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| Breast
and Ovarian Cancer |
| Genetic
Diseases in Askenazi Jews |
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By
Lauren John, MA, MLS
Reviewed
By Adam Lowe and
Beth Crawford,
MS, CGC
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Ashkenazi is the term used to describe Jews who have
ancestors from Eastern or Central Europe. Today, there
are Ashkenazi Jews all over the world and many are intermarrying.
But for centuries, political and religious factors ensured
their genetic isolation from the population at large.
As
is the case with other groups who live, or lived, in
relative isolation — such as the French Canadians
and Icelanders — scientists theorize that most
Ashkenazi Jews can trace their ancestry back to a small
number of founders. The founder theory is of particular
interest to geneticists because they theorize that even
if just a few of these founders had a mutation,
the mutation would become amplified in the population.
This theory is supported by the fact that today people
of Ashkenazi Jewish descent have a higher incidence
of a number of specific mutations, for example mutations
in the genes that increase the risk of developing breast and ovarian cancer, and mutations that cause the childhood neurological disease Tay-Sachs disease.
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Origins of the Ashkenazi Jews
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The
word Ashkenazi is derived from the Hebrew word
for "Germany". Today, the term is used to
refer to Jews who have ancestors from Eastern or Central
Europe, such as Germany, Poland, Lithuania, Ukraine,
and Russia.
Scholars
disagree about how the Jews, living for thousands of
years in the Middle East, first arrived in Eastern Europe.
Some theorize that Mediterranean Jews, ca. 800 AD, were
invited by the emperor Charlemagne to settle the Germanic
territories. Not all of the Jews traveled that far east,
however. (A separate group, the Sephardic Jews descend
from ancestors in Spain and North Africa.)
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Ashkenazi
Jews Today
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Today,
in the aftermath of World War II and the Holocaust,
most of the world's 10 million Ashkenazi Jews live in
the United States, Israel, South America, South Africa,
Australia and New Zealand. About 80 percent of the six
million ethnic Jews in the United States are of Ashkenazi
Jewish descent.
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Some
Genetic Diseases are More Common in Ashkenazi Jews
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Ashkenazi Jews are at greater risk of developing several genetic diseases rarely found in the population as a whole. For example, Type 1 Gaucher disease — a nonlethal condition in which the body lacks a gene regulating the breakdown of a particular kind of fat — is one of the more common genetic disorders among Ashkenazi Jews. An estimated one in ten Ashkenazi Jews is a carrier for Gaucher disease.
Carrying one mutated gene for the disease will not cause
symptoms. However, if both parents carry one mutated
gene, there is a one in four chance that their children
will inherit two Gaucher genes and develop the disease.
The
childhood neurological disorder Tay-Sachs disease is
also most commonly found in people of Ashkenazi descent;
an estimated 1 in 31 Ashkenazi Jews carrying a gene
for Tay-Sachs. Here again, both parents must have a
copy of the mutated gene in order to pass on the disease
to a child — and the child of two people who each
carry one Tay-Sachs mutation has a one in four chance
of receiving two mutated genes and developing the disease.
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| Recent
studies have shown that people of Ashkenazi Jewish
descent may be at greater risk for breast and ovarian
cancer than the general population. |
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In
addition, recent studies have shown that people of Ashkenazi
Jewish descent may be at greater risk for breast and
ovarian cancer than the general population. In 1995,
scientists from the National Institutes of Health (NIH)
discovered that a particular mutation in the breast
cancer gene called BRCA1 was
present in one percent of the general Jewish population.
In comparison, the percentage of people in the general
U.S. population that have any mutation in BRCA1 has
been estimated to be between 0.1 to 0.6 percent. A follow
up study in 1996 found two additional mutations (one
in the BRCA 1 gene and one in another breast cancer
gene called BRCA2) to have
a greater prevalence in the Ashkenazi Jewish population,
bringing the overall risk for carrying one of these
three mutations to 2.3 percent.
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Why
Do Ashkenazi Jews Get More Genetic Diseases? The Founder
Effect Theory
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| Most
of today's Ashkenazi Jews descend from a group of
perhaps only a few thousand people. |
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Stanford
University geneticist Neil Risch explains the range
of genetic diseases unique to the Ashkenazi Jewish population
by theorizing that most of today's Ashkenazi Jews descend
from a group of perhaps only a few thousand people — the
privileged Ashkenazi that lived 500 years ago in Eastern
Europe. The poorer Ashkenazi Jews, says Risch, had fewer
children that lived to adulthood and, as a result, they
did not pass on as many genetic conditions. Geneticists
refer to this relatively small group of ancestors as
founders.
Given
that until recently, both religious and political factors
helped to ensure that Ashkenazi Jews married other Ashkenazi
Jews, millions
of people may be able to trace their ancestry directly
to these founders. Thus, even if just a few founders
had a mutation, the
gene defect would become amplified in the population.
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Why
Do Ashkenazi Jews Get More Genetic Diseases? The Beneficial
Mutation Theory
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Other
geneticists theorize that diseases in Ashkenazi Jews
have arisen because of hidden benefits linked to the
inheritance of a single copy of a mutant gene.
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who carried the Tay-Sachs gene may have been resistant
to tuberculosis. |
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For
example, people with one copy of the Tay-Sachs gene were
discovered to be resistant to tuberculosis, a condition
common in the crowded ghetto conditions in which Ashkenazi
Jews often lived. Geneticists theorize that the people
who carried the Tay-Sachs gene may have survived better
under ghetto conditions, whereas people without the gene
would be more likely to die. As a result, the Tay-Sachs
gene continued to be passed on in greater numbers in the
Ashkenazi population.
Today,
with the tuberculosis threat largely gone, the gene
threatens the Ashkenazi population because it is linked
to serious and fatal childhood illness. But, historically,
the gene was passed on because, although it threatened
the lifespan of some children, it protected larger numbers
of Ashkenazi Jews of all ages who were at risk for tuberculosis.
Regardless
of the origin of the mutations, people of Ashkenazi
Jewish descent should be aware that they may be at increased
genetic risk for certain conditions. This knowledge
and screening has already decreased the number of children
born with the fatal Tay Sachs disease by making carrier
testing available to those who want to learn their status.
Understanding these risks can lead to more accurate
risk assessment, genetic testing, and preventive therapy.
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References
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Questions and Answers About Estimating Cancer Risk in
Ashkenazi Jews. (2000). National Cancer Institute,
Cancer Fact Sheet 3.60 http://cis.nci.nih.gov/fact/3_60.htm
Genetics and Ashkenazi Jewish Populations. (2000) University
of Pittsburgh Department of Human Genetics (http://www.pitt.edu/~edugene/Genetics.html)
Risch, N. et al. (1995). Genetic analysis of idiopathic
torsion dystonia in Ashkenazi Jews and their recent
descent from a small founder population. Nature Genetics
9(2):152-9.
Questions and Answers for Estimating Cancer Risk in
Ashkenazi Jews.(1997). National Cancer Institute,
National Institutes of Health Backgrounder May 14.
(http://rex.nci.nih.gov/massmedia/backgrounders/Ashkenazi.html)
Ashkenazi. Encyclopedia Britannica. Chicago,
IL. (2000) . http://www.britannica.com/bcom/eb/article/6/0,5716,9956+1+9835,00.html
Malcom, S., ed. (1998). Founder effects in the Jewish
populations of the world. Clinical Molecular Genetics
Society Web Site and Notes From X-Linked and Population
Genetics Courses. http://www.ich.ucl.ac.uk/cmgs/jewish.htm
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