Complex rearranged small supernumerary marker chromosomes (sSMC), three new cases; evidence for an underestimated entity?

Background: Small supernumerary marker chromosomes (sSMC) are present similar to 2.6 x 10(6) human worldwide. sSMC are a heterogeneous group of derivative chromosomes concerning their clinical consequences as well as their chromosomal origin and shape. Besides the sSMC present in Emanuel syndrome, i.e. der(22) t(11;22)(q23;q11), only few so-called complex sSMC are reported. Results: Here we report three new cases of unique complex sSMC. One was a de novo case with a dic(13 or 21;22) and two were maternally derived: a der(18) t(8;18) and a der(13 or 21) t(13 or 21;18). Thus, in summary, now 22 cases of unique complex sSMC are available in the literature. However, this special kind of sSMC might be under-diagnosed among sSMC-carriers. Conclusion: More comprehensive characterization of sSMC and approaches like reverse fluorescence in situ hybridization (FISH) or array based comparative genomic hybridization (array-CGH) might identify them to be more frequent than only similar to 0.9% among all sSMC.