Mosaic trisomy 15 and prenatal genetic counselling: a case of Prader-Willi syndrome due to maternal uniparental disomy

Introduction

Chromosome number abnormalities are frequently found in prenatal invasive testing performed because of fetal malformations and/or growth retardation detected on a scan. The risk of trisomy increases with advanced maternal age. Trisomy 15 accounts for 7.6% of all abortions for trisomy and for 1.7% of all first-trimester pregnancy losses [1]. Most of trisomy 15 pregnancies are lost during the first or second trimester due to severe malformations [2]. However, in some cases trisomy rescue takes place leading to mosaicism and, consequently, to a better pregnancy outcome.

Prader-Willi syndrome (PWS) has a prevalence of 1/10,000–1/30,000 live born children and is the first recognized genomic imprinting disorder in humans. It is caused by the loss of expression of imprinted genes on the paternal 15q11q13 region. PWS is characterized by severe hypotonia during the neonatal period, cryptorchidism in males and feeding difficulties. In later infancy affected individuals develop an excessive appetite generally leading to obesity; other features include short stature, intellectual disability and behavioral problems. Hypothalamic dysfunction has been implicated in many manifestations of the syndrome [3].

The most common cause is deletion of the 15q11.2q13 region of paternal origin (65–75%). The second most common cause is maternal uniparental disomy (UPD) of chromosome 15 (20–30%) [3]. Subtle differences have been shown between groups of people with deletions compared with matUPD: feeding difficulties, sleep disturbances, delayed speech, lower IQ and hypopigmentation are more common in the deletion group [4].

In mammals, each of the two copies of the chromosomes are inherited one from each parent, i.e. one copy from the mother and one from the father (biparental inheritance). Uniparental disomy occurs when both copies of the same chromosome are inherited from one and the same parent (uniparental). This can lead to phenotypic changes if the affected chromosome contains imprinted regions, such as the 15q11q13 region or 11p15.5. UPD is generally the consequence of a trisomy rescue mechanism [5].

We describe an additional case of prenatal mosaic trisomy 15 with postnatal diagnosis of PWS.

Case presentation

We present a case of a male infant born to a non-consanguineous healthy couple. The mother was 35 years old and the father 38. They had a first trimester miscarriage prior to this pregnancy. Family history was unremarkable. The pregnancy was being followed at an external clinic. First trimester triple screen gave back a low risk for common aneuploidies. Fetal scan at 20 weeks gestational age (GA) revealed choroid plexus cysts, cardiomegaly with multiple ventricular septal defects (VSD), and a single umbilical artery. Amniocentesis for karyotype was performed and the result was: 47,XY,+15[4]/46,XY[46] implying a low level mosaicism for trisomy 15 (8%). Quantitative fluorescent polymerase chain reaction (QF-PCR) for common aneuploidies was normal as no chromosome 15 markers were included in the assay. Array comparative genomic hybridization (aCGH) confirmed the mosaic trisomy 15: arr(15)x2-3 (CytoChip Focus Constitutional microarrays).

The couple was counseled on the poor prognosis of a fetus with trisomy 15. However, they decided to continue the pregnancy. Fetal magnetic resonance imaging (MRI) at 34 weeks’ GA showed cardiomegaly and septal defects. Due to the fetal heart anomalies the patient was referred to our tertiary care hospital for delivery and postnatal cardiac management. The baby was born after a spontaneous vaginal delivery at 40 + 1 weeks GA. His birth weight was 2352 g [<1^st centile; −2.69 standard deviation (SD)], height 43 cm (<1^st centile; −4.46 SD) and head circumference 34 cm (<1^st centile; −0.73 SD). On physical examination he was hypotonic and had right cryptorchidism. He also showed brachycephaly, upslanting palpebral fissures, thin upper lip with downturned corners of the mouth and dysplastic rotated ears. His hands had three deep horizontal palmar creases (Figure 1). Echocardiography showed a 10 mm atrial septal defect, a 5 mm muscular ventricular septal defect and a patent ductus arteriosus. He was released after 6 days of monitoring with transient naso-gastric tube feeding due to poor sucking. At age 10 days he was readmitted with a urinary tract infection due to Escherichia coli and was diagnosed with vesico-uretheral reflux grade II. The infection was treated with intravenous gentamicin but the baby developed respiratory distress and required non-invasive mechanic ventilation and eventually pulmonary artery banding.

Figure 1:

At 3 months of age: note the upslanting palpebral fissures and the thin lips with downturned corners of the mouth.

His epicanthal folds are familial/racial.

Transcranial echography was non-specific. He passed the newborn hearing screening. At the moment he is 9 months old. He has a mild motor delay and is receiving physiotherapy while awaiting definitive heart surgery.

Results

In the neonatal ward routine cytogenetic analysis was performed on blood, urine and skin. Blood and urine samples showed a normal male karyotype (46, XY) but a very low level mosaicism for trisomy 15 was found on cultured skin fibroblasts obtained during heart surgery: 2–3% (of 400 metaphases) (Figure 2). On suspicion of PWS we requested methylation-specific multiplex ligation-dependent probe amplification (MS-MLPA) of the Prader-Willi-Angelman syndrome (PWAS) critical region 15q11q13 in blood lymphocytes. The result confirmed the abnormal methylation pattern with a normal copy number, corresponding to maternal uniparental disomy of chromosome 15.

Figure 2:

Postnatal karyotype of cultured fibroblasts showing trisomy 15 mosaicism: one nucleus with two chromosomes 15 and another one with three, each nucleus corresponding to two different cell lines.

FISH probe colors: Blue 15p11.2 (D15z1); red: 15q11.2 (SNRPN); green: 15q24 (PML) Vysis^® Prader-Willi TriColor Probe.

Discussion

The risk of autosomal trisomy increases with advanced maternal age due to meiosis II non-disjunction, and therefore, the extra chromosome is generally of maternal origin. Adequate genetic counseling is of paramount importance for decision-making and obstetric and pediatric management. When confronted with mosaicism of chromosome alterations, genetic counseling is challenging because of the high variability of clinical findings often depending on the proportion of the trisomic line in the different tissues of the organism, most of which are not possible to test prenatally.

No clear characterization of the clinical phenotype of mosaicism for trisomy 15 has been described despite the over 20 cases published so far [1]. Common features in live born children with mosaic trisomy 15 include low birth weight, congenital heart defects (atrial and ventricular septal defect, coarctation of the aorta, hypoplastic left ventricle, mitral atresia, patent ductus arteriosus) and other major and minor malformations such as renal, brain and genital anomalies [2].

A trisomic cell line may disappear during prenatal development following the mechanism of trisomy rescue leading to the level of mosaicism decreasing over the pregnancy and even to a complete loss of the trisomic cell line in postnatal testing, escaping detectability by routine analysis such as karyotype, fluorescence in situ hybridization (FISH) and array CGH [4]. On the other hand, the testing method may yield inconsistent results: cultured cells usually show a lower level of mosaicism than fresh samples [6].

If the trisomy involves a chromosome carrying imprinted genes, the result of trisomy rescue and uniparental disomy may be a clinically significant entity such as PWS (chromosome 15), Silver-Russell syndrome (chromosome 11) and Temple syndrome (chromosome 14) [5]. The risk of UPD is 1/3 (33%) [7]. In order to achieve a correct diagnosis further genetic testing should be offered in all cases of mosaic trisomy 15 detected in prenatal testing. MS-MLPA of the 15q11q13 region is a single diagnostic method to test for the microdeletion and in the assay for the abnormal methylation pattern of the region. Alternatively, microsatellite analysis, single nucleotide polymorphism (SNP) array or methylation-sensitive PCR are also sensitive testing methods [6], [8].

This recommendation is applicable to mosaic trisomies of other chromosomes carrying imprinted genes as UPD should be suspected especially in the context of fetal growth problems including matUPD7 (Silver-Russell syndrome), matUPD14 (Temple syndrome), patUPD14 (Kagami-Ogata), patUPD11 (Beckwith-Wiedemann syndrome) [5].

Consistent with the literature, our proband displayed some features of mosaic trisomy 15 [interuterine growth restriction (IUGR), congenital heart disease] and also characteristic signs of PWS such as hypotonia and cryptorchidism [9]. As mentioned above, individuals with paternal 15q11q13 deletions have the typical PWS phenotype, whereas individuals with maternal UPD 15 have a slightly milder phenotype with regards to cognitive function and sleep and eating disorders. However, those with maternal UPD and mosaic trisomy 15 tend to have a more severe phenotype as a consequence of higher incidence of congenital malformations, especially of the heart [10]. Establishing a correct diagnosis of PWS enables an adequate management of the neurodevelopmental and endocrine problems associated and provides adequate genetic and prognostic counseling to families.

Conclusions

Our case illustrates the need for further genetic testing in all trisomy 15 mosaicisms detected in prenatal invasive testing in order to screen for UPD and to make the diagnosis of PWS if maternal UPD15 is confirmed.

The recommendation is applicable to prenatally detected mosaic trisomies of other chromosomes carrying imprinted genes, such as 7, 11 and 14.

This may help health professionals provide early and adequate medical management and families receive more accurate genetic counseling.