Growth retardation and congenital heart disease in a boy with a ring chromosome 6 of maternal origin

Rare chromosomal structural abnormalities, including ring chromosomes, often pose challenges to clinical genetic counselling. Here, we report a newborn with congenital heart disease and developmental delay who inherited ring chromosome 6 [46,XY,r(6)(p25q27)mat] from a phenotypically normal mother. Genotypes and phenotypes were analysed by molecular cytogenetic analysis, whole-exome sequencing and literature review. Our study showed that the pathogenicity of the ring chromosome abnormality [r(6)(p25q27)] was mainly affected by chromosome imbalance, deletions of genes with haploinsufficiency, duplications of genes with triple sensitivity, parental inheritance of the imbalance and the imprinting status of the affected genes.


Background
Ring chromosomes (RC) are a specific chromosomal abnormalities, being rare genetic events caused by terminal deletions and an intrachromosomal fusion [1]. RCs were first discovered in tumour cells in 1956 [2] and later in other autosomal and sex chromosomes in clinical cases [3][4][5][6][7]. To date, all 23 human chromosomes have been reported to be involved in RC-formation, with an overall incidence between 1/30,000 and 1/60,000 [8]. Two main types of RCs have been described: (1) 46,XN,r, where normal linear homologues are replaced by fulllength rings or unbalanced rings [8]; and (2) 47,XN,+r, where the RC is supernumerary. In both cases, RC-carrying cell lines may coexist with normal cell lines in the mosaic state.
At the time of publication of this article, there have been few reports about RCs derived from chromosome 6 [9]; inheritance from a parent was not reported yet.
Here, genome-wide copy number and pedigree analysis were performed on a foetus-to-newborn case by banding cytogenetics and molecular genetics, and a hereditary RC6 abnormality was identified [r(6)(p25q27)]. Clinical consequences and implications for genetic counselling are discussed here.

Case report
A 23-year-old pregnant woman, G1P0 (gravida 1, para 0), was admitted to foetal medical centre. The pregnant woman was 142 cm tall, within weight in the normal range for height, as were her parents and husband. The couple had normal intellectual development and no abnormal family history or mutagenic exposures.
This pregnancy was conceived naturally. No noninvasive prenatal genetic testing (NIPT) was performed in the first trimester of pregnancy. However, sonography at 24+ weeks of gestation (w.o.g.) detected intrauterine  (Fig. 1A), and ventricular septum defect (Fig. 1B). Ultrasonography at 30 th w.o.g. confirmed the previous findings and additionally a foramen ovale. However, at 34 w.o.g. a second ultrasound examination revealed no abnormalities at all.
CMA analyses in the foetus (amnion and peripheral blood) gave the following result: arr  3p25.2(1,153,042_4,172,096)×2~3. SNP-array confirmed the mosaic situation of 90% of the cells carrying the ring chromosome; also a isoUPD(6) mosaicism was found for 10% of the cells, explaining the 15 cells with normal karyotype 46,XX found in cytogenetics as being due to monosomic rescue.
Whole-exome sequencing confirmed the result of SNP-array as: seq[GRCh37] dup (6) The boy was delivered by caesarean section at 39 +2 weeks of gestation. Congenital dislocation of the right knee joint occurred in the newborn (Fig. 1C), even though no knee joint abnormality was observed at any stage of pregnancy. After treatment, the dislocation of the knee and limb was normal. The newborn had a birth weight too low for gestational age of 2.150 kg, and was overall in good mental condition, without any inborn defects. However, follow-up 8 months of age showed developmental delay concerning length (64 cm) and weight (5 kg); also congenital heart malformation was diagnosed by Doppler sonography as ventricular septal defect and atrial septal defect with the enlarged diameter of pulmonary artery and left heart enlargement; also the third top valve had a micro reflux and pulmonary hypertension was detected while left ventricular systolic function was normal (Fig. 1D).
Overall, as of the date of publication of this article, there have been no abnormal phenotypes in the newborn except for growth retardation and congenital heart malformations.

Discussion
Here we report the first case of a maternally inherited RC6 r(6)(p25q27) without major clinical consequences. Yet, 9 cases have been reported in the literature with comparable de novo r(6)(p25q27), diagnosed between 2 and 13 years old. After 2013, molecular technology was applied to determine the breakpoint; for ring chromosome 6 with 6p25 to 6q27, all cases reported in the literature apart from the present one (Table 1) Table 2 and indicate that most of these genes are OMIM genes, such as DUSP22, IRF4, and FOXC1. There are currently two imprinted genes located on chromosome 6p25 ( Table 2): FAM50B and PXDC1. Both genes were paternally expressed. Even though UPD(6) was detected in 10% of the blood cells of the mother of the patient, a clinical effect is not likely due to that postzygotic rescue phenomenon.
RC formation mechanisms may include the loss and/ or acquisition of genetic material. Previous studies have shown that at least three mechanisms may lead to RCs: inv dup del rearrangements, double-strand breaks and telomeric junctions [11]. RCs are generally considered to be the result of chromosomal aberrations during meiosis or in early postzygotic phase. Two open ends are connected to form a continuous ring. This mechanism assumes that some genetic material may be lost during ring formation. Also RCs tend to be lost during mitoses and cells with 45,XN,-6 are not viable. This is the reason for IUGR observed in the patient and his mother.  1,153,042_4,172,096)×3 originating from the mother. Although other genetic effects on the congenital abnormity of the foetus cannot be excluded, the pathogenicity is mainly due to loss of RC6 during mitoses, leading to growth restrictions. Also influence of terminal deletion and duplication in chromosome 6 on heart phenotype cannot be excluded.