-  sSMC -



What are sSMC?



sSMC "can be defined as small structurally abnormal chromosomes that occur in addition to the normal 46 chromosomes" {80}

"The term (small) 'supernumerary marker chromosome' (=sSMC) has been used to refer to any unidentifiable marker chromosome and clearly covers a diverse range of cytogenetic abnormalities" {3}

"Such an 'accessory' chromosome of unknown origin is referred as a marker chromosome (mar), using the standardized human chromosome nomenclature (Paris Conference 1971), and they comprise a mixed collection of structurally rearranged chromosome regions" {2}.



Up to 2004 it was still worth what was stated in 1987 {14-15} and 1992 {16; 42}, respectively:

"For humans there are at present no uniform criteria that enable precise distinction of supernumerary chromosomes from other extra structurally abnormal chromosome"{14}

Several attempts have been made to correlate specific marker chromosomes with a clinical picture. This has resulted in the description of a few specific syndromes, e.g. i(18p)-syndrome, i(9p)-syndrome, the Pallister-Killian syndrome = i(12p)-syndrome and the cat-eye syndrome. However, most markers have not been fully characterized. {16; 44}.

I.e. sSMC are a morphologically heterogeneous group of structural abnormal chromosomes:
different types of inverted duplicated chromosomes, minute chromosomes and ring chromosomes can be detected (see Figure below).

sSMC shapes

Thus, the description of sSMC as ‘markers’, makes sense and should be maintained, even after their identification by molecular cytogenetics.



We suggested the following cytogenetic definition of sSMC {see as well 120-121}

sSMC are structurally abnormal chromosomes that cannot be identified unambiguously by conventional banding cytogenetics alone, and are equal in size or smaller than a chromosome 20 of the same metaphase spread (see Fig. 1). sSMC can be present 1) in a karyotype of 46 normal chromosomes, (2) in a numerically abnormal karyotype (like Turner- or Down-syndrome) or (3) in a structurally abnormal but balanced karyotype (e.g. Robertsonian translocation or ring chromosome formation.

In contrast, a SMC larger than chromosome 20 usually can be identified based on chromosome-banding.

Even though cases with isochromosome 5p, 8p and 9p are not included in the group of sSMC according to that definition, they are included in this page.




What else?

At least minute sSMC evolve by trisomic rescue as recently shown in two cases {112-113}.

If among sSMC B-chromosomes are hidden is discussed in the literature {162}

Also their way of formation may be related to Howel Joly Body formation {164}

sSMC might be helpful as gene vectors in future {171}