Testing to confirm a clinical diagnosis

When a genetic test is requested to confirm a clinical diagnosis
in a child or adult, specialist genetic counselling may not be
requested until after the test result. It is therefore the
responsibility of the clinician offering the test to inform the
patient (or the parents, if a child is being tested) before the test
is undertaken, that the results may have genetic as well as
clinical implications. Confirming the diagnosis of a genetic
disorder in a child, for example, may indicate that younger
siblings are also at risk of developing the disorder. For late
onset conditions such as Huntington disease, it is crucial that
samples sent for diagnostic testing are from patients already
symptomatic, as there are stringent counselling protocols for
presymptomatic testing

Presymptomatic testing

Genetic testing in some late onset autosomal dominant
disorders can be used to predict the future health of a well
individual, sometimes many decades in advance of onset of
symptoms. For some conditions, such as Huntington disease,
having this knowledge does not currently alter medical
management or prognosis, whereas for others, such as familial
breast cancer, there are preventative options available. For adult
onset disorders, testing is usually offered to individuals above
the age of 18. For conditions where symptoms or preventative
options occur in late childhood, such as familial adenomatous
polyposis, children are involved in the testing decision.
Presymptomatic testing is most commonly done for individuals
at 50% risk of an autosomal dominant condition. Testing
someone at 25% is avoided wherever possible, as this could
disclose the status of the parent at 50% risk who may not want
to have this information. There are clear guidelines for
provision of genetic counselling for presymptomatic testing,
which include full discussion of the potential drawbacks of
testing (psychological, impact on the family and financial), with
ample opportunity for an individual to withdraw from testing
right up until disclosure of results, and a clear plan for follow up.

Carrier testing

Testing an individual to establish his or her carrier state for an
autosomal or X linked recessive condition or chromosomal
rearrangement, will usually be for future reproductive, rather
than health, implications. Confirmation of carrier state may
indicate a substantial risk of reproductive loss or of having an
affected child. Genetic counselling before testing ensures that
the individual is informed of the potential consequences of
carrier testing including the option of prenatal diagnosis. In
the presence of a family history, carrier testing is usually
offered in the mid-teens when young people can decide
whether they want to know their carrier status. For autosomal
recessive conditions such as cystic fibrosis, some people may
wish to wait until they have a partner so that testing can be
done together, as there will be reproductive consequences only
if both are found to be carriers.

Prenatal testing

The availability of prenatal genetic testing has enabled many
couples at high genetic risk to embark upon pregnancies that
they would otherwise have not undertaken. However, prenatal
testing, and the associated option of termination of pregnancy,
can have important psychological sequelae for pregnant women
and their partners. In the presence of a known family history,
genetic counselling is ideally offered in advance of pregnancy
so that couples have time to make a considered choice. This
also enables the laboratory to complete any family testing
necessary before a prenatal test can be undertaken.
Counselling should be provided within the antenatal setting
when prenatal genetic tests are offered to couples without a
previous family history, such as amniocentesis testing after a
raised Down syndrome biochemical screening result. To help
couples make an informed choice, information should be
presented about the condition, the chance of it occurring, the
test procedure and associated risks, the accuracy of the test,
and the potential outcomes of testing including the option of
termination of pregnancy. Couples at high genetic risk often
require ongoing counselling and support during pregnancy.
Psychologically, many couples cope with the uncertainty by
remaining tentative about the pregnancy until receiving the test
result. If the outcome of testing leads to termination of a
wanted pregnancy, follow-up support should be offered. Even if
favourable results are given, couples may still have some anxiety
until the baby is born and clinical examination in the newborn
period gives reassurance about normality. Occasionally,
confirmatory investigations may be indicated.

Common chromosomal disorders

Abnormalities of the autosomal chromosomes generally cause
multiple congenital malformations and mental retardation.
Children with more than one physical abnormality and
developmental delay or learning disability should therefore
undergo chromosomal analysis as part of their investigation.
Chromosomal disorders are incurable but most can be reliably
detected by prenatal diagnostic techniques. Amniocentesis or
chorionic villus sampling should be offered to women whose
pregnancies are at increased risk – namely, couples in whom
one partner carries a balanced translocation, women identified
by biochemical screening for Down syndrome and couples who
already have an affected child. Unfortunately, when there is no
history of previous abnormality the risk in many affected
pregnancies cannot be predicted before the child is born.

Translocation Down syndrome

About 5% of cases of Down syndrome are due to translocation,
in which chromosome 21 is translocated onto chromosome 14
or, occasionally, chromosome 22. In less than half of these cases
one of the parents has a balanced version of the same
translocation. A healthy adult with a balanced translocation
has 45 chromosomes, and the affected child has 46
chromosomes, the extra chromosome 21 being present
in the translocation form. The risk of Down syndrome in
offspring is about 10% when the balanced translocation is
carried by the mother and 2.5% when carried by the father. If
neither parent has a balanced translocation, the chromosomal
abnormality in an affected child represents a spontaneous,
newly arising event, and the risk of recurrence is low (1%).
Recurrence due to parental gonadal mosaicism cannot be
completely excluded.

Other autosomal trisomies

Trisomy 18 (Edwards syndrome)
Trisomy 18 has an overall incidence of around 1 in 6000 live
births. As with Down syndrome most cases are due to
nondisjunction and the incidence increases with maternal age.
The majority of trisomy 18 conceptions are lost spontaneously
with only about 2.5% surviving to term. Many cases are now
detectable by prenatal ultasound scanning because of a
combination of intrauterine growth retardation,
oligohydramnios or polyhydramnios and major malformations
that indicate the need for amniocentesis. About one third of
cases detected during the second trimester might survive to
term. The main features of trisomy 18 include growth
deficiency, characteristic facial appearance, clenched hands
with overlapping digits, rocker bottom feet, cardiac defects,
renal abnormalities, exomphalos, myelomeningocele,
oesophageal atresia and radial defects. Ninety percent of
affected infants die before the age of 6 months but 5% survive
beyond the first year of life. All survivors have severe mental
and physical disability. The risk of recurrence for any trisomy is
probably about 1% above the population age-related risk.
Recurrence risk is higher in cases due to a translocation where
one of the parents is a carrier.

Trisomy 13 (Patau syndrome)

The incidence of trisomy 13 is about 1 per 15 000 live births.
The majority of trisomy 13 conceptions spontaneously abort in
early pregnancy. About 75% of cases are due to nondisjunction,
and are associated with a similar overall risk for recurrent trisomy
as in trisomy 18 and 21 cases. The remainder are translocation
cases, usually involving 13;14 Robertsonian translocations. Of
these, half arise de novo and half are inherited from a carrrier
parent. The frequency of 13;14 translocations in the general
population is around 1 in 1000 and the risk of a trisomic
conception for a carrier parent appears to be around 1%. The
risk of recurrence after the birth of an affected child is low but
difficult to determine. Prenatal ultrasound scanning will detect
abnormalities leading to a diagnosis in about 50% of cases. Most
liveborn affected infants succumb within hours or weeks of
delivery. Eighty percent die within 1 month, 3% survive to 6
months. The main features of trisomy 13 include structural
abnormalities of the brain, particularly microcephaly and
holoprosencephaly (a developmental defect of the forebrain),
facial and eye abnormalities, cleft lip and palate, postaxial
polydactyly, congenital heart defects, renal abnormalities,
exomphalos and scalp defects. Survivors have very severe mental
and physical disability, usually with associated epilepsy, blindness
and deafness.

Chromosomal mosaicism

After fertilisation of a normal egg nondisjunction may occur
during a mitotic division in the developing embryo giving rise
to daughter cells that are trisomic and nulisomic for the
chromosome involved in the disjunction error. The nulisomic
cell would not be viable, but further cell division of the trisomic
cell, along with those of the normal cells, leads to chromosomal
mosaicism in the fetus. Alternatively a chromosome may be lost
from a cell in an embryo that was trisomic for that
chromosome at conception. Further division of this cell would
lead to a population of cells with a normal karyotype, again
resulting in mosaicism.

mosaic abnormality

The clinical effect of a mosaic abnormality detected
prenatally is difficult to predict. Most cases of mosaicism for
chromosome 20 detected at amniocentesis, for example, are
not associated with fetal abnormality. The trisomic cell line is
often confined to extra fetal tissues, with neonatal blood and
fibroblast cultures revealing normal karyotypes in infants
subsequently delivered at term. In some cases, however, a
trisomic cell line is detected in the infant after birth and this
may be associated with physical abnormalities or developmental
delay.

Mosaicism

Mosaicism for a marker (small unidentified) chromosome
carries a much smaller risk of causing mental retardation if
familial, and therefore the parents need to be investigated
before advice can be given. Chromosomal mosaicism detected
in chorionic villus samples often reflects an abnormality
confined to placental tissue that does not affect the fetus.
Further analysis with amniocentesis or fetal blood sampling may
be indicated together with detailed ultrasound scanning.

translocations

Robertsonian translocations occur when two of the acrocentric
chromosomes (13, 14, 15, 21, or 22) become joined together.
Balanced translocation carriers have 45 chromosomes but no
significant loss of overall chromosomal material and they are
almost always healthy. In unbalanced translocation karyotypes
there are 46 chromosomes with trisomy for one of the
chromosomes involved in the translocation. This may lead to
spontaneous miscarriage (chromosomes 14, 15, and 22) or
liveborn infants with trisomy (chromosomes 13 and 21).
Unbalanced Robertsonian translocations may arise
spontaneously or be inherited from a parent carrying a
balanced translocation.

Reciprocal translocations

Reciprocal translocations involve exchange of chromosomal
segments between two different chromosomes, generated by
the chromosomes breaking and rejoining incorrectly. Balanced
reciprocal translocations are found in one in 500–1000 healthy
people in the population. When an apparently balanced
recriprocal translocation is detected at amniocentesis it is
important to test the parents to see whether one of them
carries the same translocation. If one parent is a carrier, the
translocation in the fetus is unlikely to have any phenotypic
effect. The situation is less certain if neither parent carries the
translocation, since there is some risk of mental disability or
physical effect associated with de novo translocations from
loss or damage to the DNA that cannot be seen on
chromosomal analysis. If the translocation disrupts an
autosomal dominant or X linked gene, it may result in a
specific disease phenotype.

balanced translocation

the balanced translocation whose offspring would be at risk.
Abnormalities resulting from an unbalanced reciprocal
translocation depend on the particular chromosomal fragments
that are present in monosomic or trisomic form. Sometimes
spontaneous abortion is inevitable; at other times a child with
multiple abnormalities may be born alive. Clinical syndromes
have been described due to imbalance of some specific
chromosomal segments. This applies particularly to terminal
chromosomal deletions. For other rearrangements, the likely
effect can only be assessed from reports of similar cases in the
literature. Prediction is never precise, since reciprocal
translocations in unrelated individuals are unlikely to be
identical at the molecular level and other factors may influence
expression of the chromosomal imbalance. The risk of an
unbalanced karyotype occurring in offspring depends on the
individual translocation and can also be difficult to determine.
An overall risk of 5–10% is often quoted. After the birth of one
affected child, the recurrence risk is generally higher (5–30%).
The risk of a liveborn affected child is less for families
ascertained through a history of recurrent pregnancy loss
where there have been no liveborn affected infants.
Pregnancies at risk can be monitored with chorionic villus
sampling or amniocentesis.

Deletions

Chromosomal deletions may arise de novo as well as resulting
from unbalanced translocations. De novo deletions may affect
the terminal part of the chromosome or an interstitial
region. Recognisable syndromes have been delineated for the
most commonly occurring deletions. The best known of these
are cri du chat syndrome caused by a terminal deletion of the
short arm of chromosome 5 (5p-) and Wolf–Hirschhorn
syndrome caused by a terminal deletion of the short arm of
chromosome 4

Microdeletions

Several genetic syndromes have now been identified by
molecular cytogenetic techniques as being due to chromosomal
deletions too small to be seen by conventional analysis. These
are termed submicroscopic deletions or microdeletions
and probably affect less than 4000 kilobases of DNA. A
microdeletion may involve a single gene, or extend over several
genes. The term contiguous gene syndrome is applied when
several genes are affected, and in these disorders the features
present may be determined by the extent of the deletion. The
chromosomal location of a microdeletion may be initially
identified by the presence of a larger visible cytogenetic
deletion in a proportion of cases, as in Prader–Willi and
Angelman syndrome, or by finding a chromosomal
translocation in an affected individual, as occured in William
syndrome.

A microdeletion on chromosome

A microdeletion on chromosome 22q11 has been found in
most cases of DiGeorge syndrome and velocardiofacial
syndrome, and is also associated with certain types of isolated
congenital heart disease. With an incidence of 8 per 1000 live
births, congenital heart disease is one of the most common
birth defects. The aetiology is usually unknown and it is
therefore important to identify cases caused by 22q11 deletion.
Isolated cardiac defects due to microdeletions of chromosome
22q11 often include outflow tract abnormalities. Deletions have
been observed in both sporadic and familial cases and are
responsible for about 30% of non-syndromic conotruncal
malformations including interrupted aortic arch, truncus
arteriosus and tetralogy of Fallot.

DiGeorge syndrome

DiGeorge syndrome involves thymic aplasia, parathyroid
hypoplasia, aortic arch and conotruncal anomalies, and
characteristic facies due to defects of 3rd and 4th branchial
arch development. Velocardiofacial syndrome was described as
a separate clinical entity, but does share many features in
common with DiGeorge syndrome. The features include mild
mental retardation, short stature, cleft palate or speech defect
from palatal dysfunction, prominent nose and congenital
cardiac defects including ventricular septal defect, right sided
aortic arch and tetralogy of Fallot.

Sex chromosome abnormalities

Numerical abnormalities of the sex chromosomes are fairly
common and their effects are much less severe than those
caused by autosomal abnormalities. Sex chromosome
abnormalites are often detected coincidentally at amniocentesis
or during investigation for infertility. Many cases are thought to
cause no associated problems and to remain undiagnosed. The
risk of recurrence after the birth of an affected child is very low.
When more than one additional sex chromosome is present
learning disability or physical abnormality is more likely.

Turner syndrome

Turner syndrome is caused by the loss of one X chromosome
(usually paternal) in fetal cells, producing a female conceptus
with 45 chromosomes. This results in early spontaneous loss of
the fetus in over 95% of cases. Severely affected fetuses who
survive to the second trimester can be detected by
ultrasonography, which shows cystic hygroma, chylothorax,
asictes and hydrops. Fetal mortality is very high in these cases.

Turner syndrome

The incidence of Turner syndrome in liveborn female
infants is 1 in 2500. Phenotypic abnormalities vary considerably
but are usually mild. In some infants the only detectable
abnormality is lymphoedema of the hands and feet. The most
consistent features of the syndrome are short stature and
infertility from streak gonads, but neck webbing, broad chest,
cubitus valgus, coarctation of the aorta, renal anomalies and
visual problems may also occur. Intelligence is usually within
the normal range, but a few girls have educational or
behavioural problems. Associations with autoimmune
thyroiditis, hypertension, obesity and non-insulin dependent
diabetes have been reported. Growth can be stimulated with
androgens or growth hormone, and oestrogen replacement
treatment is necessary for pubertal development. A proportion
of girls with Turner syndrome have a mosaic 46XX/45X
karyotype and some of these have normal gonadal
development and are fertile, although they have an increased
risk of early miscarriage and of premature ovarian failure.
Other X chromosomal abnormalities including deletions or
rearrangements can also result in Turner syndrome.

triple X syndrome

The triple X syndrome occurs with an incidence of 1 in 1200
liveborn female infants and is often a coincidental finding. The
additional chromosome usually arises by a nondisjunction error
in maternal meiosis I. Apart from being taller than average,
affected girls are physically normal. Educational problems are
encountered more often in this group than in the other types
of sex chromosome abnormalities. Mild delay with early motor
and language development is fairly common and deficits in
both receptive and expressive language persist into adolescence
and adulthood.

Klinefelter syndrome

The XXY karyotype of Klinefelter syndrome occurs with an
incidence of 1 in 600 liveborn males. It arises by
nondisjunction and the additional X chromosome is equally
likely to be maternally or paternally derived. There is no
increased early pregnancy loss associated with this karyotype.
Many cases are never diagnosed. The primary feature of the
syndrome is hypogonadism. Pubertal development usually
starts spontaneously, but testicular size decreases from
mid-puberty and hypogonadism develops. Testosterone
replacement is usually required and affected males are infertile.
Poor facial hair growth is an almost constant finding. Tall
stature is usual and gynaecomastia may occur. The risk of
cancer of the breast is increased compared to XY males.
Intelligence is generally within the normal range but may be
10–15 points lower than siblings. Educational difficultes are
fairly common and behavioural disturbances are likely to be
associated with exposure to stressful environments. Shyness,
immaturity and frustration tend to improve with testosterone
replacement therapy.

XYY syndrome

The XYY syndrome occurs in about 1 per 1000 liveborn male
infants, due to nondisjunction at paternal meiosis II. Fetal loss
rate is very low. The majority of males with this karyotype have
no evidence of clinical abnormality and remain undiagnosed.
Accelerated growth in early childhood is common, leading to
tall stature, but there are no other physical manifestations of
the condition apart from the occasional reports of severe acne.
Intelligence is usually within the normal range but may be
about 10 points lower than in siblings and learning difficulties
may require additional input at school. Behavioural problems
can include hyperactivity, distractability and impulsiveness.
Although initially found to be more prevalent among inmates
of high security institutions, the syndrome is much less strongly
associated with aggressive behaviour than previously thought
although there is an increase in the risk of social
maladjustment.