Prenatal diagnosis of a small supernumerary, XIST-negative, mosaic ring X chromosome identified by fluorescence in situ hybridization in an abnormal male fetus

Special cases in Cornelia de Lange syndrome: The Spanish experience

Cornelia de Lange Syndrome (CdLS) is an autosomal dominant (NIPBL, SMC3, and RAD21) or X-linked (SMC1A and HDAC8) disorder, characterized by distinctive craniofacial appearance, growth retardation, intellectual disability, and limb anomalies. In 2005, the Spanish CdLS Reference Center was started and now we have more than 270 cases in our database. In this special issue, we describe some of the unique or atypical patients studied by our group, whose clinical features have contributed to the expansion of the CdLS classical phenotype, helping clinicians to diagnose it. We include the case of a male with unilateral tibial hypoplasia and peroneal agenesis who had a mutation in NIPBL; we also describe one patient with a mutation in NIPBL and somatic mosaicism identified by new generation sequencing techniques; we also include one patient with CdLS and Turner syndrome; and last, an interesting patient with a duplication of the SMC1A gene. Finally, we make a short review of the splicing mutations we have found in NIPBL regarding the new knowledge on the physiological variants of the gene. © 2016 Wiley Periodicals, Inc.

Could a patient with SMC1A duplication be classified as a human cohesinopathy?

The disorders caused by mutations in genes encoding subunits and accessory proteins of cohesin complex are collectively termed as cohesinopathies. The best known cohesinopathy is Cornelia de Lange Syndrome (CdLS), which is a multisystem developmental disorder characterized by facial dysmorphism, limb malformations, growth and cognitive impairment. Mutations in five genes, encoding subunits of the cohesin complex (SMC1A, SMC3, RAD21) and its regulators (NIPBL, HDAC8), are responsible for ∼ 70% of CdLS cases. We describe a 16-year-old boy with facial dysmorphism, growth retardation, intellectual disability, hirsutism and small hands, who has a small Supernumerary Marker Chromosome (sSMC) present in mosaic form. sSMC is composed of two duplicated segments encompassing 17 genes including SMC1A gene, at the regions Xp11.22 and Xp11.21q11.1. Clinical comparison between our patient with a previously reported individual with a SMC1A duplication and four male carriers of similar sSMC reported in databases, suggest that they all share clinical features related to cohesinopathies. Although our patient does not have the classical CdLS craniofacial phenotype, he has pre and postnatal growth retardation, intellectual disability and mild musculoskeletal anomalies, features commonly seen in patients with cohesinopathies.

[Strategies to identify supernumerary chromosomal markers in constitutional cytogenetics]

Supernumerary marker chromosomes (SMCs) are defined as extrastructurally abnormal chromosomes which origin and composition cannot be determined by conventional cytogenetics. SMCs are an heterogeneous group of abnormalities concerning all chromosomes with variable structure and size and are associated with phenotypic heterogeneity. The characterisation of SMCs is of utmost importance for genetic counselling. Different molecular techniques are used to identify chromosomal material present in markers such as 24-colour FISH (MFISH, SKY), centromere specific multicolour FISH (cenMFISH) and derivatives (acroMFISH, subcenMFISH), comparative genomic hybridisation (CGH), arrayCGH, and targeted FISH techniques (banding techniques, whole chromosome painting...). Based on the morphology of SMC with conventional cytogenetic and clinical data, we tried to set up different molecular strategies with all available techniques.

Small supernumerary marker chromosomes – progress towards a genotype-phenotype correlation

Small supernumerary marker chromosomes (sSMC) are still a major problem in clinical cytogenetics as they are too small to be characterized for their chromosomal origin by traditional banding techniques, but require molecular cytogenetic techniques for their identification. Apart from the correlation of about one third of the sSMC cases with a specific clinical picture, i.e. the i(18p), der(22), i(12p) (Pallister Killian syndrome) and inv dup(22) (cat-eye) syndromes, most of the remaining sSMC have not yet been correlated with clinical syndromes. Recently, we reviewed the available >1600 sSMC cases (Liehr T, sSMC homepage: http://mti-n.mti.uni-jena.de/~huwww/MOL_ZYTO/sSMC.htm). A total of 387 cases (including the 45 new cases reported here) have been molecularly cytogenetically characterized with regard to their chromosomal origin, the presence of euchromatin, heterochromatin and satellite material. Based on analysis of these cases we present the first draft of a basic genotype-phenotype correlation for sSMC for all human chromosomes apart from the chromosomes Y, 10, 11 and 13.

Small supernumerary marker chromosomes (sSMC) in humans

Small supernumerary marker chromosomes (sSMC), defined as additional centric chromosome fragments too small to be identified or characterized unambiguously by banding cytogenetics alone, are present in 0.043% of newborn children. Several attempts have been made to correlate certain sSMC with a specific clinical picture, resulting in the description of several syndromes such as the i(18p)-, der(22)-, i(12p)- (Pallister Killian syndrome) and inv dup(22)- (cat-eye) syndromes. However, most of the remaining sSMC including minute-, ring-, inverted-duplication- as well as complex-rearranged chromosomes, have not yet been correlated with clinical syndromes, mostly due to problems in their comprehensive characterization. Here we present an overview of sSMC, including the first attempt to address problems of nomenclature and their modes of formation, problems connected with mosaicism plus familial occurrence. The review also discusses the frequency of sSMC in prenatal, postnatal, and clinical cases, their chromosomal origin and their association with uniparental disomy. A short review of the up-to-date approaches available for sSMC characterization is included. Clinically relevant correlations concerning the presence of a specific sSMC and its phenotypic consequences should become available soon.

Prenatal diagnosis ofde novo proximal interstitial deletion of 9q and review of the literature of uncommon aneuploidies associated with increased nuchal translucency

OBJECTIVES

To present the prenatal diagnosis and molecular cytogenetic analysis of de novo proximal interstitial deletion of 9q and to review the literature of uncommon aneuploidies associated with increased nuchal translucency (NT).

CASE

Obstetric ultrasound at 11 weeks' gestation revealed an increased NT thickness of 6.6 mm in a 31-year-old primigravid woman. At 13 weeks' gestation, repeat ultrasound examinations revealed a normal NT thickness of 1.8 mm. The subcutaneous nuchal fluid accumulation was no longer present at the following ultrasound scans. An amniocentesis was performed at 18 weeks' gestation.

RESULTS

Cytogenetic analysis revealed a karyotype of 46,XX,del(9)(q21.1q22.2). The parental karyotypes were normal. At 21 weeks' gestation, a 442-g female fetus was delivered with low-set ears, hypertelorism, and a thick nuchal fold. The parental origin of the interstitial deletion of 9q was analyzed with polymorphic DNA markers. With the microsatellite markers D9S238 (9q13), D9S889 (9q21.11), and D9S253 (9q22.2), two alleles inherited from the parents were seen in the proband, but with markers D9S1780 (9q21.31), D9S303 (9q21.32), D9S252 (9q21.33), and D9S316 (9q22.1), only one maternal allele was present. The deletion was of paternal origin.

CONCLUSIONS

Fetuses with uncommon aneuploidies may manifest increased NT in the first trimester. The present case provides evidence for a correlation between increased NT and interstitial 9q deletion. Prenatal identification of increased NT should alert subtle structural chromosome aberrations and prompt high-resolution karyotyping.