Abstract
Small supernumerary maker chromosomes (sSMC) and uniparental disomy (UPD) are rare, and a combination of both is rarely encountered. Accordingly, only 46 sSMC cases UPD have been reported. Despite of its rareness, UPD has to be considered, especially in prenatal cases with sSMC. Here, the authors reviewed all sSMC cases with UPD (sSMCU+) and compared them to sSMC without UPD (sSMCU−), which resulted in the following correlations: 1) every sSMC, irrespective of its chromosomal origin, may be principally connected with UPD; 2) mixed hetero- and iso-UPD (hUPD/iUPD) can be observed most often in sSMCU+ cases followed by complete iUPD, complete hUPD, and segmental iUPD; 3) UPD of chromosomes 6, 7, 14, 15, 16, and 20 is most often reported in sSMCU+; 4) maternal UPD was approximately nine times more frequent than paternal UPD; 5) if mosaic with a normal cell line, acrocentric-derived sSMC had a three times higher chance of occurrence than the corresponding nonmosaic sSMC cases; 6) UPD in connection with a parentally inherited sSMC is, if existent at all, a rare event; and 7) the gender type and shape of sSMC had no effect on UPD formation. Overall, sSMCU+ cases may have a story to tell about chromosome number control mechanisms in early embryogenesis.
Keywords
Conventional banding cytogenetics alone is not sufficient to identify or characterize small supernumerary marker chromosome (sSMC), a structurally abnormal chromosome that is (in general) equal in size or smaller than chromosome 20 of the same metaphase spread. Phenotypes associated with an sSMC are variable, from clinically normal to severely affected. Beside chromosomal imbalance caused by sSMC, other factors such as mosaicism, mode of inheritance (de novo or parental), uniparental disomy (UPD), or a chromosome pair derived from only one parent present in a disomic cell line (UPD) of sSMC’s sister chromosomes have to be observed (Liehr et al. 2004).
UPD can be identified by microsatellite analysis, molecular cytogenetics, or single nucleotide polymorphism (SNP)–based array techniques (review in Liehr 2010); however, in the latter approach, UPD cannot be distinguished from deletions.
The UPD concept was introduced in 1980 into human genetics by Eric Engel (1980), and in 1987, the first case of UPD was proven by molecular methods (Créau-Goldberg et al. 1987), even though cases having a UPD were reported before (Liehr 2010). To date, approximately 1600 reports of UPD are available (Liehr 2010, 2011b). It is now an important diagnostic (Eggerman et al., 2005) and prognostic factor for special syndrome (Weksberg and Squire 1976; Halit et al. 2008) and tumorigenesis (Tuna et al. 2009).
As mentioned previously, UPD is something cytogeneticists have to consider as a possibility during routine sSMC diagnostics (Liehr 2004, 2011a). The question as to whether UPD is coincidence or consequence is still a matter of discussion (Kotzot 2002; Liehr, Mrasek, et al. 2006).
Here, we reviewed all sSMC cases with UPD (sSMCU+) available from the literature (http://www.med.uni-jena.de/fish/sSMC/00START-UPD.htm) and compared them to sSMC without UPD (sSMCU−) followed by a discussion about the mode of formation of UPD in connection with sSMC.
Cases with sSMC and UPD
According to the literature (Liehr 2011a, 2011b), 46 cases with sSMC and UPD (sSMCU+) are reported (Table 1). Examples for 13 of 24 chromosomes are available by now: 20 reported for chromosome 15; 6 for chromosome 7; 4 for chromosome 14; 3 for chromosome 20; 2 for chromosome 6, 12, 16, and 22; and 1 each for chromosome 1, 3, 4, 9, and 10. As expected (Liehr et al. 2010), 87% of cases have been reported for maternal UPD, whereas only 6 of 46 cases (13%) have a paternal one.
Details on the 46 Reported sSMCU+ Cases according to Liehr 2011a
Abbreviations: Add. Cytog. Aberr., additonal cytogenetic aberrations; DN, de novo; F, female; H, heterodisomy (hUPD); H & I, mixed form, hUPD and iUPD; I, isodisomy (iUPD); Inv dup, inverted duplicated; M, male; Mar, marker; Min, centric minute; Mos, mosaic; NA, not available; sSMC, small supernumerary marker chromosome; UPD, uniparental disomy.
Case number (column 1), mosaicism (column 2), origin of UPD (column 3), type of UPD (column 4), gender of sSMC carrier (column 5), sSMC shape (column 6), origin of sSMC (column 7), and presence of additional cytogenetic aberrations (column 8) are listed. 47,+der(22)t(11;22)(q23;q11) cases are excluded.
Of the total 46 cases studied, 11 cases were nonmosaic (i.e., had the sSMC in all studied cells); however, in the remainder, an sSMC was present in 8% to 88% of the metaphase spreads. For 13 cases, no information was available if a hetero (hUPD) or an isodisomy (iUPD) was detected. Two cases presented with segmental iUPD, 9 each with complete iUPD or hUPD, and the remainder 13 cases had mixed hUPD and iUPD. For five cases, the parental origin was not tested; however, in all the other cases, a de novo origin was proven. The male-to-female ratio was 25:17, as for four cases the gender was not available. The sSMC shapes (Liehr 2009) were inverted duplicated (18 cases), ring (8 cases), or centric minute (17 cases). Interestingly, three of the sSMCU+ had additional numerical (two cases) or structural (one case) chromosomal aberrations (Table 1).
Cases With sSMC and UPD (sSMCU+) Compared With sSMC and No UPD (sSMCU−)
Comparing sSMC cases with UPD (sSMCU+) and without UPD (sSMCU−) led to the following findings (Liehr and Weise 2007; Liehr 2011a, 2011b).
Chromosomal Origin
In general, sSMC can be derived from each of the 24 human chromosomes, although sSMC with no chromosomal origin determined have also been reported (Mackie Ogilvie et al. 2001; Liehr et al. 2008). Also, UPD can in principal appear for all human chromosomes (Liehr et al. 2004; Rodríguez-Santiago et al. 2010; Liehr 2011a, 2011b). Thus, it can be interpreted just as a matter of small numbers that until now, for only 13 of the 24 human chromosomes are sSMCU+ cases known. Comparing the frequencies, it is obvious that no sSMCU+ are reported if sSMCU− cases constitute less than 5% of all reported sSMC cases (see Fig. 1; note that for chromosome 18, there is only one report for UPD).
Frequency and chromosomal distribution of small supernumerary maker chromosomes with uniparental disomy (sSMCU+) and without uniparental disomy (sSMCU−) cases for all human autosomes.
In summary, irrespective of the chromosomal origin of an sSMC, in principle, UPD occurrence is always possible. Also it can be speculated that a UPD in connection with an sSMC can be expected to be much more likely than in the others for chromosomes 6, 7, 14, 15, 16, and 20 (asterisks in Fig. 1). However, it cannot be excluded that this coincidence is due to an ascertainment bias, as these chromosomes are known to underlie imprinting and are tested more frequently than others.
Mosaicism
Somatic mosaicism is also known to be present in cases with sSMC. Accordingly, mosaicism is present in both cases with sSMCU+ and sSMCU−. It ranges from very low (i.e., fewer than 10% of studied cells have an sSMC) to a very high (i.e., [practically] all cells of the studied tissue have sSMC; Liehr, Mrasek, et al. 2006; Table 1). Also, cryptic mosaicism (Liehr, Mrasek, et al. 2006; Liehr et al. 2010) can be observed in both groups. Interestingly, a statistically significant difference (t-test: p = 0.001) was observed for appearance of mosaicism in sSMCU− (52%; Liehr et al. 2010) and sSMCU+ cases (76%; Table 1). As the mosaicism frequencies of acrocentric versus non– acrocentric-derived sSMCU− differ, these two groups were considered separately: the mosaic rate of acrocentric sSMCU− is 28% versus 69% in sSMCU+ (statistically significant difference; t-test: p = 0.001), and the mosaic rate of non–acrocentric sSMCU− is 82% versus 85% in sSMCU+ (t-test: p = 0.730). This means that acrocentric-derived sSMC tend to present a UPD more likely if mosaic, whereas for non–acrocentric-derived sSMC, there is no correlation with mosaicism.
Male-to-Female Ratio
In sSMCU−, the male-to-female ratio is practically 1:1 (Liehr 2006). In the 42 sSMCU+ cases in which the gender was reported, the ratio was very similar (1.5:1). This is not a statistically significant difference (t-test: p = 0.251).
Parental Origin of UPD
In 6 of the 46 cases (13%) with sSMCU+, both normal sister chromosomes of the sSMC are of paternal origin. This fits with the well-known fact that, in general, maternal UPD appears nine times more frequently than paternal UPD (Liehr et al. 2004). Besides the fact that aneusomies are more likely to be contributed from the female side, some enzymatic content in the male- and female-derived pronuclear compartment was also suggested to be an important factor. It can be argued that since the oocyte has a less active machinery to eliminate chromosomal mistakes than the spermatocyte does, at the stage of pronuclei, an elimination of a paternally-derived additional chromosome could be more likely than a maternally-derived one (Liehr et al. 2004).
Descent of sSMCU+
All by now reported sSMCU+ are de novo (Table 1). It is important to note that UPD in connection with a parentally inherited sSMC is, if existent at all, is a rare event.
sSMC Shape
As recently reviewed, sSMC can form three basic types of shapes: a ring structure (r), an inverted duplication (inv dup), and a centric minute (min) structure (Liehr 2009). All three shapes can be found in sSMCU+, as well as irrespective of the gender (Table 1). As shown in Table 2, there is no significant difference between cases with and without UPD in connection with sSMC shape.
UPD-Associated and Non–UPD-Associated sSMC Are Compared for Acrocentric and Non–Acrocentric-Derived sSMCa
Abbreviations: Inv dup, inverted duplicated; Min, centric minute; r, ring; sSMC, small supernumerary marker chromosome; sSMCU+, small supernumerary marker chromosome with UPD; sSMCU−, small supernumerary marker chromosome without UPD; UPD, uniparental disomy.
There is no influence of sSMC shape on UPD formation.
In addition, it is noteworthy that until now, no UPD was reported in cases with neocentric sSMC (Liehr et al. 2007) or in sSMC formed by the McClintock mechanism (Baldwin et al. 2008). For complex sSMC (Trifonov et al. 2008), only one patient with maternal heterodisomy of chromosome 22 has been reported to date in Emanuel syndrome cases (Dawson et al. 1996).
hUPD or an iUPD in sSMCU+ Cases
The two subtypes of UPD recognizable by molecular analysis are heterodisomy (hUPD), defined as an inheritance of both chromosomes from one parental pair, and isodisomy (iUPD), that is, inheritance of two copies of the same chromosomes from one parent. Disease can be caused if hUPD and iUPD affect a gene underlying genomic imprinting (expression of a gene that depends on parental origin). In addition, iUPD, independently of imprinting, can result in a functional reduction to homozygosity and thus can cause a recessive disease to occur in the offspring of one carrier patient. According to literature, monosomic rescue cases should always be iUPD, but to continue in the same statement, hUPD and iUPD can be observed as mixed forms mostly. Overall, either meiotic I or II errors and/or postzygotic events contribute to UPD formation (Liehr et al. 2004; Liehr 2010).
For 33 sSMCU+ cases, information was available if hetero- or isodisomic (Table 1). Of 33, 6% had segmental iUPD, 27% each complete iUPD or hUPD, and 67% mixed hUPD and iUPD. However, for cases with complete iUPD and hUPD, it has to be considered that many cases have been tested with only a small number of microsatellite markers. According to the observed possible differences of iUPD and hUPD presence, different mechanisms must contribute to UPD formation in cases with sSMC. However, as tested in Tables 3 and 4, there is no influence of sSMC mosaicism or gender on the UPD type that is formed.
A Comparison of UPD Type and Mosaicism Showed That There Is No Relation between Presence of Mosaicism and UPD Formation
Abbreviations: H, heterodisomy (hUPD); I, isodisomy (iUPD); H & I, mixed form, hUPD and iUPD; sSMC, small supernumerary marker chromosome; UPD, uniparental disomy.
UPD Formation Is Not Influenced by Gender of sSMC Carrier
Abbreviations: H, heterodisomy (hUPD); I, isodisomy (iUPD); H & I, mixed form, hUPD and iUPD; sSMC, small supernumerary marker chromosome;.
How Does UPD Form Together with an sSMC?
As outlined before, sSMCU+ can be connected with (segmental) iUPD, combined iUPD and hUPD, or hUPD alone. In adaptation and extension of the suggestions of Kotzot (2002), at least the following mechanisms of formation are possible (see Fig. 2), even though other, more complicated, ones may also happen.
Schematic depiction of small supernumerary maker chromosomes with uniparental disomy (sSMCU+) formation; only the chromosomes involved in UPD formation are symbolized as bars. Dark-red and light-red bars represent the maternally derived chromosomes, blue ones the paternal derived one. Not included are models for segmental isodisomy (iUPD) formation; structural rearrangements must therefore be postulated. (A) Heterodisomic and mixed iso/heterodisomic sSMCU+ cases can form by two mechanisms (A-1 and A-2; for details, see text). (B) Isodisomic sSMCU+ cases can form by five mechanisms (B-1 to B-5; for details, see text).
Heterodisomy and combined hetero- and isodisomy can be formed easily by two mechanisms (Fig. 2A), both starting with a meiosis 1 error. The first variant (Fig. 2A-1) is that a disomic gamete forms a trisomic zygote, trisomic rescue takes place, and either a mosaic 47,XN,+A/47,XN,+mar/46,XN or 47,XN,+mar/46,XN is formed. The second possibility is that one heterodisomic gamete meets another one, which carries only an sSMC instead of the corresponding sister chromosome. This can be either due to the presence of an inherited sSMC or due to the partial chromosome fragmentation during meiosis (Fig. 2A-2). Either a nonmosaic case with karyotype 47,XN,+mar is formed or trisomic rescue happens to the sSMC and a mosaic 47,XN,+mar/46,XN constitutes.
For iUPD in sSMC cases, at least five mechanisms are conceivable. Those shown in Figure 2B-1 and B-2 are the same as described for hUPD before (Fig. 2A-1 and A-2). The only difference here is that a meiosis 2 error led to an isodisomic zygote. Besides (Fig. 2B-3), a combination of monosomic and trisomic rescue may happen. Finally (Fig. 2B-4 and B-5), somatic erroneous monosomic followed by trisomic rescue may appear. Not included in Figure 2 is a model for segmental iUPD formation in connection with an sSMC. Structural rearrangements of sSMC sister chromosomes must therefore be postulated.
For formation of sSMCU+ with hUPD or mixed hUPD/iUPD, model A-1 is most likely to be taking place. One of the models A-2, B-2, or B-3 will take place in those cases with Emanuel syndrome with UPD of chromosome 22. According to the number of errors necessary to end up with an sSMCU+, iUPD should be formed most frequently according to model B-1, followed by B-4 and/or B-5.
Multiple sSMCs (Liehr, Starke, et al. 2006) are formed most probably another way. However, it can only be speculated here, as only a few of these multiple sSMCs have been studied for UPD at all, and all studies done were without UPD (Liehr 2011a, 2011b), that one possible explanation would be triploidy rescue and another explanation multiple trisomy rescue.
Also noteworthy is that 3 of 46 (6.5%) of sSMCU+ cases have an additional numerical chromosomal aberration (Table 1). In sSMCU−, this can be found in only 43 cases (1.3%), 35 of which have an additional chromosome 21 (Liehr 2011b). Thus, in sSMCU+ cases overall, there seems to be a higher rate of numerical chromosome aberrations and/or chromosome instability compared with sSMCU−.
Conclusion and Outlook
Comprehensive characterization of unusual events such as single (including neocentric) or multiple sSMC can provide rare insights into mechanisms taking place in early embryogenesis. This option should be taken by studying sSMC not only by molecular cytogenetics but also by molecular approaches such as tests for UPD. Here we could show that 1) every sSMC, irrespective of its chromosomal origin, may be principally connected with UPD; 2) mixed hUPD/iUPD can be observed most often in sSMCU+ cases followed by complete iUPD, complete hUPD, and segmental iUPD; 3) UPDs of chromosomes 6, 7, 14, 15, 16, and 20 are most often reported in sSMCU+; 4) maternal UPD is ˜9 times more frequent than paternal UPD in sSMCU+ cases; 2) acrocentric-derived sSMCs tend to present a UPD ˜2.5 times more likely if mosaic with a normal cell line than corresponding nonmosaic sSMC cases; 6) UPD in connection with a parentally inherited sSMC is, if existent at all, a rare event; and 7) sSMC shape and gender have no influence on UPD formation. Still the question as to whether UPD exists in inherited sSMC, neocentric sSMC, and multiple sSMC is not definitely answered. Besides clinical impact, insights into the presence of absence of UPD in multiple sSMCs should contribute to models of formation and may be to yet unknown chromosome elimination processes in the zygote during first cell cleavages.
Footnotes
Acknowledgements
Supported in parts by the DAAD.
The authors declared no potential conflicts of interest with respect to the authorship and publication of this article.
The authors received no financial support for the research and authorship of this article.