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Monthly Archives: March 2010

Mitochondrial DNA: Not As Pure As Once Thought

The analysis of mitochondrial DNA (mtDNA) has been a useful forensic technique since it was first admitted as evidence in a US court in Tennessee v. Wade in 1996. Unlike nuclear DNA, the DNA used for DNA fingerprinting, mtDNA does not match a crime scene sample to a particular individual. That is, it can’t point the finger directly at one person. What it reveals is the maternal lineage of the individual or crime scene sample in question. Often this is all that is necessary to prove that a particular individual was at the crime scene, but it is not the “smoking gun” that nuclear DNA is.

But now some new evidence casts a different light on this test. Before addressing that, let’s look at an excerpt from my book Howdunnit: Forensics so we can understand exactly what mitochondrial DNA is.


From Howdunnit: Forensics:

The DNA used for standard DNA testing is nuclear DNA and it can be extracted from any nucleated cell. But cells also contain non-nuclear DNA. This DNA is found within the mitochondria, which are small organelles that reside within the cytoplasm of the cell and serve as the cell’s energy production center. A small amount of DNA is found within the mitochondria, but each cell has many mitochondria.

Mitochondrial DNA has several characteristics that make it unique. These include the fact that it is passed from generation to generation by the maternal linage, mutates rarely, is found in places where nuclear DNA doesn’t exist, and is exceptionally hardy.

Your mtDNA is inherited unchanged from your mother and only from your mother. And she received hers from her mother, and her mother from her mother, and so on. Why is this? At fertilization, the egg supplies the cell and half the DNA while the sperm supplies only half the DNA. The sperm cell itself breaks down and disappears after passing its genetic material into the nucleus of the egg cell. This means that the actual cell and all the cell components (including the mitochondria) of the developing zygote come from the mother. As the cell divides and multiplies, these mitochondria are copied and passed on, generation after generation so that all the cells of the body contain identical mtDNA.

Since, mtDNA undergoes a significant mutation approximately once every 6500 years, it is unchanged over many generations. This means that your mtDNA is virtually identical to your mother’s, your great-great grandmother, and your maternal ancestors from 1000 years ago. Thus, anyone’s maternal linage can be accurately traced over many generations and this fact can be used to prove if two people share the same maternal linage.

That is the current state of mtDNA. Now to the new study:

Research reported in the March 4 issue of Nature suggests that mitochondrial DNA is not as pure as we once thought. I should point out that nothing in nature is absolute and there are variabilities in virtually any natural system you want to discuss. This holds true for mtDNA. But the entire basis for using mitochondrial DNA is that it varies little in a given individual so that the mitochondrial DNA found throughout the body remains consistent. This new study suggests that this might not be so. They found approximately 7% variability in skeletal muscle mitochondria and a disturbing 90% in kidney tissue. What does all this mean?

It does not mean that the individual will not share mitochondrial DNA with all of his maternal ancestors, nor that a non-relative would have mtDNA that matched the maternal linage in question, but rather that all of his mitochondrial DNA will not be identical and that this variability itself varies from organ to organ and tissue to tissue within a person’s body. So a sample taken from a skeletal muscle will be less variable than would one taken from a kidney.

This means that if possible the same tissue should be used for comparison. That is, if the suspects mother’s or grandmother’s mtDNA is obtained from a blood sample then the suspect’s mtDNA should be obtained from the same type of sample. This should lessen the variability, or at least make the degree of variability more predictable, and this should in turn make interpreting the test much easier. But this is still supposition and the final word will come only when further research is done.

One word of caution: never draw firm conclusions from a single study. Scientists do not do that, or at least should not, and therefore, though this study is intriguing and does raise some interesting questions, the truth will require further study by other scientists and only then can we know what all this actually means. But it is intriguing.

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Posted by on March 9, 2010 in DNA, High Tech Forensics

 

Dr. James C. Whorton and THE ARSENIC CENTURY

My guest today is Dr. James C. Whorton, author of THE ARSENIC CENTURY. Dr.
Whorton is a Professor Emeritus in the Department of Bioethics and Humanities at the University of Washington School of Medicine. He has written extensively on the history of medicine. It is my great pleasure to welcome him to The Writer’s Forensics Blog.

DPL: Dr. Whorton, thanks for being with us today. Tell us a little about arsenic and how it affects those unfortunate enough to ingest it.

JCW: First, the ‘arsenic’ we all think of as a deadly poison is not the element arsenic, which is relatively innocuous, but the compound arsenic trioxide, a substance that can be taken into the body not only by oral ingestion, but also by inhalation, through abraded skin, and by absorption through the mucus membranes of the rectum, vagina, and urethra.  It can also be conveyed through the milk of nursing mothers.  Once in the body, it produces a sharp, burning sensation in the stomach and esophagus (usually about 30 to 60 minutes after ingestion), and then profuse vomiting and diarrhea lasting for hours.  Ultimately, the poison damages the heart and other viscera, but typically death comes only after 12 to 24 hours, or even longer.  Statistics from the 1800s suggest that about half of those poisoned died.

DPL: What is it about arsenic that has allowed it to remain an effective method of murder even in this day of toxicological sophistication?

JCW: Arsenic is colorless and tasteless, easily confused for sugar or flour.  This allows it to be mixed into foods and beverages without arousing suspicion, and also conduces to accidental poisoning.  In the nineteenth-century, when arsenic was the most common substance used to poison rats and mice, it was present in most households and frequently mistaken for harmless white foodstuffs.  Accidental deaths from arsenic greatly outnumbered those from homicide and suicide.  Restrictions were placed on the sale of arsenic in 1851 in Britain, and accidental death became less common.

DPL: When was the earliest known use of arsenic for murder and when did
testing for it first enter the forensic arena?

JCW: Arsenical compounds known as orpiment and realgar (both sulfides of arsenic, but less toxic than arsenic trioxide) were used for murder in antiquity:  Nero is believed to have murdered a political rival with one of the sulfides, for example.  With the appearance of arsenic trioxide during the middle ages, however, a much more effective agent became available, though it was used mostly by the aristocracy to eliminate political opposition.  With the expansion of mining in the eighteenth century, supplies of arsenic (a byproduct of the smelting process) became much greater and its price dropped to the point anyone could afford it.  By the nineteenth century, arsenical murder had been democratized; during the 1800s, in fact, most arsenical homicides were committed by the laboring classes.  There were tests for arsenic dating to the 1700s, but they were unreliable when applied to organic material (food, vomit, feces).  A reliable method came along only during the 1830s, with the introduction of the Marsh test (accomplished by an English chemist named James Marsh).  The Marsh test made it easier to detect murder, so brought about a slow decrease in arsenic’s use.  Arsenic murder became much less common in the twentieth century, but is still occasionally used to kill.

DPL: Your book deals with accidental, suicidal, and homicidal arsenic poisoning during the 19th century. What were the most common sources of accidental poisoning at that time and do any of these remain a concern today?

JCW: Besides the accidents resulting from mistaking arsenic for flour or sugar, there were numerous cases of death and sickness caused by green arsenic compounds that became popular in the 1800s as pigments used to color all manner of domestic products:  wallpapers, wrapping paper, candies, candles, even many articles of clothing (it was arsenic IN old lace that was the true problem).  These pigments are no longer used, having been replaced by synthetic organic compounds during the second half of the nineteenth century.  The chief threat from arsenic today is chromated copper arsenate (CCA), used as a wood preservative (decks, playground equipment) into the early 2000s.  There is also a severe problem with arsenic in water supplies in some parts of the world, particularly Bengal and Bangladesh.

DPL: Wasn’t arsenic called Inheritance Powder during Victorian times?

JCW: Arsenic became known as ‘la poudre de succession,’ or inheritance powder, in France in the 1600s after a rash of poisonings by several noblewomen.  It continued to be used to accelerate inheritances into the nineteenth century, but mostly by the lower and middle classes to cash in on life insurance.

DPL: What famous poisoners employed arsenic as their chosen weapon?

JCW: Lucrezia Borgia is perhaps the name people most readily associate with arsenical murder, though it was actually her brother Cesare who was the guilty one (fifteenth century).  In the sixteenth century, Catherine de Medici was the most famous, while in the seventeenth century, it was a Sicilian woman named Toffana (who was believed to have killed hundreds, if not thousands, with a special preparation she sold to women eager to get rid of the men in their lives).  The prevalence of Italian poisoners resulted in the coining of the verb ‘to Italianate,’ i.e., to murder with arsenic, in the 1600s.  During the nineteenth century, the most famous arsenical killers were England’s Mary Ann Cotton, who killed three husbands and a dozen or more of her children in the 1860s and 1870s; Madeleine Smith, a Glasgow woman who likely killed her lover in the 1850s, but who was not convicted; and Florence Maybrick, an American woman living in Liverpool, who was accused of poisoning her husband but probably didn’t–she was convicted anyway, but eventually pardoned.

DPL: What drew you to this subject and led to your writing this book?

JCW: I did my doctoral dissertation, in 1969, on the public health problem of pesticide residues on foods in the days before DDT, i.e., 1860-1945.  The most popular insecticides during that period were arsenic compounds (lead, copper, and calcium arsenates).  In doing research on those early insecticides, I came across a good bit of discussion on the presence of arsenic in other products and decided that one day I would get back to the topic.  It only took me three decades to do so.  In writing the book, I decided to concentrate on Great Britain because the problem was most severe there (for a variety of reasons), and because I am a great fan of British beer and pubs–doing research in London and Edinburgh was thus a lot more fun, for me at least, than doing it in America.

DPL: Any other cool things about arsenic you want to share with us?

JCW: I would only add that the description of arsenic symptoms given in the first question applies to acute poisoning.  That was the common form of murder before the nineteenth century, giving a lethal dose of three or more grains at one time.  During the nineteenth century, slow poisoning became more popular, killing by repeated doses none of which was great enough to kill by itself, but that cumulatively finished off the victim over a period of weeks.  That was a form that was more difficult to detect, so improved the killer’s odds of escaping.  It was a matter of how big a risk-taker a person was versus how much patience she or he had (I put ‘she’ first because the majority of arsenic poisoners in the nineteenth century were women; men tended to rely on brute force rather than the subtlety of poison).

DPL: Thanks so much for being with us today. I’m sure my readers will employ
some of this new found knowledge in their stories.

THE ARSENIC CENTURY

 

King Tut, Malaria, and DNA

King Tutankhamun has captured the world’s imagination from the moment Howard Carter unearthed his tomb in 1922. That he had opened the tomb of royalty was never questioned. The gold and jewels found within were astounding, particularly the jewel encrusted golden death mask. The Boy King ascended to the throne at age 9 and reigned for 9 years until his death in 1324 BC.

His life and death have not been without controversy. There have been many theories about who his parents were and how he actually died. There are many who believe that he was murdered. This speculation was mostly fueled by a hole found in the young man’s skull. Most now believe that this defect was postmortem as part of the embalming process.

Recently researchers have been conducting a series of DNA and radiological tests on Tut’s remains and they have uncovered some fascinating results. DNA evidence shows that he was indeed the son of the great Akhenaten, the Egyptian King who replaced Egyptian polytheism with a monotheistic religion. Even before this testing, most scholars believed that Akhenaten was his father but the identity of his mother had always been questioned. This new DNA evidence now suggests that she was his aunt, Akhenaten’s sister. This would not be unexpected as throughout history royal families have consistently inbred as a way of preserving linage, land, and wealth and the ancient Egyptians were no exception. An interesting finding was that the corpses of two fetuses were also found within Tut’s tomb and DNA testing suggests that one of these had been Tut’s own daughter.

The investigators also uncovered other things about the Boy King’s existence. He was plagued with a number of injuries, deformities, and diseases. Rather than a handsome young man, as he if often portrayed, it appears that he might have suffered from a cleft palate, a club foot, a spinal deformity, and Marfan’s Syndrome, entities that seemed to have run in his family. In addition he may have had an uncommon bone disorder known as Kohler’s Disease, which causes weakening of the bones over time. A healed leg fracture was also revealed. Most likely he had difficulty walking because of these musculoskeletal problems and this seems to be supported by the fact that a large number of walking canes were found in his tomb. Perhaps as many as 130. Since royalty is often buried with objects that are personally important to them, these canes must have been useful to the young man.

As for the cause of his death, DNA evidence suggested the presence of a malignant form of malaria known as Plasmodium falciparum in the young King’s brain This was also found in other mummies around that time.

Malaria (mal–aria) means “bad air.” In not so ancient times it was felt that the disease came from breathing air that was contaminated with some toxin or evil spirit. In various civilizations, city inhabitants would move out of swampy areas and up into the mountains in order to avoid living in the air around the swamps. This happened in ancient Rome as the wealthy moved into the hills to avoid the diseases that were felt to linger in the lower areas of the city. Swamp air has a distinctive odor and therefore it was logical for them to assume that this aromatic air might have something to do with malaria, which was much more common in these areas than it was at higher altitudes. We now know that the malarial mosquito lives in the swamps and it is this organism that passes the Plasmodium parasite from person to person. They obviously didn’t know that in ancient Egypt or even in Renaissance Europe.

Malaria infects 300 to 500 million and kills one million people each year, mostly children in the African continent where the disease is endemic (infects a large portion of the population). Those that don’t die, often become carriers of the disease. This means that they themselves are not gravely ill but they harbor the organism within their bloodstream. When a mosquito feeds on them it picks up the parasite and then, when it bites an uninfected person, passes the disease along. Millions of people carry a malarial organism within their bodies.

There are several types of malaria but that caused by Plasmodium falciparum is the most aggressive and the most dangerous. Since King Tut had the DNA of this type within his body and brain tissue, it is reasonable to assume that he was infected with the Plasmodium falciparum organism and indeed this may have caused his death. But maybe not.

The reason this question can’t be conclusively answered is that malaria might very well have been endemic in ancient Egypt and therefore finding it within Tut might or might not be related to his death. But the fact that he was infected with the more malignant type and the fact that the organism had invaded his brain certainly makes this as the cause of his death a distinct possibility. This is something that further research will have to determine.

For me, the fact that DNA can be extracted from a 3300-year-old mummy and analyzed this way is fascinating, maybe even astounding.

Fyrther Reading:

JAMA Article

NY Times Article

Dallas Morning News Article

 
 
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