Clinical and cytogenetic manifestations of subtelomeric aberrations: Report of six cases

Screening for Subtelomeric Rearrangements in Thai Patients with Intellectual Disabilities Using FISH and Review of Literature on Subtelomeric FISH in 15,591 Cases with Intellectual Disabilities

We utilized fluorescence in situ hybridization (FISH) to screen for subtelomeric rearrangements in 82 Thai patients with unexplained intellectual disability (ID) and detected subtelomeric rearrangements in 5 patients. Here, we reported on a patient with der(20)t(X;20)(p22.3;q13.3) and a patient with der(3)t(X;3)(p22.3;p26.3). These rearrangements have never been described elsewhere. We also reported on a patient with der(10)t(7;10)(p22.3;q26.3), of which the same rearrangement had been reported in one literature. Well-recognized syndromes were detected in two separated patients, including 4p deletion syndrome and 1p36 deletion syndrome. All patients with subtelomeric rearrangements had both ID and multiple congenital anomalies (MCA) and/or dysmorphic features (DF), except the one with der(20)t(X;20), who had ID alone. By using FISH, the detection rate of subtelomeric rearrangements in patients with both ID and MCA/DF was 8.5%, compared to 2.9% of patients with only ID. Literature review found 28 studies on the detection of subtelomeric rearrangements by FISH in patients with ID. Combining data from these studies and our study, 15,591 patients were examined and 473 patients with subtelomeric rearrangements were determined. The frequency of subtelomeric rearrangements detected by FISH in patients with ID was 3%. Terminal deletions were found in 47.7%, while unbalanced derivative chromosomes were found in 47.9% of the rearrangements.

Update on Kleefstra Syndrome

Kleefstra syndrome is characterized by the core phenotype of developmental delay/intellectual disability, (childhood) hypotonia and distinct facial features. The syndrome can be either caused by a microdeletion in chromosomal region 9q34.3 or by a mutation in the euchromatin histone methyltransferase 1 (EHMT1) gene. Since the early 1990s, 85 patients have been described, of which the majority had a 9q34.3 microdeletion (>85%). So far, no clear genotype-phenotype correlation could be observed by studying the clinical and molecular features of both 9q34.3 microdeletion patients and patients with an intragenic EHMT1 mutation. Thus, to further expand the genotypic and phenotypic knowledge about the syndrome, we here report 29 newly diagnosed patients, including 16 patients with a 9q34.3 microdeletion and 13 patients with an EHMT1 mutation, and review previous literature. The present findings are comparable to previous reports. In addition to our former findings and recommendations, we suggest cardiac screening during follow-up, because of the possible occurrence of cardiac arrhythmias. In addition, clinicians and caretakers should be aware of the regressive behavioral phenotype that might develop at adolescent/adult age and seems to have no clear neurological substrate, but is rather a so far unexplained neuropsychiatric feature.

Delineation of subtelomeric deletion of the long arm of chromosome 6

Pure subtelomeric deletion of the long arm of chromosome 6 is rare. The frequency of this deletion accounts for approximately 0.05% of subjects with intellectual disability and developmental delay with or without dysmorphic features. Common phenotypes associated with this deletion include intellectual disability, developmental delay, dysmorphic features, seizure, hypotonia, microcephaly and hypoplasia of the corpus callosum. The smallest overlapped region is approximately 0.4 Mb, and contains three known genes. Of these genes, TBP has been considered as a plausible candidate gene for the phenotype in patients with a subtelomeric 6q deletion. Analysis of the breakpoints in 14 cases revealed a potential common breakpoint interval 8.0-9.0 Mb from the chromosome 6q terminus where the FRA6E fragile site exists and the PARK2 gene is located. This suggests that breakage at the FRA6E fragile site may be the mechanism behind chromosome 6q subtelomeric deletion in some of the cases.

1q44-qter trisomy: clinical report and review of the literature

Subtelomeric rearrangements are one of the main causes of multiple congenital anomalies and mental retardation, and they are detected in 5% of patients. We report on a 6.5-year-old boy with mental retardation, dysmorphic features, and behavioral problems, who revealed 1q44-qter trisomy and 22q13.3-qter monosomy due to a maternal cryptic translocation t(1;22). We compared the clinical and cytogenetic data of our patient with those of another case presenting a pure 22qter monosomy and with those of all 1qter trisomy cases reported in the international literature. To the best of our knowledge, the subterminal 1q trisomy found in the present case has been reported in only 12 patients to date (including five familial cases). This report aims to contribute to our understanding of 1q44-qter trisomy.

Structural autosomal imbalance and oesophageal defects: Addendum to the article by Felix et al. (2007)

Information about approximately 25 additional cases of oesophageal defects in patients with structural autosomal imbalance (in addition to 30 cases already mentioned in the article by Felix et al.) may facilitate search for the genes responsible for these defects.

Systematic screening for subtelomeric anomalies in a clinical sample of autism

High-resolution karyotyping detects cytogenetic anomalies in 5-10% of cases of autism. Karyotyping, however, may fail to detect abnormalities of chromosome subtelomeres, which are gene rich regions prone to anomalies. We assessed whether panels of FISH probes targeted for subtelomeres could detect abnormalities beyond those identified by karyotyping in 104 individuals with Pervasive Developmental Disorders (PDDs) drawn from a general clinical population. Four anomalies were detected by karyotyping, while no additional anomalies were detected by subtelomere FISH or by probes targeted for 15q11.2q13 or 22q11.2 in subgroups of our sample. We conclude that while karyotyping may be more broadly indicated for autism than previously supposed, subtelomere FISH appears less likely to be a useful screening tool for unselected PDD populations.

Screening of subtelomeric rearrangements in autistic disorder: identification of a partial trisomy of 13q34 in a patient bearing a 13q;21p translocation

Within the framework of a FISH screening protocol to detect cryptic subtelomeric rearrangements in autistic disorder (AD), a patient bearing three copies of the subtelomeric portion of the q arm of chromosome 13 has been identified. Beside AD, the patient also has severe mental retardation and displays several dysmorphic features. Further FISH analyses revealed that the trisomy was caused by the translocation of a 13q subtelomeric fragment to the acrocentric tip of one chromosome 21 [46,XY.ish der(21) t(13;21) (q34;p13)(D13S1825+)]. Gene dosage experiments carried out with three multiallelic polymorphisms of the subtelomeric region of chromosome 13q showed that the putative length of the triplicate region does not exceed 300 kb about, that is, the distance from telomere to the first normally inherited marker. In addition, gene dosage analysis performed on the derivative chromosome 21, did not reveal loss of the most telomeric protein-encoding genes on 21p. The potential relationship between a postulated increased expression of genes on 13q34 and the complex phenotype in this trisomic patient is discussed.

Subtelomeric chromosome aberrations: still a lot to learn

Subtelomeric chromosome aberrations: still a lot to learn.Cryptic subtelomeric chromosome aberrations are a significant cause of mental retardation (MR). More than 4000 patients have been investigated, and the mean overall prevalence of subtelomeric rearrangements has been found to be 5.2%. In order to contribute to knowledge on the clinical presentation of subtelomeric rearrangements, we retrospectively studied patients with unexplained MR who had been evaluated for subtelomeric abnormalities by different fluorescence in situ hybridization (FISH) techniques. Hundred and two patients had an unexplained combination of MR with dysmorphism, congenital anomalies, and/or a positive family history and were investigated by total subtelomeric (TS) FISH (89/102), or by total painting (TP) in an obligate carrier in the case of familial MR (13/102). In 59 additional patients, a sequence-specific FISH was performed on clinical indication. In the 102 patients studied by TS or TP, six pathogenic aberrations (5.9%) were found in addition to one polymorphism. In total, eight clinically significant subtelomeric aberrations were found in the 161 index patients; four of these eight aberrations were familial. We report on the clinical presentation of all patients with an aberration and review the relevant literature. Factors complicating the interpretation of subtelomeric rearrangements are discussed, such as the occurrence of variants, clinical variability, and limited knowledge of the phenotype.

Calibration of 6q subtelomere deletions to define genotype/phenotype correlations

Testing for subtelomere abnormalities in patients with idiopathic mental retardation has become a useful diagnostic tool. However, limited data exist regarding genotype/phenotype correlations for specific subtelomere imbalances. We have ascertained five patients with 6q subtelomere deletions either as a result of an isolated deletion or as a result of an unbalanced translocation, and developed a molecular ruler assay utilizing BAC or PAC clones and determined the size of the deleted regions to range from <0.5 to 8 Mb. To establish genotype/phenotype correlations for distal 6q, we compared the clinical features of these patients to previously reported cases of 6q subtelomere and cytogenetically visible deletions and found that they shared multiple abnormalities, suggesting that the causative genes may lie in the region of the smallest 6q subtelomeric deletion, approximately 400 kb from the telomere. However, multiple unique features were present only in patients with cytogenetically visible 6q deletions, indicative that genes involved in the development of these features may lie more proximally on 6q. These initial studies demonstrate the ability to develop genotype/phenotype correlations for subtelomere rearrangements, which will aid in the diagnosis and prognosis of these patients and may help narrow the search for relevant developmental genes.

The genetics of autism

Autism spectrum disorders (ASD) are among the most heritable of all neuropsychiatric disorders. Discovery of autism susceptibility genes has been the focus of intense research efforts over the last 10 years, and current estimates suggest that 10 to 20 different interacting genes are involved. Evidence from twin and family studies demonstrates increased risk in family members not only for autistic disorder, but also for a milder constellation of similar symptoms referred to as the broader phenotype. In addition, several genetic syndromes and chromosomal anomalies have been associated with ASD. Large family studies using linkage-analysis techniques have demonstrated several chromosomal regions thought to harbor genes related to the disorder. Finally, specific candidate genes based on function and location have been explored; these studies are reviewed here.