A familial unbalanced subtelomeric translocation resulting in monosomy 6q27-->qter

The phenotypic spectrum of terminal 6q deletions based on a large cohort derived from social media and literature: a prominent role for DLL1

BACKGROUND

Terminal 6q deletions are rare, and the number of well-defined published cases is limited. Since parents of children with these aberrations often search the internet and unite via international social media platforms, these dedicated platforms may hold valuable knowledge about additional cases. The Chromosome 6 Project is a collaboration between researchers and clinicians at the University Medical Center Groningen and members of a Chromosome 6 support group on Facebook. The aim of the project is to improve the surveillance of patients with chromosome 6 aberrations and the support for their families by increasing the available information about these rare aberrations. This parent-driven research project makes use of information collected directly from parents via a multilingual online questionnaire. Here, we report our findings on 93 individuals with terminal 6q deletions and 11 individuals with interstitial 6q26q27 deletions, a cohort that includes 38 newly identified individuals.

RESULTS

Using this cohort, we can identify a common terminal 6q deletion phenotype that includes microcephaly, dysplastic outer ears, hypertelorism, vision problems, abnormal eye movements, dental abnormalities, feeding problems, recurrent infections, respiratory problems, spinal cord abnormalities, abnormal vertebrae, scoliosis, joint hypermobility, brain abnormalities (ventriculomegaly/hydrocephaly, corpus callosum abnormality and cortical dysplasia), seizures, hypotonia, ataxia, torticollis, balance problems, developmental delay, sleeping problems and hyperactivity. Other frequently reported clinical characteristics are congenital heart defects, kidney problems, abnormalities of the female genitalia, spina bifida, anal abnormalities, positional foot deformities, hypertonia and self-harming behaviour. The phenotypes were comparable up to a deletion size of 7.1 Mb, and most features could be attributed to the terminally located gene DLL1. Larger deletions that include QKI (> 7.1 Mb) lead to a more severe phenotype that includes additional clinical characteristics.

CONCLUSIONS

Terminal 6q deletions cause a common but highly variable phenotype. Most clinical characteristics can be linked to the smallest terminal 6q deletions that include the gene DLL1 (> 500 kb). Based on our findings, we provide recommendations for clinical follow-up and surveillance of individuals with terminal 6q deletions.

Isolated Chromosome 6q27 Terminal Deletion Syndrome

Any change in either the short (p) or long (q) arm of chromosome six can result in a variety of disorders. 

A two-year-old female child came to us with a history of sudden onset generalized tonic-clonic seizure. She had a syndromic face with frontal bossing and palpable thinning of the right lower lip and an apparent facial asymmetry while crying due to the hypoplasia of the right depressor angularis oris. Her joints were hypermobile and hypotonic. Chromosomal karyotyping exhibited a normal female karyotype, but pathogenic microarray genetic evaluation showed a loss of approximately 783 kb of the 6q27 terminus. She was diagnosed with chromosome 6q27 terminal deletion and managed with anti-seizure medications. 

Chromosome 6q27 terminal deletion can present with an array of structural and developmental anomalies. It is, therefore, necessary to understand the typical phenotypic and distinctive clinical features of congenital chromosome 6q27 terminal deletion syndrome for early diagnosis and intervention.

A retrospective study by oligonucleotide array-CGH analysis in 50 fetuses with multiple malformations

OBJECTIVE

To retrospectively define the frequency and the nature of submicroscopic chromosomal imbalances among fetuses with multiple congenital anomalies (MCA).

METHODS

We used oligonucleotide arrays to perform comparative genomic hybridization after termination of pregnancy in 50 polymalformated fetuses with a normal karyotype. These fetuses presented with at least three significant malformations (42 cases) or a severe brain anomaly (eight cases).

RESULTS

We identified a deleterious copy number variation (CNV) in five fetuses (10%). De novo genomic imbalances identified in this study consisted of a 6qter deletion in a fetus with brain and renal malformations, a mosaicism for a 8p tetrasomy in a fetus with agenesis of corpus callosum, growth retardation, mild facial dysmorphic features, and vertebral anomalies, a 17p13.3 deletion in a fetus with a complex brain malformation, and a partial 11p trisomy in a fetus with severe growth retardation and oligoamnios. In one case, we identified a partial 17q trisomy resulting from malsegregation of a cryptic-balanced translocation.

CONCLUSIONS

This study shows that array comparative genomic hybridization (aCGH) is particularly effective for identifying the molecular basis of the disease phenotype in fetuses with multiple anomalies. Our study should help to define clinical relevant regions that would need to be included in targeted arrays designed for prenatal testing.

TBP as a candidate gene for mental retardation in patients with subtelomeric 6q deletions

Monozygotic twin brothers with a subtelomeric 6q deletion presented with mental retardation, microcephaly, seizures, an enlarged cisterna magna, dimpling at elbows, a high arched palate and a thin upper lip. The same subtelomeric deletion was detected in the mother of the patients, presenting with a milder phenotype. We narrowed down the breakpoint to a region of approximately 100 kb and estimated the size of the terminal deletion to be 1.2 Mb. This region contains four known and seven putative genes. Comparison of the deletion with other reported patients showed TBP was the most plausible candidate gene for the mental retardation in this syndrome. We verified that the TBP gene expression was halved in our patients using real-time PCR. Cognitive and behavioural tests performed on previously described heterozygous tbp mice suggested that TBP is potentially involved in cognitive development.

Delineation of a 2q deletion in a girl with dysmorphic features and epilepsy

In recent years, the spectrum of available methods for the characterization of chromosomal aberrations has significantly increased. Micro-array technologies now allow the rapid fine mapping of small genomic imbalances. Here we used various technologies to characterize a de novo translocation t(2;15) in a girl with dysmorphic features, severe developmental delay and frequent seizures. Multiplex-FISH (M-FISH) excluded the involvement of other chromosomes than chromosomes 2 and 15. We used an oligonucleotide array containing more than 10.000 SNPs, that is, the GeneChip Mapping 10K 2.0 SNP Affymetrix array, and readily fine-mapped a deletion in chromosomal region 2q24.1 --> 2q31.1. The extent of this deletion was verified with multicolor BAC-clone hybridizations. The deletion has a size of about 13 Mb and is within a gene rich region containing about 76 genes. Interestingly, several of these genes are ion channel genes or genes involved in neuron differentiation, so that the frequently occurring seizures are probably due to loss or haploinsufficiency of one or more of these genes.