Τομέας Υγείας - Μητέρας - Παιδιού
Library of the School of Health Sciences

2014-07-23

2014

Μανωλάκος Εμμανουήλ

Καθ. Καναβάκης Εμ., Καθ. Κίτσιοθ-Τζέλη Σ., Καθ. Μαύρου Αρ.

Χαρακτηρισμός υπεράριθμων χρωμοσωμάτων-δεικτών στον προγεννητικό έλεγχo

Greek

Small supernumerary marker chromosomes (sSMCs) are defined as structural
chromosomal abnormalities of equal or smaller size than chromosome 20 that can
not be characterised and identified by classical cytogenetic techniques. A sSMC
can appear in a karyotype with (1) 46 normal chromosomes, (2) numerical
aneuploides (e.g. Turner syndrome or Down syndrome) or (3) in balanced
translocations. The sSMCs appear with an incidence of 0.075% in random cases of
prenatal screening while merely 0.044% in postnatal cases. The incidence of
sSMCs in cases of prenatal screening with having ultrasound abnormalities is at
least five times greater than in neonates and at least three times greater whom
compared to random prenatal screening cases. Furthermore 0.288% of patients
with neuro/developmental retardation have a sSMC. Approximately 70% of de novo
sSMCs in prenatal screening cases have no phenotypical manifestations. The risk
for phenotypic abnormalities in cases of prenatal screening detection amounts
to approximately 13%. That is depends on the origin of the marker chromosome as
well as on whether this is appearing for the first time in that embryo (de
novo) or familial. The risk can vary from 7% when the de novo sSMC comes from
chromosomes 13, 14, 15, 21 and 22 and is up to 28% for non-acrocentric
chromosomes. In addition more than 98% of hereditary sSMCs are without
phenotypic consequence. In prenatal screening, however, 16% of sSMCs have
clinical consequences. In particular the risk for phenotypic abnormalities for
sSMCs that originate from acrocentric chromosomes amounts to 10.9% and up to
14% for non-acrocentric sSMCs.
We do not know a great deal about on to the exact mechanism of sSMC formation
and in particular it is unclear where, when and why, an sSMC is created during
gametogenesis or embryogenesis. However there are various models of how the
various types of sSMCs are created. These theories are based on findings from
UPD (uni parental disomy) studies and on the observation that sSMCs can be
created from incomplete correction of a trisomy. In general a sSMC is created
by the additive effect of two or more incidents during gametogenesis or
embryogenesis.
Usually the presence of an sSMC in a patient is confirmed using chromosomal
banding from peripheral blood sample, amniotic fluid or chorionic villus
samples. If cytogenetic analysis reveals an sSMC, then this has to be further
characterised using molecular cytogenetic techniques. Today with the
development of molecular cytogenetics and the use of array-CGH almost all
chromosomal imbalances can be characterised.
The aim of this study was to fully investigate sSMCs that are found in prenatal
samples so that the right prognosis as well as genetic counselling can be
given. The ultimate goal of the study was to correlate the genotype with the
phenotype and in particular with the ultrasound findings as well as the
clinical picture of the neonates in cases of continued pregnancies. The
research work focused on characterising and identifying marker chromosomes in
samples of prenatal screening and concurrently studying the presence or absence
of euchromatic regions utilising classical cytogenetic and molecular
cytogenetic techniques (fluorescent in situ hybridization (FISH) and array-
CGH). At the same time the presence or absence of uniparental disomy (UPD) was
studied, focusing on marker chromosomes whose origin came from chromosomes
involved in monogenic disomy. For this reason a molecular analysis for the
presence or absence of UPD was performed in sSMC cases with a high risk for
UPD.
A total 42 sSMCs where successfully characterised, drawn from approximately
50,000 prenatal samples of amniotic fluid and chorionic villi. The results of
the current study confirmed the contribution of molecular cytogenetic
techniques (FISH and Array-CGH) towards the precise characterisation of sSMCs
identified during prenatal screening. By utilising FISH and array-CGH we were
able to successfully characterise all 42 sSMCs that where detected using
classical cytogenetic techniques. On top of the chromosomal origin of the sSMC
we identified the presence or absence, size of euchromatin as well as its
constituent genes. In total 19/42 (45%) of cases this study are connected with
important clinical findings, resulting in accurate genetic counselling for the
couples.
In the current study if we had utilised only array-CGH technique we would have
missed 16 cases of sSMCs that were comprised exclusively from heterochromatic
material. In 12/16 cases the sSMC originated from chromosomes for which UPD
analysis was necessary. Conversely non utilisation of array-CGH would not
reveal the complexity of some sSMCs. In addition it are us the possibility for
a detailed characterisation of these sSMCs as a result of which a clearer view
of the prognosis and genetic counselling was obtained. The extended practice of
array-CGH will reveal more surprises and an possibly differentiate the opinion
for the exact composition of supernumerary chromosomal markers.
In conclusion molecular cytogenetics (FISH, array-CGH) in combination with
other molecular techniques (UPD) can provide valuable information regarding the
chromosomal origin and composition of marker chromosomes in prenatal diagnosis.
With the advent of new technologies, like array-CGH, it will be possible to
fully characterise the genetic components of every sSMC and in special cases to
describe and predict the possible clinical phenotype.

Supernumerary markers chromosome, Prenatal diagnosis, Molecular caryotype, Amniocentesis, Diagnosis

No

0

Yes

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