An unbalanced half-cryptic translocation involving the 6q subtelomeric region and 2p25.3 in a child with mental retardation: uses and limitations of fluorescence in situ hybridization

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.

6q Terminal Deletion Syndrome Associated with a Distinctive EEG and Clinical Pattern: A Report of Five Cases

PURPOSE

Mental retardation, facial dysmorphisms, and neurologic and brain abnormalities are features of 6q terminal deletions. Epilepsy is frequently associated with this chromosome abnormality, but electroclinical findings are not well delineated. We report five unrelated patients with 6q terminal deletions and a peculiar clinical, EEG, and neuroradiologic picture of epilepsy, mental retardation, and colpocephaly.

METHODS

These three male and two female patients underwent general and neurologic examinations, repeated awake and sleep EEGs, and brain magnetic resonance imaging (MRI). A cytogenetic study and fluorescent in situ hybridization (FISH) with chromosome-specific subtelomeric probes were carried out in all cases.

RESULTS

All subjects had seizures characterized by vomiting, cyanosis, and head and eye version, with and without loss of consciousness. In four cases, EEGs showed posterior spike-and-wave complexes, which were activated by sleep. No patient had status epilepticus or prolonged seizures. Brain MRI revealed colpocephaly and dysgenesis of the corpus callosum and brainstem in four patients; three of them also had hypertrophic massa intermedia. FISH analysis revealed a 6q terminal deletion in all patients, which ranged between 9 Mb (cases 2 and 3) and 16 Mb (case 4).

CONCLUSIONS

We suggest that epilepsy associated with 6q terminal deletions is a new entity. Patients with dysmorphic features associated with focal occipital epilepsy, colpocephaly, and dysgenesis of the corpus callosum, thalami, and brainstem should be considered candidates for testing for 6q subtelomere deletions.

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.

Telomeres: a diagnosis at the end of the chromosomes

In recent years, subtelomeric rearrangements have been identified as a major cause of mental retardation and/or malformation syndromes. So far, over 2500 subjects with mental retardation have been tested and reported of whom approximately 5% appeared to have a subtelomeric rearrangement. In this review, the clinical aspects of each known (submicroscopic) subtelomeric deletion will be presented and the various methods available for detecting subtelomeric abnormalities will be discussed. Not only will the patients and their families benefit from a good collection and report of the various telomeric abnormalities and their clinical phenotype, but it will also give more insight into the aetiology of mental retardation and malformation syndromes.

Unexpected retention and concomitant loss of subtelomeric regions in balanced chromosome anomalies by FISH

Florescence in situ hybridization (FISH) using subtelomeric probes has been useful in detecting cryptic telomeric chromosomal rearrangements. We report, for the first time, that cytogenetically visible chromosome rearrangements can occur between the subtelomeric and telomeric region in clinically normal individuals with balanced chromosome anomalies in which one of the breakpoints involves a terminal band region. Using FISH with subtelomeric probes, we observed in three cases with a balanced reciprocal translocations the retention and subsequent loss of subtelomeric regions. In one case with a paracentric inversion, there was a proximal relocation of a subtelomeric region. Because subtelomeric regions serve important roles in chromosome pairing, this retention and concomitant loss or relocation of a subtelomeric region could possibly further disrupt the complex meiotic configurations of these balanced chromosome rearrangements. This may then have an effect on gamete production, placing these individuals at a higher risk for miscarriages and/or abnormal outcomes for individuals with similar chromosome aberrations.

Rapid interphase analysis for prenatal diagnosis of translocation carriers using subtelomeric probes

Interphase fluorescence in situ hybridization (FISH) has become an accepted laboratory technique for the rapid and preliminary prenatal assessment of chromosome aneuploidy. The introduction of subtelomeric FISH probes now allows for the molecular-cytogenetic analysis of terminal chromosome rearrangements. In a prospective study, we examined the prenatal use of subtelomeric probes on interphase cells to rapidly detect the carrier status of a fetus when a parent carried a known reciprocal or Robertsonian chromosome translocation. Three of the cases were identified as being abnormal. All cases were confirmed by routine cytogenetic analysis. These findings clearly demonstrated the utility of this technique and these probes to rapidly and correctly identify balanced and unbalanced chromosome anomalies of a fetus that could result from parental translocations.

Mild phenotype in two siblings with distal monosomy 12p13.31-->pter

We report two sibs with trisomy for the region 2p25.1--> pter and monosomy for the region 12p13.31--> pter, due to adjacent-1 segregation of a maternal balanced reciprocal translocation, 46,XX,t(2;12)(p25.1;p13.31). These sibs presented with a mild phenotype, but nevertheless showed features of each of the contributing aneusomies. Monosomy 12p has previously been considered to have a variable and indistinct phenotype. Comparison of these patients with previous reports showed that many features, including microcephaly, facial dysmorphia, developmental and growth delay and dental and digital anomalies are frequently associated with monosomy for 12p. Many of these features are common to other aneusomies, thereby mitigating against a distinct 12p monosomy syndrome at this time. However, the combination of digital and dental anomalies may suggest the presence of this particular monosomy. The proband and his sister had some of the more non-specific features of 2p trisomy syndrome, and comparison with previous reports suggested that the characteristic 2p trisomy syndrome is more usually associated with larger or more proximal trisomies of 2p.

A review of fluorescence in situ hybridization (FISH): current status and future prospects

Fluorescence in situ hybridization (FISH) is a powerful technique for detecting DNA or RNA sequences in cells, tissues and tumors. This molecular cytogenetic technique enables the localization of specific DNA sequences within interphase chromatin and metaphase chromosomes and the identification of both structural and numerical chromosome changes. FISH is quickly becoming one of the most extensively used cytochemical staining techniques owing to its sensitivity and versatility, and with the improvement of current technology and cost effectiveness, its use will surely continue to expand. Here we review the wide variety of current applications and future prospects of FISH technology.