In the present survey, we described 15 sSMCs involving different chromosomes with a frequency of 0.073% in 20,481prenatal diagnosis cases, which was in agreement with the incidence of 0.075% previously reported [4]. The origins of sSMCs in our survey were derived from acrocentric chromosomes (42%), followed by non-acrocentric chromosomes (37%) and the Y chromosome (21%), which is in agreement with the literature [10]. It is reported that chromosome 15 was the most common origin for sSMCs in acrocentric chromosomes, accounting for ~ 30–50% [11]. Chromosome 15-derived sSMCs incorporating the PWACR are associated with developmental delay, mental retardation, ataxia, seizures and behavioural problems and patients with more copies of this region may develop a more severe phenotype [12]. In contrast, sSMCs(15) without the PWACR has no or only a minor influence on the carrier’s phenotype but is associated with a high incidence of infertility in males [13]. Patients that are positive for sSMC(15) even without the involvement of the PWACR should also undergo prenatal testing for UPD(15) [14]. In our study, three cases of sSMCs originated from chromosome 15 in total. The sSMC of case 7 included four to six copies of the PWACR and was evaluated as pathogenic; the couple decided to terminate the pregnancy. The sSMCs of cases 5 and case 6 did not involve the PWACR and were evaluated as a variant of unknown significance. Two couples continued the pregnancies and the newborns did not display any clinical phenotype at 9 or 15 months. No UPD conditions for imprinted chromosome 15 were found in these three cases.
Routine cytogenetic analysis showed mosaic sSMCs in 8 out of the 15 cases, with the sSMC cell line mosaic level ranging from 16 to 77%. sSMCs are quite unstable during mitosis leading to mosaicism, which is estimated to be present in 50% of sSMC cases, which is in accordance with our data [15]. The frequency of mosaic sSMCs derived from non-acrocentric, acrocentric and Y chromosomes was 80% (4/5), 33% (2/6) and 67% (2/3), respectively. A higher prevalence of mosaicism in non-acrocentric chromosomes than acrocentric chromosomes may be related to dose sensitivity. Liehr and Al-Rikabi found that mosaicism was the reason for the normal phenotypes in carriers of sSMC with known and well-defined syndromes. In 2% of cases, there was a normal or much less severe outcome than expected in low-mosaic samples [16].
Only around 30% of sSMC carriers are clinically impaired after birth and the pathogenicity of sSMCs is higher in prenatal cases [17]. In our survey, 14 cases of sSMC were successfully identified by SNP array and nine cases were pathogenic or likely pathogenic (9/14), four cases were a variant of unknown significance (4/14) and one case was likely benign (1/14). We speculated that sSMCs with a clinical phenotype would more likely lead to a termination of the pregnancy, thus decreasing the frequency of pathogenicity in live births. For the nine cases of pathogenicity or likely pathogenicity, seven pregnancies were terminated and two were continued. Case 10 presented with a karyotype of 47,XY,+mar with an increased nuchal translucency of 3.4 mm. A small marker chromosome was tetraploid for 2.82 Mb in the region 22q11.21(chr22: 18,648,855_21,464,764). This region correlates with cat-eye syndrome, which is a rare genetic syndrome with an incidence of around 1/150,000 live births and is caused by partial tetrasomy of chromosome 22 [18]. The classical triad of CES consists of iris colobomas, anal malformations and ear anomalies. Ultrasound findings of case 10 indicated pyelectasis during the second trimester. The pregnancy was continued and the mother gave birth to a boy at 39 + 2 weeks gestation (birth weight 2990 g, length 50 cm and head circumference 34 cm). Apgar scores were 10 (1′) and 10 (5′). Neonatal pneumonia was diagnosed 1 h after birth with arterial blood gas results of pH 7.329, PCO2 33.7 mmHg and PO2 37 mmHg.
Case 13 also showed a pathogenic sSMC but continued gestation. The karyotype was 46,X,+mar and showed a loss of 7.97 Mb for the region Yq11.222q11.23 (chrY: 20,828,795_28,799,654), which was a deletion of the Y chromosome in the AZFb+AZFc region. The father was also a carrier of an sSMC with the same morphology and presenting oligozoospermia. The AZF region that undergoes microdeletions has been mapped to Yq11.22–23 and consists of three subregions called AZFa, AZFb and AZFc, which are associated with male oligo/azoospermia and accounts for 10–12% of phenotypically normal men with idiopathic infertility [19]. The most frequent deletion type is of the AZFc region (~ 80%) followed by AZFa (0.5–4%), AZFb (1–5%) and AZFb+c (1–3%) [20]. Deletions of the entire AZFa region are associated with an invariable clinical phenotype of Sertoli cell-only syndrome and azoospermia, whereas deletions of the AZFc region are compatible with residual spermatogenesis and severe oligozoospermia may even be transmitted naturally to the male offspring in rare cases [21]. Furthermore, there is a ~ 50% chance of retrieving spermatozoa by testicular sperm extraction and conceiving children by intracytoplasmic sperm injection for men with azoospermia and AZFc deletion [22]. AZFb+c deletions are similar to the complete deletions of the AZFa region; however, spermatid arrest and even crypto/oligozoospermia have been reported in only three cases [23, 24]. Case 13 chose to continue the pregnancy and a boy was born naturally at 39 + 4 weeks. The boy was followed-up to 1 year and his development and growth were normal.
Approximately 77% of sSMCs are de novo while 23% are inherited, either maternally (16%) or paternally (7%) [2]. In our study, three cases were inherited, two of which maternally and one paternally. Case 9 was at high risk of trisomy 21(1/180) and presented with a mosaic karyotype of 47,XY,+mar[14]/46,XY[16]. The sSMC was close to the centromere and included a tetraploid gain of 4.18 Mb of the 21q11.2q21.1 region, which is a dose-insensitive region and does not contain the Down syndrome critical region (DSCR). Duplication of the DSCR due to duplication of 0.6–8.3 Mb within human chromosome 21q22 is sufficient to induce the major phenotypes of Down syndrome, i.e. mental retardation, congenital heart disease, characteristic facial appearance, and probably the hand and dermatoglyphic abnormalities [25]. Patients excluding the DSCR present mild and non-specific phenotypes, such as joint hyperlaxity, hypotonia and brachycephaly hypertelorism, epicanthic folds, strabismus and mildly dysmorphic ears [26]. The mother of case 9 without any abnormal clinical phenotype was also a carrier of an sSMC with a mosaic karyotype of 47,XX,+mar[11]/46,XX[39], which is a triploid gain for the same region of 21q11.2q21.1 but with a different morphology, suggesting that the sSMC underwent recombined duplication during through two generations. If the additional euchromatic material is one small copy near the centromere, it can be tolerated. Whereas if the additional euchromatic material is too large or involves between four and six copies, an abnormal phenotype occurs [17]. We estimated the sSMC as likely a benign variant and would have no or only a minor influence on the carrier’s phenotype, thus the women chose to continue the pregnancy. A boy was born naturally at 39 + 6 weeks and was followed-up for 3 years with development and growth both normal.
Another consequence of sSMC is the risk for uniparental disomy and 1.3% of sSMC cases present with UPD [27, 28]. In cases with UPD, 40 of 46 cases (87%) of cases had a maternal UPD and only 13% were paternal [29]. Thus far, only five chromosomes have been defined as imprinted based on the associated clinical phenotypes: chromosomes 6, 7, 11, 14 and 15. In our study, two cases of sSMC were combined with UPD (UPD(1) and UPD(22)). Chromosomes 1 and 22 are concerned imprinting disease impossibility, but the potential of unmask recessive alleles has been described for several diseases [30]. Case 1 had UPD for the entire chromosome 1. The foetus, therefore, was at increased risk for recessive genetic diseases and rare disorders [31]. We deemed the sSMC and UPD(1) to both be a variant of unknown significance and the formation was likely via trisomy rescue. For case P11, the SNP array detected an additional chromosome abnormality, i.e. a loss of heterozygosity at segmental chromosome 22q12.2q13.2(chr22: 29,841,642_43,483,242). Six clinically normal cases with UPD(22) mat and three cases of recessive gene activation due to UPD(22) mat have been reported [28]. We classified the sSMC as likely pathogenic and UPD(22) as a variant of unknown significance. These data suggested that some sSMC patients may have additional chromosomal UPD anomalies and thus could be underestimated without advanced molecular techniques.
Case 3 presented an abnormal ultrasound indicating nasal bone hypoplasia and a mosaic karyotype of 47,XY,+mar[7]/46,XY[38]. The sSMC was a 24.99 Mb gain of the 3q26.31q29 region [arr 3q26.31q29(172862469_197851444)×2~3[30%]], with a neocentromere. Neocentric sSMCs constitute one of the smallest groups of reported sSMCs and have a centromeric constriction but without detectable alpha-satellite DNA [32]. Neocentric sSMCs carry newly derived centromeres that are apparently formed within interstitial chromosomal sites and have no centromeric function, but it is unclear how the neocentromere is acquired and formed on an acentric fragment [33]. More than 90% of cases of a neocentric sSMC are associated with an adverse clinical outcome, which is mainly due to the size of the imbalanced chromosome dosage. In total, 10 neocentric sSMCs of chromosome 3 have been reported, nine of which were derived from the distal tip of the long arm [17]. Typical phenotypes of neocentric sSMC(3) are dysmorphic features (64%), streaky hyperpigmentation (55%), mental retardation (45%), kidney problems (36%, and polydactyly (27%) [28]. We classified the sSMC of case 3 as pathogenic, thus the pregnancy was terminated.
The shortcoming of this paper was that interphase fluorescence in situ hybridisation (FISH) analyses were not applied to identify the morphology due to a limited amount of specimens and time in prenatal diagnosis, which was not indispensable for pathogenicity assessment and genetic counselling. The disadvantage of chromosomal microarray analysis for sSMC characterisation is if the sSMC contains only heterochromatin, it may not be identified. In this study, the failure to detect the chromosomal origin of cases 12 is likely due to the same reason. Meanwhile, low-level mosaicism (< 20%) can also be missed. The combined application of traditional and molecular cytogenetic analyses has a critical role in precisely characterising sSMCs, including the mosaic form, molecular components, and shape of the sSMCs, which offers more information for genetic counselling.
