-  sSMC -





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as provided by the patient supportgroup 'Unique'



Small supernumerary marker chromosomes (sSMC)


The constitutional karyotype of human consists of 46 chromosomes (46,XX or 46,XY).
Each chromosome is present twice, apart from the X- and Y-chromosome in male.
In summary there are 24 different human chromosomes (1-22 and X and Y). See Fig. 1.

However, about 2.7 million individuals of the whole human population do not have 46 but 47 chromosomes,
i.e. 46 normal chromosomes and one small supernumerary marker chromosome (47,XX,+mar or 47,XY,+mar).


These small supernumerary marker chromosomes (= sSMC) can originate from
each of the 24 human chromosomes and have different shapes. See Fig. 2.


About 70% of the cases with sSMC are de novo, 30% are inherited within a family.
About 30% of the carriers of a sSMC are clinically abnormal, 70% are clinically normal.
Thus, the name 'patient with a sSMC' summarizes a very heterogeneous group of persons/patients .

The main problem in connection with sSMC appears, when the diagnosis of the sSMC presence is made prenatally.
Up to now there has not been a possibility to make clear predictions about the outcome of the pregnancy. See Fig. 3.


Presently, research is going on towards a genotype-phenotype correlation of sSMC.
This page collects all published cases with an sSMC to come nearer to this goal.

As shown below in Fig. 4 on the example of chromosome 2: there are hints that it is possible in principle (!)*** to distinguish benign from malignant sSMC.

*** The whole story is not as simple as it is shown in this diagram for chromosome 2 as additional factors like uniparental disomy (UPD) or additional complex or cryptic rearrangements are possible, which may influence clinical outcome.


At present, we are setting up a collection of sSMC cases.

For this purpose we are looking for carriers of sSMC who would be willing to spend 10ml of peripheral blood to establish from that an immortalized cell line available for further sSMC-research (in case of interest please contact Dr. Liehr in Jena, Germany to obtain an informed consent and additional information).

All sSMC studies carried out in the Institute of Human Genetics Jena (Germany) are approved by the ethical commission of the Friedrich Schiller University (FSU) Jena, Germany - internal code 1457-12/04.



Figure 1.   46 normal chromosomes plus a small supernumerary marker chromosome = sSMC (arrow head)


sSMC shapes

Figure 2.   Three possible different shapes of an sSMC - schematic drawing.


sSMC frequ

Figure 3.   Until recently the only clinical correlations known between sSMC presence and clinical outcome based on data from 1998.
This data is still broadly used in genetic counseling.



Figure 4.  Potentially gene-dosage insensitive regions are labelled in green; potentially genedoasge sensitive ones are labelled in red.