The standard test for suspected chromosomal rearrangements is cytogenetic analysis at a 400-550 band resolution. This has been available for many years, but cannot routinely detect rearrangements smaller than 5 megabases (Mb) and much larger abnormalities may be missed if they occur in regions where the banding pattern is not distinctive. The suggestion that a substantial proportion of MR might be a result of smaller chromosomal anomalies, undetectable using conventional cyto-genetic analysis, gave the impetus to develop new molecular diagnostic assays which have a higher level of resolution and reliability.

A major advance came with the discovery that the very end portions of human chromosomes (the telomeres and adjacent subtelomeric regions) can be cloned by functional complementation in yeast. This paved the way for the identification of telomere-specific microsatellite markers and for generating telomere-specific cosmids, bacteriophage P1 artificial chromosomes (PACs), and bacterial artificial chromosomes (BACs).

Wilkie,[1] in 1993, first suggested that subtelomeric rearrangements might be responsible for MR. Screening telomeres for rearrangements held a number of attractions. First, the majority of translocations involve chromosome ends and therefore an assay that targeted telomeres would detect these with 100% sensitivity regardless of size. Second, the regions adjacent to telomeres are gene-rich so rearrangements involving these would be more likely to have phenotypic consequences than rearrangements in many other parts of the genome. Finally, rearrangements involving telomeres were emerging as an important cause of human genetic diseases. For example, Wolf-Hirschhorn, Cri du Chat, and Miller-Dieker syndromes and alpha-thalassemia with mental retardation of chromosome 16 were all known to be a result of the unbalanced products of subtelomeric translocations. In 1995, the results of a pilot study that used polymorphic loci to study the subtelomeric regions of 28 chromosomes in 99 mentally retarded individuals were reported by Flint et al.[2] The findings suggested that at least 6% of idiopathic MR might be explained by submicroscopic rearrangements involving telomeres. The first extended study was reported by Knight et al.[3] who used a novel multitelomere fluorescence in situ hybridization (FISH) approach.[4] The results confirmed those of the pilot study and found that subtelomeric rearrangements contributed to the MR in 7.4% cases with moderate to severe MR with dysmorphic features, in about half of those with a family history, and also in 0.5% cases with mild MR.[3]

Subtelomeric Testing Methods

The complex structure of subtelomeric regions and the practical difficulties of testing 41 telomeres in a single procedure proved particularly challenging when it came to developing a robust, cost-effective diagnostic technique with a high degree of sensitivity and specificity.1-5-1 However, subtelomeric rearrangements have now been detected using at least nine different approaches: 1) multitelomere FISH;[3] 2) multiplex or multicolor FISH, including M-TEL[6,7] and combined binary ratio labeling fluorescence in situ hybridization (S-COBRA FISH);[8] 3) telomere spectral karyotyping (telomere SKY);[9,10] 4) primed in situ labeling (PRINS);[11] 5) high-resolution chromosome analysis;[12] 6) comparative genome hybridization (CGH) to chromosomes;[13,14]

7) scanning short tandem repeat polymorphisms;1-15-18-1

8) locus copy number measurement by hybridization with amplifiable probes (MAPH)[19-21] and multiplex ligation-dependent probe amplification (MLPA);[22] and 9) telo-mere array CGH.[23,24] Of all the available methods, the FISH-based approaches still provide the gold standard for the detection of copy number changes and for balanced rearrangements (Fig. 1). However, neither these nor any of the alternative methods can be singled out as ideal; concerns over cost or sensitivity and specificity or both are constantly recurring issues. The advantages and drawbacks of each method are summarized briefly in Table 1.

Subtelomeric Anomalies and Unexplained Mental Retardation

Since the initial findings, the results of testing subtelomeric regions of almost 3300 individuals with MR have been reported. These can be divided broadly into three categories: 1) case reports; 2) studies of few, although highly selected patient sets; and 3) studies of smaller or larger sample sets referred because the patients have unexplained MR (includes mild or moderate to severe MR, with or without dysmorphic features and normal karyo-types at a 550-band level). The latter allow an estimate of the overall frequency of subtelomeric anomalies associated with MR. Of the 2690 unselected unexplained MR cases reported, 136 have subtelomeric rearrangements, giving a frequency of ~ 5% in this group. The results can be further subdivided, at least for some studies, according to the degree of MR. Combining these, subtelomeric anomalies are found in 0.3% of those with mild MR and 7.9% with moderate to severe MR. Overall, the results show that small, subtelomeric anomalies are more likely to be present in those with moderate to severe MR, those with dysmor-phic features, and those with a positive family history, but as addressed in the following section, these features are present in a substantial number of MR referrals.

Who Should Be Tested?

Patients with unexplained MR account for approximately 15% of all referrals passing through genetics and

Getting Started With Dumbbells

Getting Started With Dumbbells

The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.

Get My Free Ebook

Post a comment