section 23.9
The Human Genome Project
539
1
. Restriction sites:
sequences of DNA digested by
restriction enzymes;
2.
Variable-number tandem repeats (VNTRs):
sequences of bases that are repeated in tandem
almost without change up to 40 copies;
3.
Simple sequence repeats (SSRs):
sequences of
bases, most commonly CA, that are repeated dozens
of times and occur at thousands of sites in the
genome; and
4.
Single-nucleotide polymorphisms (SNPs):
sites in
human DNA that differ by a single base pair.
Because SSRs and SNPs occur so frequently in the
genome, they can be used to distinguish one chromosome
from another in an individual. An SSR may occur 19 times
at one location in a particular chromosome and
2 1
times
in a homologous chromosome. Since an SSR is inherited
like a gene, the variability can be followed from person to
person in a pedigree. The length of a particular SSR can be
determined by gel electrophoresis without any knowledge
of the exact sequence. SNP differences occur about once
in every thousand bases in DNA. That is, at a position on a
chromosome where an A-T base pair occurs, a G-C pair
might occur on the homologous chromosome.
Because only a small percentage of the total number
of nucleotides in the human genome is actually used to
encode information for the synthesis of different proteins,
techniques have been developed to search for and map
genes that are expressed. Knowing the location of these
protein-encoding genes is extremely useful as mutated
forms of these expressed genes are likely implicated in
many diseases.
Two techniques have contributed to the identification
of many active genes in the human genome.
Expressed
sequence tags (ESTs)
for as many as 50,000 human genes
have been generated by isolating messenger RNAs and
copying them into DNA. These DNA probes can then be
used to identify the location of the expressed genes on the
various chromosomes by hybridization. Specific chromo-
somal sites also can be identified by using PCR primers to
amplify a small segment of a chromosomal locus. The sites
identified by using PCR amplification are called
sequence
tagged sites (STSs).
Completing the sequencing of the human genome is
only the first step in the revolution in medicine that many
are predicting will result in the 21st century. However,
knowing the base sequence, chromosomal position, and
even the function of the protein that is encoded in the gene
is only the first step in using the information to benefit pa-
tients with inherited defects (Figure 23-17). The sequence
information does not specify how and when a gene is
turned or or off, in what cells the gene is active or inactive,
or in what stage of development expression of the gene
is needed. The expression of most genes is modulated in
complex ways and coordinated by numerous intracellular
and extracellular factors. How a particular protein affects
the biochemistry, physiology, and phenotype of an indi-
vidual must be understood before therapeutic messures
can be developed. Often inherited defects are manifested
during fetal development and the damage is irreversible by
the time the child is born. Some of the anticipated medical,
social, and ethical consequences deriving from the human
genome project are described below.
Genetic testing
: Hundreds of genetic tests are avail-
able that can be used to determine if a person carries
a defective gene and to estimate the risks of develop-
ing a disease as well as the risk of passing the gene on
to progeny. In many instances, determining the risk of
passing on genes for Hungtington’s disease, cystic fibro-
sis, hemophilia, Duchenne’s muscular dystrophy, or sickle
cell anemia provides prospective parents with informa-
tion useful in making reproductive choices. However, an
asymptomatic person with a family history of Hunting-
ton’s disease faces a difficult choice. Finding out while
young that you carry a gene that eventually will produce
a lethal, neurological disease creates enormous emotional
and psychological stress since there is no treatment or cure
that can be offered. Faced with the choice of genetic test-
ing for Huntington’s disease, many possible carries of the
gene opt not to be tested.
Genetic tests also can determine if a person carries
genes that increase the chances of developing breast can-
cer, colon cancer, and other polygenic-multifactorial dis-
eases. Again, in most cases, only marginally helpful med-
ical help can be offered to individuals who test positive for
cancer-causing susceptibility genes. Some young women
who are homozygous for the
BRCA1
gene have elected
to undergo prophylactic bilateral mastectomy to reduce
the risk of breast cancer later in life. If one tests positive
for colon cancer susceptibility genes, the only recourse
is frequent examination of the colon and immediate re-
moval of any polps that appear. Clearly, the consequences
of many of the genetic tests currently available create
serious problems for those who discover that they carry
disease-causing genes of one kind or another. As hundreds
of additional genetic tests become available, patients and
physicians will have to make difficult choices regarding
the risks and benefits of genetic testing.
Genetic discrimination:
Profound social, ethical, and
financial questions are raised by wide spread use of genetic
testing and identification of persons at risk for diseases.
Employers and insurance companies would like to have ac-
cess to such information since it would be in their financial
interest to know who is likely to become sick or disabled.
