on this topic; the list thins out as the CD numbers go higher, and the external references are few. To my astonishment, I found a related page on Facebook. Neither makes the grade.

Where should patients get information about these kinds of things? Or doctors, for that matter?

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TV Meets Real Life Oncology, and Anticipating the MCATs

Yesterday I wrote on some tough decisions facing a TV show‘s protagonist. She’s got metastatic melanoma and might participate in a clinical trial when the show resumes.

Now imagine you’re an oncologist, or a real patient with this killing disease – you really need to be on top of new developments, to understand the pros and cons, because someone’s life depends on it.

If you’re the doctor in the relationship, you need keep abreast of current information for all the other tumors types of patients in your care: what are the new findings, if any, what are the limitations of the data. You need to know how the advances apply to an individual person who, most likely, has another condition or two, like high blood pressure or, say, osteoporosis.

Oncologists ought to be familiar with new drugs, and how those compare to old ones, and the side effects, and the distinctions between tumors with and without BRAF mutations. They should know what BRAF stands for.

Melanoma is one form of skin cancer. We understand now there are breast cancer subtypes – with distinct behavior and responsiveness to treatments, with and without inherited and acquired genetic mutations (BRCA-1 and -2 were identified well over 10 years ago; there’s much more known now), dozens of lymphoma forms and innumerable leukemia subtypes. Lung cancer, prostate cancer, brain cancer… Each is a group of diseases.

But the science physicians apply in their work doesn’t just apply in oncology. Even in traditionally “softer” fields of medicine, like pediatrics, doctors need to know how congenital diseases are diagnosed with newer, cheaper methods for testing mutations; in gynecology, doctors need to know about inherited clotting dispositions; in psychiatry, doctors give medicines with complex metabolic effects that involve, or should involve, some grounding in modern neuroscience.

This is why we need to keep the MCAT hard. (I’ll write more on this current issue in medical education, soon.)

Have a great weekend!

ES

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Back to Basics – But Which Ones?

A front-page story on the Humanities and Medicine Program at the Mount Sinai School of Medicine, here in Manhattan, recently added to the discussion on what it takes to become a doctor in 2010. The school runs a special track for non-science majors who apply relatively early in their undergraduate years. Mount Sinai doesn’t require that they take MCATs or the usual set of premedical science courses – some college math, physics, biology, chemistry and organic chemistry – before admission.

The idea of the program is two-fold: first, that the traditional med school requirements are a turn-off, or barrier, to some young people who might, otherwise, go on to become fine doctors; second, that a liberal arts education makes for better, communicative physicians and, based on the numbers published in a new article, a greater proportion who choose primary care.

Today Orac, a popular but anonymous physician-scientist blogger, considers the issue in a very long post. His view, as I understand it, is that if doctors don’t know enough science they’ll be vulnerable to misinformation and even quackery.

On the side of the spectrum, perhaps, Dr. Pauline Chen, a surgeon who puts her name on her blog and essays. In a January column, “Do You Have the Right Stuff to Be a Doctor?” she challenged the relevance of most medical schools’ entry requirements.

I see merit on both sides:

It seems fine, even good, for some students to enter medical school with backgrounds in the humanities. Knowledge of history, literature, philosophy, art history, anthropology and pretty much any other field can enhance a doctor’s capability to relate to people coming from other backgrounds, to recognize and describe nonparametric patterns and, perhaps, deliver care. Strong writing and verbal skills can help a doctor be effective in teaching, get grants and publish papers and, first and foremost, communicate well with patients and colleagues.

Still, there’s value in a doctor’s having a demonstrated aptitude in math and science. Without the capacity to think critically in math and science, physicians may not really understand the potential benefits and limitations of new medical findings. What’s more, doctors should grasp numbers and speak statistics well enough so they can explain what often seems like jumbled jargon to a patient who’s about to make an important decision.

Thinking back on my years in medical school, residency, fellowship, research years and practice in hematology and oncology, I can’t honestly say that the general biology course I took – which included a semester’s worth of arcane plant and animal taxonomy – had much value in terms of my academic success or in being a good doctor. Chemistry and organic chemistry were probably necessary to some degree. Multivariable calculus and linear algebra turned out to be far less important than what I learned, later on my own, about statistics. As for physics and those unmappable s, p, d and f orbitals whereabout electrons zoom, I have no idea how those fit in.

What I do think is relevant was an advanced cell biology course I took during my senior year.  That, along with a tough, accompanying lab requirement, gave me what was a cutting-edge, 1981 view of gene transcription and the cell’s molecular machinery. Back then I took philosophy courses on ethical issues including autonomy – those, too, proved relevant in my med school years and later, as a practicing physician. If I could do it again, now, I’d prepare myself with courses (and labs) in molecular biology, modern genetics, and college-level statistics.

My (always-tentative) conclusions:

1. We need doctors who are well-educated, and gifted, in the humanities and sciences. But for more of the best and brightest college students to choose medicine, we (our society) should make the career path more attractive – in terms of lifestyle, and finances.

(To achieve this, we should have salaried physicians who do not incur debt while in school, ~European-style, and who work in a system with reasonable provisions for maternity leave, medical absences, vacation, etc. – but this is a large subject beyond the scope of this post.)

2. There may not be one cookie-cutter “best” when it comes to premedical education. Rather, the requirements for med school should be flexible and, perhaps, should depend on the student’s ultimate goals. It may be, for instance, that the ideal pre-med fund of knowledge of a would-be psychiatrist differs from that of a future orthopedist or oncologist.

3. We shouldn’t cut corners or standards in medical education to save money. As scientific knowledge has exploded so dramatically in the past 30 years or so, there’s more for students to learn, not less. Three years of med school isn’t sufficient, even and especially for training primary care physicians who need be familiar with many aspects of health care. If admission requirements are flexible, that’s fine, but they shouldn’t be lax.

Critical thinking is an essential skill for a good doctor in any field. But that kind of learning starts early and, ideally, long before a young person applies to college. To get that right, we need to go back to basics in elementary and high school education. If students enter college with “the right stuff,” they’ll have a better understanding of health-related topics whether they choose a career in medicine, or just go to visit the doctor with some reasonable questions in hand.

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DNA Comes Home, or Maybe Not

Earlier this month employees at most of 7500 Walgreens pharmacies geared up to stock a new item on their shelves: a saliva sampler for personal genetic testing. On May 11, officials at Pathway Genomics, a San Diego-based biotech firm, announced they’d sell over-the-counter spit kits for around $25 through an arrangement with the retailer. A curious consumer could follow the simple package instructions and send their stuff in a plastic tube, provided in a handy box with pre-paid postage, for DNA analysis.

DNA helix structure (Wikimedia Commons)

Once the sample’s in the lab, the cost of genomics testing depends on what, and how much, you want to know. Pathway offers a variety of options. A pre-pregnancy planning report would check to see if you carry mutations for each of 37 inherited diseases – conditions as varied as beta thalassemia, cystic fibrosis and familial Mediterranean fever – for $179. A profile of tests for genes involved in metabolizing specific drugs, such as Plavix and Coumadin, goes for $79. A vaguer, health conditions panel suggests a propensity to develop particular diseases including Type II diabetes and melanoma. This series runs $179 if purchased separately, but might be had for less through a discounted package rate. A genetics ancestry evaluation lists for $249.

Within two days, after some controversy and a receipt of a letter from the FDA Office of In Vitro Diagnostic Device Evaluation and Safety addressed to James Plante, Founder and CEO of Pathway (dated May 10), Walgreens nixed the plan. Now, Congress wants to know more about direct-to-consumer personal genomics testing. On May 19, the House Committee on Energy and Commerce sent letters to Plante and the CEOs of two major competitors – Navigenics and 23andMe. House Committee Chairman Henry Waxman and colleagues have some questions about how samples are processed and the accuracy of the results:

“The Committee is requesting information from the companies on several aspects of the tests:  How the companies analyze test results to determine consumers’ risk for any conditions, diseases, drug responses, and adverse reactions; the ability of the companies’ genetic testing products to accurately identify any genetic risks; and the companies’ policies for the collection, storage, and processing of individual genetic samples collected from consumers.

The Federal Trade Commission has cautioned consumers about genetic testing kits since 2006.  Still, personal genomics tests are readily through available on-line sales. You can get the 23andMe “DNA Test for Health and Ancestry Information” from the manufacturer or at Amazon.com for $499. Navigenics takes a distinct approach by marketing its genetic tests strictly as a laboratory service for medical practitioners and so, thus far, avoids some rules regarding in vitro diagnostic tools.

New York State, my home, is one region where Walgreens wouldn’t have sold the kits in stores. That’s because of stricter state laws regarding genetic testing.

Dan Vorhaus, writing for the Genomics Law Blog, provides a considered analysis:

At present, whether a genetic test is subject to FDA regulation largely depends on how it is developed and marketed. The literature, as well as current FDA regulatory policy, divides genetic tests into two primary categories:

(i) in vitro diagnostic test kits (also sometimes referred to as IVD kits or, simply, as genetic test kits), which may be sold by their manufacturers directly to consumers, testing laboratories, clinicians or other approved recipients, depending on the device; and

(ii) laboratory developed tests (or LDTs, also sometimes referred to as “home brew” assays), which are not sold directly to the general public or to physicians; rather, a testing service (as opposed to the actual test itself) is marketed, and samples (e.g., of saliva) are collected and submitted to the laboratory for evaluation.

The FDA regulates IVD kits as medical devices…

Up until now personal genomics testing companies have had few constraints, and little profit. What’s clear from the recent news is that we’ll be hearing more about these kits – their manufacture, distribution, accuracy and interpreting results. And that doctors, for our part, have some serious studying to do. Whether the test results go directly to patients, or not, they’re sure to raise many legitimate questions. We’ll need some solid answers about the testing process in itself, besides meaningful responses about what’s found in our DNA.

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News, Information, Facts and Fiction

This morning I was in the gym, half-watching CNN as I did my usual exercises. Mathew Chance, a senior international correspondent based in Moscow, recapped the horrific scene involving explosions at two metro stations at the peak of rush hour. Chance reported that the bombers were both women. Most of the other facts surrounding the tragedy remain uncertain, he said. John Roberts, one of the CNN hosts, asked about any claims of responsibility for the terrorist attacks.

“Well, in fact, we had some information earlier today,” Chance responded. “…there had been a claim of responsibility…But that information appears to be incorrect.”

Wow!  Now, there’s an AM Fix.

Can information be wrong? Of course it can, we all know. There’s good evidence for this in my medical textbooks, among other reliable sources.

Lately, and especially since I started this blog, I’ve been thinking a lot about the nature of information – how we define it, how and if it might be distinguished from data, and what separates information and opinion.

“Information is the lifeblood of modern medicine,” wrote Dr. David Blumenthal in a carefully-designated “perspective” piece in the February 4 issue of the New England Journal of Medicine. He continued:

Health information technology (HIT) is destined to be its circulatory system. Without that system, neither individual physicians nor health care institutions can perform at their best or deliver the highest-quality care, any more than an Olympian could excel with a failing heart…

OK, so information needs to get around. It’s kind-of like blood; we can’t thrive without it. We won’t win any gold medals in health-care delivery before implementing the Health Information Technology for Economic and Clinical Health (HITECH) Act.

I agree on the essentialness of information in medical practice and decision-making. But that brings us back to the crucial issue of its nature – how people, doctors, scientists, news reporters or anyone, literate or otherwise, can tell if something’s true or untrue.

Last year in journalism school at Columbia University I took a course called “Evidence and Inference.” We went as far back as Plato’s cave, and as far forward as the New York Times’ 2002 reporting on possible evidence for weapons of mass destruction in Iraq. The point of the exercise, in sum, was that it’s sometimes hard, even for inquisitive journalists, scholars and scientists, to tell fact from fiction.

(Rest assured, I didn’t need a graduate course at Columbia to learn that much, although I did enjoy going back to school.)

Last week’s cover story in the Economist, on “Spin, Science and Climate Change,” drew my attention to some parallels between the Climategate controversy and distrust regarding other areas of scientific and medical knowledge. In a briefing within, the author or authors write:

…In any complex scientific picture of the world there will be gaps, misperceptions and mistakes. Whether your impression is dominated by the whole or the holes will depend on your attitude to the project at hand. You might say that some see a jigsaw where others see a house of cards. Jigsaw types have in mind an overall picture and are open to bits being taken out, moved around or abandoned should they not fit. Those who see houses of cards think that if any piece is removed, the whole lot falls down. When it comes to climate, academic scientists are jigsaw types, dissenters from their view house-of-cards-ists.

The authors go on to consider some ramifications of a consensus effect. (There’s an interesting discussion on this, which relates to a herding effect, in a recent post by Respectful Insolence).  Meanwhile, house-of-card-ists, dubbed doubters, emphasize errors from confirmational bias, or the tendency of some people to select evidence that agrees with their outlook.

There’s far more to consider on this subject – how we perceive and represent information – than I might possibly include in today’s post. So let’s just call this the start of a long conversation.

Getting back to medical lessons – the problem is that most of us can’t possibly know what’s really right. (Yes, I mean doctors too.) Few know enough of the relevant and current facts, or even the necessary terms, to make decisions about, say, which therapy is best for Ewing’s sarcoma in a four-year-old child or whether a new drug for Parkinson’s is worth a try in your dad’s case. Even for those of us who know something about statistics, it’s tricky.

Ultimately, I think it comes down to a matter of trust in the people who provide us information. It’s about knowing your source, whether that’s Deep Throat, a person reporting from the street in Moscow early this morning, or your personal physician.

Well, it’s a holiday for me over the next few days. I’ll read some history first, and then some fiction.

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