Background: The majority of small supernumerary marker chromosomes (sSMCs) are derived from one single chromosome. Complex sSMCs instead consist of two to three genomic segments, originating from different chromosomes. Additionally, discontinuous sSMCs have been seen; however, all of them are derived from one single chromosome. Here, we reported a 41 year-old patient with infertility, hypothyroidism, rheumatism, and degenerative spine and schizoaffective disorder, being a carrier of a unique, complex, and discontinuous sSMC. Methods: The sSMC was characterized in detail by banding and molecular cytogenetics including fluorescence in situ hybridization (FISH) and array-comparative genomic hybridization (aCGH), as well as by optical genome mapping (OGM). Results: The neocentric sSMC characterized here contained seven portions of five different chromosomes and was present in ~50% of both peripheral blood cells and buccal mucosa cells. aCGH and OGM revealed gains of 8q12.3q12.3, 8q22.3−8q23.1, 9q33.3−9q34.11, 14q21.1−14q21.1, 14q21.1−14q21.2, 15q21.2−15q21.2, and 21q21.1−21q21.1. Furthermore, glass-needle based microdissection and reverse FISH, as well as FISH with locus-specific probes confirmed these results. The exact order of the involved euchromatic blocks could be decoded by OGM. Conclusions: Among the >7000 reported sSMCs in the literature, this is the only such complex, discontinuous, and neocentric marker with a centric minute shape.

OBJECTIVE: To characterize small supernumerary marker chromosomes (sSMC) in infertile males RESEARCH QUESTION: Are molecular cytogenetic methods still relevant for the identification and characterization of sSMC in the era of next-generation sequencing?
METHODS: In this paper, we report five males with oligoasthenozoospermia or azoospermia with a history of recurrent pregnancy loss in partnership in four cases. R-banding karyotyping and fluorescence in situ hybridization (FISH) analysis were performed and showed sSMC in all five cases. Microdissection and reverse-FISH were performed in one case.
RESULTS: One sSMC, each, was derived from chromosome 15 and an X-chromosome; two sSMC were derivatives of chromosome 22. The fifth sSMC was a ring chromosome 4 complemented by a deletion of the same region 4p14 to 4p16.1 in one of the normal chromosomes 4. All markers were mosaics except one of sSMC(22).
CONCLUSIONS: Through this study, we emphasize the necessity of a proper combination of high-throughput techniques with conventional cytogenetic and FISH methods. This could provide a personalized diagnostic and accurate results for the patients suffering from infertility or RPL. We also highlight FISH analyses, which are essential tools for detecting sSMC in infertile patients. In fact, despite its entire composition of heterochromatin, sSMC can have effects on spermatogenesis by producing mechanical perturbations during meiosis and increasing meiotic nondisjunction rate. This would contribute to understand the exact chromosomal mechanism disrupting the natural and the assisted reproduction leading to offer a personalized support.

The formation of small supernumerary marker chromosomes (sSMC) still remains enigmatic. However, it is suggested that majority of all kinds of de novo sSMC (inverted duplication-, ring-, and centric-minute-shaped ones) are products of incomplete trisomic rescue during early embryogenesis. Recent work, based on molecular cytogenetics, suggests that these trisomic rescue events are going together with chromothripsis, directed against the supernumerary chromosome to be degraded. Here we present a protocol how to characterize so-called discontinuous sSMC by means of fluorescence in situ hybridization (FISH).

BACKGROUND: Angelman syndrome (AS) is a neurodevelopmental disorder. AS patients concomitant with sSMC are rather rare events. It will provide more useful and proper information for genetic counseling to identify the sSMC origin.
METHODS: A 27-year-old woman was referred for genetic counseling and prenatal diagnosis at 26 weeks of gestation due to her elder daughter, diagnosed as Angelman syndrome (AS) with an interstitial deletion in one of the chromosomes 15, carrying a small supernumerary marker chromosome (sSMC). The G-banding results of the woman and her current fetus both were 47,XX,+mar. In this paper, fluorescence in situ hybridization (FISH) results showed that there was no deletion of chromosome 15 in the woman and fetus. We demonstrated that the proband\'s sSMC was maternally inherited and was an inv dup(22)(q11.1) , and that the deletion in 15q11.2-q13.1 was de novo.
CONCLUSIONS: Taking into account above results and normal phenotypes of the proband\'s mother, in this case we suggest that the sSMC don\'t increase the recurrence risk of AS. After prenatal diagnosis, the woman chose to continue the pregnancy, and finally gave birth to a normal female infant.

Small supernumerary marker chromosomes (sSMC) are found about four times more frequently in subfertile compared to the general population. The reason for this finding is still unclear. However, a connection of interphase architecture and genome function is suggested. And as we found in a previous study the presence of sSMC influences the nuclear architecture of peripheral blood cells and fibroblasts, we hypothesized that sSMC could have similar effects in sperm cells possibly leading to infertility. Here we applied for the first time 3-dimensional interphase fluorescence in situ hybridization (3D-FISH) to characterize the position of an extra-chromosome with respect to its sister- and selected other chromosomes (6, 15, 18, 19, 21, X, and Y) in sperm. Two sSMC carrier brothers with the identical sSMC derived from chromosome 15 were studied. One of the brothers was fertile and the other brother was infertile. Deviations from the normal positioning of chromosomes 21 and Y were seen in both brothers and for chromosomes 19 and X only in the infertile brother. Most striking were high rates of nullisomy and/or disomy for chromosomes 15, including sSMC (15), and 18 exclusively seen in the infertile brother. Overall, further evidence is provided that sSMC influence the nuclear architecture of a cell, including sperm. Further studies are necessary in sperm of fertile and infertile sSMC carriers to elaborate if the detected aneuploidy like that seen in the infertile brother is due to sSMC presence and disturbance of nuclear architecture.

Somatic mosaicism is something that is observed in everyday lives of cytogeneticists. Chromosome instability is one of the leading causes of large-scale genome variation analyzable since the correct human chromosome number was established in 1956. Somatic mosaicism is also a well-known fact to be present in cases with small supernumerary marker chromosomes (sSMC), i.e. karyotypes of 47,+mar/46. In this study, the data available in the literature were collected concerning the frequency mosaicism in different subgroups of patients with sSMC. Of 3124 cases with sSMC 1626 (52%) present with somatic mosaicism. Some groups like patients with Emanuel-, cat-eye- or i(18p)- syndrome only tend rarely to develop mosaicism, while in Pallister-Killian syndrome every patient is mosaic. In general, acrocentric and non-acrocentric derived sSMCs are differently susceptible to mosaicism; non-acrocentric derived ones are hereby the less stable ones. Even though, in the overwhelming majority of the cases, somatic mosaicism does not have any detectable clinical effects, there are rare cases with altered clinical outcomes due to mosaicism. This is extremely important for prenatal genetic counseling. Overall, as mosaicism is something to be considered in at least every second sSMC case, array-CGH studies cannot be offered as a screening test to reliably detect this kind of chromosomal aberration, as low level mosaic cases and cryptic mosaics are missed by that.