|
 Genomic
imprinting refers to a process that distinguishes paternally derived chromosomes
from maternally derived chromosomes. Imprinting plays a critical role
in gene expression, mammalian development, and human disease. However,
the biological requirement for imprinting remains a mystery. In the first
2 columns on the topic, we will review how imprinting was initially identified,
present some hypotheses about the mechanisms of imprinting, and speculate
on the evolutionary forces maintaining this phenomenon. The subsequent
2 columns will discuss the molecular bases for 2 disorders in which imprinting
is involved, namely, Prader-Willi and Angelman syndromes.
Genomic
imprinting might be considered the exception that proves the rule. Until
the late 1980s, Gregor Mendelıs laws of inheritance were thought to be
inviolate. All autosomal genes were believed to be expressed equally,
regardless of whether they were inherited from the mother or the father.
For most genes, this is true. However, it is now recognized that a small
subset of genes are violators of Mendelıs laws and are expressed differently
depending on the parent from whom they are inherited.
Genomic
imprinting refers to the normal process whereby specific genes or DNA
segments are reversibly modified during gametogenesis in a parent-specific
fashion. Although research on exactly how this occurs is not yet understood,
one modification that is believed to play a role is the reversible addition
of methyl groups to specific cytosine residues within the DNA sequence,
a process that occurs differently in generation of the egg and the sperm.
Genomic imprinting is called an epigenetic phenomenon, since the gene
structurethe actual sequence of nucleotidesis not affected as occurs
during the mutations that were discussed in previous columns. Rather,
the ³imprint² is erased during gametogenesis and must be reapplied in
a gender-specific manner. For example, in the normal situation, a methylated
gene inherited by a male from his mother will be unmethylated during spermatogenesis,
and this unmethylated gene, when passed on to his daughter, must be remethylated
during oogenesis.
As
a result of this differential methylation, the maternally inherited copy
of an imprinted DNA segment differs from the paternally inherited copy.
What has recently been discovered is that these differences may also be
reflected in differences in gene expression, even though the nucleotide
sequences of the 2 segments are identical. The imprinting process leads
to an inactivation of either the paternally or maternally inherited copy
of some genes within some of the bodyıs cells. Genetic mutations that
change this pattern and lead to either increased or decreased expression
of imprinted genes may upset the normal amounts of proteins that are expressed.
The
discovery of genomic imprinting as a normal mechanism of genetic regulation
has provided dramatic insights into previously puzzling human diseases.
Within the past decade, mutations of imprinted genes on several different
chromosomes have been found to cause a wide range of phenotypic effects.
This genetic mechanism underlies 3 well-known genetic disorders, namely,
Prader-Willi, Angelman, and Beckwith-Wiedemann syndromes.
Prader-Willi
and Angelman syndromes were the first disorders recognized as occurring
because of genomic imprinting. Both disorders affect approximately 1 in
10,000 persons. Prader-Willi syndrome is a multisystem disorder characterized
by neonatal hypotonia and failure to thrive, followed by early childhoodonset
hyperphagia and resultant obesity. Symptoms also include short stature,
typical facial features, small hands and feet, and hypogonadotropic hypogonadism.
Mild mental retardation is present as is a characteristic neurobehavioral
profile with temper tantrums, obsessive-compulsive disorder, and occasionally
psychosis. Hypothalamic dysfunction is thought to underlie many of the
major features of this disorder.
In
contrast, Angelman syndrome is a disorder featuring microcephaly, severe
mental retardation, minimal or absent speech, hyperactivity, sleep disturbance,
seizures, hypertonia, and gait ataxia. Often described are characteristic
jerky movements, a flexed arm posture, and a happy disposition with frequent
inappropriate outbursts of laughter.
The
first insight into the causation of Prader-Willi syndrome and Angelman
syndrome came in the 1980s. Both conditions were noted to have a cytogenetically
detectable deletion involving the proximal long (q) arm of one copy of
chromosome 15. Curiously, the chromosome 15 deletions seen in both patient
groups seemed to involve the identical chromosomal region, extending from
bands 15q11 to 15q13.
That
2 such strikingly diverse phenotypes could be caused by the same chromosomal
deletion was unexplainable by classical concepts of inheritance. In the
late 1980s, new molecular genetic techniques resolved this apparent paradox
by revealing that the chromosome 15 deletions in these syndromes differed
in a fundamental way, namely, by their parent of origin. The deletions
occurring in patients with Prader-Willi syndrome were found to involve
the paternally inherited copy of chromosome 15, while those in Angelman
syndrome involved the same region but in the maternally inherited copy.
In both cases, the relevant parents had normal chromosomes themselves.
This
parent-of-origin effect on the phenotype is now known to be attributable
to the phenomenon of genomic imprinting. The chromosome 15q11-q13 region
contains multiple genes that are differentially expressed, depending on
the parent of origin. As was mentioned, one mechanism that is thought
to be involved is methylation within the chromosomal region that prevents
the expression of the relevant gene(s). The extent of methylation is under
the control of a specific DNA sequence within this region called the imprinting
center. Other mechanisms that may play a role in the differential expression
of genes will be discussed in the subsequent column.
Although
it is not yet known which genes are critical for the phenotypic effects
in Prader-Willi syndrome, it is clear that this disorder occurs when there
is loss of expression of genes in the 15q11-q13 region that are normally
expressed only from the paternally inherited copy of the chromosome. While
the same genes are also present on the maternally inherited copy, they
are not expressed. It is now known that this loss of paternal gene expression
can occur by 3 different genetic mechanismsdeletion, uniparental disomy,
and mutations to the imprinting center (Fig.
1).
In
approximately 70% of cases, Prader-Willi syndrome is caused by a paternal
interstitial chromosome 15 deletion. These sporadically occurring deletions
may or may not be microscopically visible by high-resolution chromosome
analysis. In about 28% of cases, Prader-Willi syndrome results from maternal
uniparental disomy, whereby affected individuals inherit both copies of
chromosome 15 from their mother and none from their father, usually as
the consequence of abnormal chromosome segregation during meiosis. In
these cases, the disorder occurs in spite of the presence of 2 complete
and normal copies of chromosome 15, because the relevant genes are silent
on both maternally inherited copies and the paternally inherited copy
that would be active is missing.
In
the remaining cases (2%) of Prader-Willi syndrome, there is an intrinsic
abnormality in the mechanism by which imprinting occurs. This sometimes
involves a detectable mutation in the imprinting center and can be inherited
in an autosomal dominant manner. As a result, both the maternal and paternal
copies of chromosome 15 have a maternal imprint, resulting in lack of
expression of the paternally inherited genes. In such situations, it appears
that the imprint from the paternal grandmother could not be erased in
the paternal germline. This is the only genetic category of Prader-Willi
syndrome associated with a significant recurrence risk. Rarely, a translocation
involving a chromosome break within the 15q11-q13 region will disrupt
normal imprinting.
Despite
the fact that several maternally imprinted genes have been identified
in the 15q11-q13 region, there is currently no consensus as to which of
these are critical to Prader-Willi syndrome. One gene that may play an
important role in this disorder is a small nuclear ribonucleoprotein (called
SNRPN) involved in the processing of messenger RNA. This disorder will
be discussed in greater detail in a subsequent column.
Most
cases of Angelman syndrome result from similar but opposite phenomena
to those that occur in Prader-Willi syndrome, namely maternal chromosome
15 deletions, paternal uniparental disomy, or imprinting defects (Fig.
1). As with Prader-Willi syndrome, the majority (70%) of cases result
from a visible or submicroscopic chromosome 15 deletion, although in Angelman
syndrome the deletion involves the maternally inherited chromosome. Similarly,
a small proportion (2%5%) of cases of Angelman syndrome are due to abnormalities
that affect the imprinting process. Only about 2% of cases of Angelman
syndrome result from paternal uniparental disomy, probably because abnormal
segregation of chromosomes is less likely in the male than in the female
germline.
This
leaves approximately 25% of patients with Angelman syndrome who have neither
deletion, nor uniparental disomy, nor imprinting defects. Unlike in Prader-Willi
syndrome, however, a single causative gene for Angelman syndrome has recently
been identified. This gene, UBE3A, is a ubiquitin-protein ligase, which
normally functions to add ubiquitin to proteins to target them for degradation.
UBE3A is expressed only from the maternally inherited chromosome 15 and
is specifically imprinted within the brain. Many, although not all, patients
with Angelman syndrome who do not have deletions, uniparental disomy,
or imprinting defects have now been shown to have mutations in the maternally
inherited copy of UBE3A. When a UBE3A mutation or an imprinting center
mutation causes Angelman syndrome, it is associated with a 50% recurrence
risk to the parents of the affected individual. Finally, there are some
individuals with Angelman syndrome in whom the underlying genetic mechanism
of their disorder has not been identified.
Advances
in the understanding of the genetic abnormalities responsible for Prader-Willi
syndrome and Angelman syndrome have led to the development of highly sensitive
and specific diagnostic tests for these disorders. The one test that can
detect Prader-Willi syndrome due to any cause and Angelman syndrome due
to all detectable causes except a UBE3A mutation is known as methylation
analysis. This is a DNA-based test that can distinguish between the maternal
methylated and paternal unmethylated copies of a gene within the 15q11-q13
region. An exclusively maternal methylation pattern occurs in individuals
with Prader-Willi syndrome, whereas an exclusively paternal pattern is
seen in most cases of Angelman syndrome.
If
an abnormal methylation pattern is identified, then additional tests are
available to determine the specific genetic mechanism underlying either
disorder in a given patient. High- resolution chromosome analysis with
a fluorescence in situ hybridization (FISH) probe for a gene within 15q11-q13
(usually SNRPN) can identify those cases of Prader-Willi or Angelman syndrome
arising from the characteristic chromosome 15 deletion. Chromosome analysis
is also a valuable diagnostic test in patients who are being evaluated
for Prader-Willi syndrome and Angelman syndrome because sometimes other
chromosome anomalies are found to be the cause of their mental retardation
and neurobehavioral dysfunction.
Testing
known as microsatellite analysis is also available to verify the presence
of uniparental disomy by analyzing the pattern of DNA sequence variations
on the copy of chromosome 15 inherited from each parent. This testing
requires that DNA samples be obtained from the patient and both parents
in order to determine whether polymorphisms in the affected individual
are present in both parents or only in one. Finally, research-based testing
for imprinting center mutations is available for patients with either
disorder who have abnormal methylation studies without evidence of a chromosome
15 deletion or uniparental disomy and for UBE3A mutations in patients
with suspected Angelman syndrome who have a normal methylation analysis.
Some
clinical differences have recently been identified between patients who
have uniparental disomy versus those who have a deletion. To a large degree,
these differences are thought to result from the loss of other, nonimprinted
genes in patients who have deletions.
Beckwith-Wiedemann
syndrome is another genetic disorder related to genomic imprinting. It
is an overgrowth condition featuring large body size, large tongue, omphalocele,
organomegaly, neonatal hypoglycemia, and a predisposition to embryonic
tumors, especially Wilms tumor. Although the molecular basis of this disorder
is not fully understood, it is known to arise from abnormal dosage of
specific imprinted genes located on chromosome 11p15, including an insulin-like
growth factor gene. Several different genetic mutations have been identified
as causing Beckwith-Wiedemann syndrome, including chromosomal abnormalities,
uniparental disomy, and mutations within a single gene.
Other
medical conditions are now recognized to arise from abnormalities of genomic
imprinting. For example, a small proportion of cases of Russell-Silver
syndrome, a growth disorder leading to very short stature and body asymmetry,
results from uniparental disomy of chromosome 7. Similarly, transient
neonatal diabetes is caused by uniparental disomy of chromosome 6, and
a pattern of multiple anomalies is found in the presence of uniparental
disomy of chromosome 14.
Only
a small number of human genes have thus far been shown to be imprinted,
but discovery of the imprinting mechanism as essential to normal human
development represents an important step forward in the genetics of childhood
psychiatric illnesses. Indeed, it has helped to broaden the understanding
of genetic disorders that do not follow Mendelıs laws of inheritance and
has forced an expansion of thinking about the complexities of human development.
Finally, it is hoped that research in this area will provide further insights
that may ultimately lead to the development of novel treatments for these
disorders.
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