BackgroundComplex small supernumerary marker chromosomes (sSMC) constitute one of the smallest subgroups of sSMC in general. Complex sSMC consist of chromosomal material derived from more than one chromosome; the best known representative of this group is the derivative chromosome 22 {der(22)t(11;22)} or Emanuel syndrome. In 2008 we speculated that complex sSMC could be part of an underestimated entity.ResultsHere, the overall yet reported 412 complex sSMC are summarized. They constitute 8.4% of all yet in detail characterized sSMC cases. The majority of the complex sSMC is contributed by patients suffering from Emanuel syndrome (82%). Besides there are a der(22)t(8;22)(q24.1;q11.1) and a der(13)t(13;18)(q11;p11.21) or der(21)t(18;21)(p11.21;q11.1) = der(13 or 21)t(13 or 21;18) syndrome. The latter two represent another 2.6% and 2.2% of the complex sSMC-cases, respectively. The large majority of complex sSMC has a centric minute shape and derives from an acrocentric chromosome. Nonetheless, complex sSMC can involve material from each chromosomal origin. Most complex sSMC are inherited form a balanced translocation in one parent and are non-mosaic. Interestingly, there are hot spots for the chromosomal breakpoints involved.ConclusionsComplex sSMC need to be considered in diagnostics, especially in non-mosaic, centric minute shaped sSMC. As yet three complex-sSMC-associated syndromes are identified. As recurrent breakpoints in the complex sSMC were characterized, it is to be expected that more syndromes are identified in this subgroup of sSMC. Overall, complex sSMC emphasize once more the importance of detailed cytogenetic analyses, especially in patients with idiopathic mental retardation.

A case of prenatally detected cri du chat syndrome (5p-) is reported. Amniocentesis was performed following an abnormal ultrasound finding of isolated moderate bilateral ventriculomegaly. The karyotype showed a terminal deletion of the short arm of chromosome 5 including the critical region 5p15 for cri du chat syndrome. This was confirmed by fluorescence in situ hybridisation (FISH). Isolated mild ventriculomegaly may be a non-specific marker for cri du chat syndrome.

We report a prenatal case of a chromosome 21-derived minute supernumerary marker, found as a mosaic along with a trisomy 21 cell line at amniocentesis. Follow-up analysis of other fetal tissues confirmed the mosaicism and also disclosed a normal cell line. It is likely that the marker reflects a mutation event that resulted in trisomy rescue early in embryonic development. Had the trisomy 21 cell line not been found at amniocentesis, a low risk of an abnormal phenotype (approximately 5%) would have been assigned. We suggest that the risk associated with minute non-euchromatic marker chromosomes should be revised to account for the possibility of mosaicism with potentially aneuploid populations and/or uniparental disomy (UPD). The finding of any marker chromosome should prompt a thorough investigation for aneuploid cell lines. In the case of small markers with no euchromatin, the given risk of adverse phenotypic effects is not likely to be associated with the marker per se but with the possible presence of a cryptic aneuploid cell line from which the marker may have arisen.

We report on a neonate with a disorder of sex development, Prader 3-4 external genitalia and a palpable structure in the right inguinal canal suggestive of gonadal tissue. Chromosome studies on blood lymphocytes showed monosomy of chromosome X. Laparoscopy identified a streak-like gonad on the left side, unicorn uterus and a dysgenetic testis on the right, attached to a Fallopian tube. Because of the unilateral palpable gonad and the presence of ambiguous genitalia we investigated further for the presence of Y material. Quantitative fluorescent PCR analysis of material from the dysgenetic gonad and skin fibroblasts revealed the presence of chromosome Y-derived sequences, suggesting sex chromosome mosaicism. In 45,X/46,XY mosaicism, chromosome studies carried out on peripheral lymphocytes do not always reflect the proportion of cell lines in the gonads. The detection of Y chromosome material in a dysgenetic gonad is extremely significant, due to the high risk of malignant transformation.