Mosaic Trisomy 5: Prenatal Genetic Diagnosis and Outcomes of a New Case

Abstract

Chromosomal mosaicism is defined as the presence of two or more different cell lines in an organism that originate from the same embryo. Trisomy of chromosome 5 is one of the most severe forms of autosomal trisomy and only seven cases of mosaic trisomy 5 have been reported to date. Mosaicism at prenatal level constitutes a challenge in genetic counseling, particularly in the case of mosaic trisomy 5, due to its low incidence. We report the case of a girl with a prenatal diagnosis of mosaic trisomy 5. The pre- and postnatal genetic tests (noninvasive prenatal testing, array comparative genomic hybridization, karyotype in amniotic fluid cells, karyotype in peripheral blood, and uniparental disomy analysis) revealed the fetal chromosomal status and indicated etiology giving rise to the mosaicism, suggesting a prezygotic meiotic error corrected through late trisomic rescue in the zygote.

Introduction

Chromosomal mosaicism is defined as the presence of two or more different cell lines in an organism that originates from the same embryo. ¹ In amniocyte cultures for prenatal cytogenetic diagnosis, the frequency of chromosomal mosaicism has been reported to be ∼0.2%. ²

Mosaicisms arise from cell division errors or segregation errors. Such errors may be due to meiotic or mitotic nondisjunction, anaphase lag, postzygotic duplication, chromosomal rearrangements due to errors in phase G2 (isochromosome of postzygotic origin), or deletion/duplication of a chromosomal fragment, as well as other causes. ^{3 4}

A mitotic error in the epiblast during embryonic development can give rise to mosaicism present in the embryonic tissue and in the amniotic compartment. If the mitotic error occurs in the extraembryonic tissue, mosaicism will be confined to the placenta. ^{2 5} When trisomic rescue occurs after meiotic error in a very early stage in a cell giving rise to the internal cellular mass and some extraembryonic tissues, the embryo will have a normal chromosomal profile, while the placenta will exhibit mosaic trisomy. When occurring in later stages, however, the fetus could present with mosaic trisomy and the placenta with pure trisomy. ^{2 5} In cases where trisomy rescue takes place after a segregation error of meiotic origin, there is a 33.3% probability that uniparental disomy (UPD) will occur. ^{3 6}

Trisomy of chromosome 5 is one of the most severe forms of autosomal trisomy, and only seven cases of mosaic trisomy 5 have been reported to date. ^{7 8} Likewise, few cases of mosaic trisomy 5 have been diagnosed in the prenatal period ⁹ ; such a scenario constitutes a challenge for genetic counseling, due to the difficulty of estimating its repercussions upon the phenotype and subsequent development of the newborn infant. ⁷

Noninvasive prenatal testing (NIPT) is a diagnostic tool that analyzes maternal blood for free fetal DNA originating from cells of the placental syncitiotrophoblast, ¹⁰ where a chromosomal alteration confined to the placenta will give rise to false-positive results. ^{5 11} Another technique is the array comparative genomic hybridization (CGH) test, which studies copy number variations (CNVs), but is unable to detect low-grade mosaicisms (less than 25%). ¹²

We present the case of a girl with a prenatal diagnosis of mosaic trisomy 5. NIPT detected pure trisomy of chromosome 5 and cytogenetic study of the amniotic fluid determined the presence of mosaicism of chromosome 5. Both results explained the fetal alterations detected at ultrasound examination. Preterm delivery occurred and the infant showed growth retardation. The patient is currently under follow-up, with an adequate course.

Clinical Report

This was the first pregnancy of a 32-year-old woman. Combined triple screening in week 13 + 0 indicated high risk (1/88) for trisomies 13/18. In turn, NIPT in week 14 + 2 revealed low risk for trisomies 13, 18, and 21, but reported the presence of trisomy of chromosome 5. Amniocentesis was rejected by the mother.

Ultrasound in week 21 + 1 showed type I intrauterine growth restriction (IUGR), with concordant fetal biometry in week 19 + 5, accompanied by cardiac anomalies in the form of horizontal orientation of the heart (cardiac axis 53 degrees [normal value 45 ± 10 degrees]) ( Fig. 1 ), diminished left atrial size, no visualization of pulmonary venous drainage, and a slight decrease in left pulmonary artery size.

Fig. 1.

Echocardiographic study in week 21 + 1. Note the mild horizontal orientation of the cardiac axis.

In week 23 + 2, in view of the important IUGR ( p  < 1), amniocentesis was performed, revealing the presence of two cell lines with formula mos 47, XX, +5 [12]/46, XX [83].

Some of the echocardiographic parameters improved: symmetrical growth of both branches of the pulmonary artery, with improvement of the horizontal orientation of the heart in week 29.

Delivery took place in week 35 + 1, resulting in a newborn female weighing 1,480 g ( p  < 3), 41 cm in length ( p  < 3), and with a cranial circumference of 30 cm (p3).

The phenotypic study at 3 months of age revealed features of IUGR with small palpebral length, macrocephalia (p85), a dysplastic right ear, and esotropia of both eyes as evidenced by the Hirschberg's test. There were no other apparent external malformations (normal abdomen, normally formed genitalia, normal skeleton, and stable hips). The neurological examination was normal, with adequate reflexes and good contact with the environment.

The initial echocardiographic findings were normal, with pulmonary branch acceleration being observed at posterior control, with no clinical alterations.

Materials and Methods

Screening of the First Trimester

The combined first-trimester screening is performed in two steps sequentially:

• Week 9 + 2: Pregnancy-associated plasma protein-A (PAPP-A) and free human chorionic gonadotropin β subunit (β-HCG) were assayed by electrochemoluminescence (COBAS 6000; Roche, Basel, Switzerland).

• Week 13: Ultrasound and risk estimation. It used the SsdwLab 6.1 algorithm (SBP SOFT 2007 S.L.; Girona, Spain).

NIPT

The extracted fetal DNA was analyzed by noninvasive prenatal screening TrisoNIM BASIC, using a full sequencing counting method (NIMGenetics; Madrid, Spain). It used the NIFTY Test algorithm (BGI; Shenzhen, China).

Array CGH

Array CGH was performed using 60K from Agilent Technologies (Santa Clara; California, United States), with CytoGenomics software from Agilent Technologies (Santa Clara). It used the ADM2 6.0 algorithm, reference assembly GRCh37.

Cytogenetic Study

The standard culture technique was performed, seeding three independent flasks for the cytogenetic study of the amniotic fluid cells. G-banding was performed and 95 informative metaphases in total were analyzed, studying all culture flasks.

For the peripheral blood study, the standard G-banding and culture technique was performed. Fifty metaphases were analyzed.

The microscopic analysis was made with Metafer and Ikaros software from MetaSystems (Altlussheim, Germany).

Uniparental Disomy Study

UPD study was performed using microsatellite markers (D5S614; D5S807; D5S1349; D5S1465; D5S2505). ABI3130xI Genetic Analyzer from Applied Biosystems (Foster City, California, United States), GeneMapper from Thermo Fisher (Waltham, Massachusetts, United States).

Results

The results of screening (week 9 + 2), in multiples of the median (MoMs), were: PAPP-A 0.63 and free β-HCG 0.07 (reference interval: 0.5–2 MoM).

NIPT revealed pure trisomy of chromosome 5, with an estimated fetal fraction of 8%.

The amniotic fluid karyotype study showed: mos 47, XX, +5 [12]/46, XX [83] ( Fig. 2 ). ¹ The presence of the abnormal trisomic line was detected in all culture flasks.

Fig. 2.

Karyotypes in amniotic fluid. The two cell lines of the patient are seen: the euploid line 46, XX (left) and the trisomic line 47, XX, +5 (right). The arrow indicates the extra chromosome 5.

On the contrary, array CGH did not reveal the presence of CNV, formula arr(1-22, X)x2. ¹

The peripheral blood karyotype was 46,XX. The UPD study of chromosome 5 identified a normal microsatellite marker profile consistent with normal biparental inheritance. Low-grade mosaicism confined to tissue could not be confirmed.

Discussion

Mosaic trisomy 5 is an infrequent condition, and the available information is limited. The reported clinical cases reflect diverse phenotypes ( Table 1 ), dependent on the percentage of mosaicism and the histological distribution of the aneuploid line, which in turn depend on the molecular events giving rise to mosaicism.

Table 1. Review of the phenotypic findings related to trisomy of chromosome 5 found in the literature.

	Richkind et al (1987) 15	Casamassima et al (1989) 16	Sciorra et al (1992) 17	Brzustowicz et al (1994) 18	Villa et al (2007) 9	Reittinger et al (2017) 8	Present case
Gender	Male	Male	Male	Male	Female	Male	Female
Age at examination	6 mo	7 mo	At birth	2 y	At birth	Neonate	3 mo
Genetics results	Trisomy 5 in 80% of amniocytes, cord blood normal	Trisomy 5 in 23% of amniocytes, fetal blood cells normal	Trisomy 5 in 25% of amniocytes, normal fetal blood cells, and cord blood lymphocytes, 20% of fetal skin fibroblasts had trisomy 5	Normal male karyotype, possible paternal isodisomy causing spinal muscular atrophy	Trisomy 5 in 64.2% of placental cells, fetal peripheral blood cells normal	Amniocentesis karyotype: 47, XY, +5 (5/15 cells) (33%). Postnatal karyotype: 46, XY. Postanatal CMA: 179 Mb homozygosity of chromosome 5 (UDP5), otherwise normal	NIPT: total trisomy 5. Amniocentesis karyotype: 47, XX, +5 (12/95 cells) (13%). Normal amniotic fluid array CGH, peripheral blood lymphocytes karyotype, and UPD analysis
IUGR	−	−	+	−	+	+	+
Craniofacial dysmorphisms	−	−	Dysmorphic facies (high square forehead, hypertelorism, prominent nasal bridge)	−	NA	Tall forehead, low anterior hairline, hypertelorism, low-set ears, prominent nose and midface	Macrocephalia. Dysplastic right ear. Small palpebral length
Renal anomalies	−	−	NA	NA	NA	Unilateral multicystic kidney	−
Brain anomalies	−	−	−	NA	−	−	−
Urogenital/anal anomalies	−	−	NA	NA	Anteriorly placed anus	Hypospadias, perineal fistula	−
Congenital heart disease	−	−	Ventricular septal defect	NA	Interatrial shunt	Atrial and ventricular septal defect, polyvalvular	−
Digit anomalies	−	−	Proximally placed left thumb	NA	Clinodactyly of fifth right toe	Bilateral bifid thumbs with nails.	−
Other	−	−	Eventration of the diaphragm. Growth/developmental delays	Type III spinal muscular atrophy	Weight, length and head circumference <10th centile	Decreased subcutaneous fat with loose skin. Low-lying conus medullaris	Both eyes exotropia by Hirschberg's test

Abbreviations: CGH, comparative genomic hybridization; CMA, chromosomal microarray; IUGR, intrauterine growth restriction; UPD, uniparental disomy.

Source: Modified from Reittinger et al. ⁸

Notes: The (+) sign indicates the presence of the characteristic reported by the authors, while (−) indicates its absence, and NA indicates that the characteristic was not commented (not available).

In NIPT, where the free circulating fetal DNA is of placental origin, the result was pure trisomy of chromosome 5. The amniotic fluid study, in which the analyzed cells are derived from the epiblast, revealed the presence of true mosaic trisomy 5. The normal array CGH result was attributable to technical limitations of the test, which does not detect reliably mosaicism less than 25%. ¹²

The discrepancy between the peripheral blood karyotype and the amniotic fluid karyotype may be due to trisomic rescue in the embryo. The possibility of UPD therefore must be considered. Since the result obtained was consistent with normal biparental inheritance of chromosome 5, UPD would be discarded as the origin of the phenotypic alterations of the patient. However, our analysis is unable to inform of the existence of partial UPD; the observed phenotype therefore may have been caused by the presence of an isozygous recessive mutation. ^{13 14}

The distribution of the cell lines depends on the moment during pregnancy in which the cell division error occurs, the affected cell line, and the type of causal error. ⁵

In our case, we observed the presence of pure trisomy in placental tissue and since early postzygotic events would be characterized by complete concordance between the fetal karyotype and the placental karyotype, ² mitotic error in the zygote could be ruled out.

Having discarded postzygotic mitotic error, this chromosomal discrepancy between the fetal karyotype and the placental karyotype suggests a prezygotic meiotic error corrected by late trisomic rescue in the zygote. ⁵

The phenotype of our patient was determined by the affected tissues and the percentage of mosaicism in each tissue, since the absence of mosaic trisomy 5 seen in peripheral blood does not guarantee its absence in other tissues. ⁵ This idea is reinforced by the mild phenotype of the patient compared with the case presented by Reittinger et al, ⁸ where the phenotype was more pathological, with a higher level of mosaicism and UPD.

Conclusion

In presenting this case of mosaic trisomy 5, we hope to contribute further information on the genetic diagnosis of the disorder, improved knowledge of the possible phenotypic alterations, and better understanding of this rare syndrome, since its detection may pose a dilemma in prenatal counseling.