Submicroscopic terminal deletion of 1p36.3 and Xp23 hidden in complex chromosome rearrangements: independent mechanism of telomere restitution on the two chromatids

Prenatal findings in 1p36 deletion syndrome: New cases and a literature review

OBJECTIVE/METHOD

1p36 deletion syndrome is considered to be the most common deletion after 22q11.2 deletion. It is characterized by specific facial features, developmental delay, and organ defects. The primary objective of the present multicenter study was to survey all the cases of 1p36 deletion diagnosed prenatally by French cytogenetics laboratories using a chromosomal microarray. We then compared these new cases with the literature data.

RESULTS

Ten new cases were reported. On average, the 1p36 deletion was diagnosed at 19 weeks of gestation. The size of the deletion ranged from 1.6 to 16 Mb. The 1p36 deletion was the only chromosomal abnormality in eight cases and was associated with a complex chromosome 1 rearrangement in the two remaining cases. The invasive diagnostic procedure had always been prompted by abnormal ultrasound findings: elevated nuchal translucency, structural brain abnormality, retrognathia, or a cardiac defect. Multiple anomalies were present in all cases.

DISCUSSION

We conclude that 1p36 deletion is not associated with any specific prenatal signs. We suggest that a prenatal observation of ventriculomegaly, congenital heart defect, or facial dysmorphism should prompt the clinician to consider a diagnosis of 1p36 deletion syndrome.

Chromosome 1p36 deletion syndrome: prenatal diagnosis, molecular cytogenetic characterization and fetal ultrasound findings

OBJECTIVE

To present prenatal diagnosis and molecular cytogenetic characterization of de novo partial monosomy 1p (1p36.23→pter) and partial trisomy 20p (20p12.1→pter) associated with ventriculomegaly, ventricular septal defect and midface hypoplasia.

MATERIALS, METHODS AND RESULTS

A 31-year-old, primigravid woman was referred for amniocentesis at 20 gestational weeks because of ventriculomegaly, ventricular septal defect, and midface hypoplasia. Amniocentesis revealed an aberrant derivative chromosome 1, or der(1). Parental karyotypes were normal. Spectral karyotyping analysis revealed that the der(1) contained a segment of chromosome 20 in the distal end of the short arm of chromosome 1. Array comparative genomic hybridization demonstrated an 8.4-Mb distal 1p deletion and a 14-Mb distal 20p duplication. The karyotype was 46,XX,der(1)t(1;20)(p36.23;p12.1)dn. Polymorphic DNA marker analysis determined the paternal origin of the aberrant chromosome. The pregnancy was subsequently terminated. A 462-g malformed female fetus was delivered at 22 gestational weeks with a prominent forehead, midface hypoplasia, a flat nasal bridge, low-set ears, a long philtrum, a pointed chin and micrognathia.

CONCLUSION

Spectral karyotyping, fluorescence in situ hybridization and array comparative genomic hybridization are useful for the prenatal investigation of the nature of a de novo aberrant derivative chromosome. Partial monosomy 1p (1p36.23→pter) and partial trisomy 20p (20p12.1→pter) are associated with ventriculomegaly, ventricular septal defect and midface hypoplasia on prenatal ultrasound. Prenatal diagnosis of ventriculomegaly, congenital heart defects and midface hypoplasia should alert clinicians to chromosome 1p36 deletion syndrome and prompt molecular cytogenetic analysis if necessary.

Molecular characterisation of a mosaicism with a complex chromosome rearrangement: evidence for coincident chromosome healing by telomere capture and neo-telomere formation

BACKGROUND

Broken chromosomes must acquire new telomeric "caps" to be structurally stable. Chromosome healing can be mediated either by telomerase through neo-telomere synthesis or by telomere capture.

AIM

To unravel the mechanism(s) generating complex chromosomal mosaicisms and healing broken chromosomes.

METHODS

G banding, array comparative genomic hybridization (aCGH), fluorescence in-situ hybridisation (FISH) and short tandem repeat analysis (STR) was performed on a girl presenting with mental retardation, facial dysmorphism, urogenital malformations and limb anomalies carrying a complex chromosomal mosaicism.

RESULTS & DISCUSSION

The karyotype showed a de novo chromosome rearrangement with two cell lines: one cell line with a deletion 9pter and one cell line carrying an inverted duplication 9p and a non-reciprocal translocation 5pter fragment. aCGH, FISH and STR analysis enabled the deduction of the most likely sequence of events generating this complex mosaic. During embryogenesis, a double-strand break occurred on the paternal chromosome 9. Following mitotic separation of both broken sister chromatids, one acquired a telomere vianeo-telomere formation, while the other generated a dicentric chromosome which underwent breakage during anaphase, giving rise to the del inv dup(9) that was subsequently healed by chromosome 5 telomere capture.

CONCLUSION

Broken chromosomes can coincidently be rescued by both telomere capture and neo-telomere synthesis.