The supernumerary cat eye syndrome (CES) chromosome is dicentric, containing two copies of 22pter→q11.2. We have found that the duplication breakpoints are clustered in two intervals. The more proximal, most common interval is the 450–650 kb region between D22S427 and D22S36, which corresponds to the proximal deletion breakpoint interval found in the 22q11 deletion syndrome (DiGeorge/velocardiofacial syndrome). The more distal duplication breakpoint interval falls between CRKL and D22S112, which overlaps with the common distal deletion interval of the 22q11 deletion syndrome. We have therefore classified CES chromosomes into two types based on the location of the two breakpoints required to generate them. The smaller type I CES chromosomes are symmetrical, with both breakpoints located within the proximal interval. The larger type II CES chromosomes are either asymmetrical, with one breakpoint located in each of the two intervals, or symmetrical, with both breakpoints located in the distal interval. The co-localization of the breakpoints of these different syndromes, plus the presence of low-copy repeats adjacent to each interval, suggests the existence of several specific regions of chromosomal instability in 22q11.2 which are involved in the production of both deletions and duplications. Since the phenotype associated with the larger duplication does not appear to be more severe than that of the smaller duplication, determination of the type of CES chromosome does not currently have prognostic value.

All reported mutations in the choroideremia (CHM) gene result in the truncation or complete absence of Rab escort protein 1 (REP1). Molecular analysis was carried out on 57 families diagnosed with CHM. Confirmation of the clinical diagnosis is important as end-stage CHM may be clinically similar to the end stages of other retinal degenerative diseases such as RP. The primary means of confirming the diagnosis of CHM is to sequence all 15 exons. An alternative method involves detection of the REP1 protein, as described in MacDonald et al. [1998]. A monoclonal antibody to REP1 does not detect truncated REP1 by immunoblot analysis, presumably due to instability and subsequent degradation of the truncated protein. This analysis provides relatively fast confirmation of the diagnosis, however, protein samples are not always available and are susceptible to degradation, affecting the accurate interpretation of results. CHM gene mutations were found in 54 of 57 families studied. The majority of mutations (>42%) were transitions and transversions. Complete deletions of the CHM gene and deletion/insertion mutations each accounted for almost 4% of the total, while over 9% had large intragenic and other partial deletions. Almost 28% of the mutations were deletions of fewer than 5 base pairs (bp) and almost 13% were splice site mutations. Despite the fact that mutations are found throughout the gene with no common mutation for the disorder, identical mutations have been characterized in unrelated individuals. The majority of these mutations are C to T transitions, changing an arginine residue (CGA) to a stop codon (TGA). Four of the five CGA codons in the CHM gene are sites of recurring mutations.

Choroideremia is a chorioretinal degeneration displaying X-linked recessive inheritance. In recent years, technological advances have increased the accessibility of genetic testing for mutations in the gene that lead to this disorder. The disorder itself, approaches for its detection and the steps and the rationale behind testing are outlined in this review. All mutations in the choroideremia gene result in the truncation or absence of the normal protein product Rab escort protein-1, which is a component of Rab geranylgeranyltransferase, an enzyme complex that mediates correct intracellular vesicular transport. Sequence analysis of the 15 exons of the choroideremia gene and adjacent splice sites is a primary method of mutation detection used by the authors' laboratory, through which a variety of mutations including nonsense mutations, insertions, deletions and splice site alterations have been detected. Alternatively, if no mutations are revealed using this approach, reverse transcription PCR, northern blot analysis or a protein truncation test can be employed to detect aberrantly spliced products. Immunoblot analysis can also be performed to confirm the absence of Rab escort protein-1 in affected males. Deletions create a practical problem in assessing the carrier status of females; linkage analysis with closely linked markers is the most practical approach in these cases.

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Cat eye syndrome (CES} is typically associated with a supernumerary bisatellited marker chromosome derived from human chromosome 22pter to 22q11.2. The region of 22q duplicated in the typical CES marker chromosome extends between the centromere and locus D22S36. We have constructed a long-range restriction map of this region using pulsed-field gel electrophoresis and probes to I0 loci (11 probes}. The map covers -3.6 Mb. We have also used 15 loci to construct a yeast artificial chromosome contig, which encompasses about half of the region critical to the production of the CES phenotype {centromere to D22S57]. Thus, the CES critical region has been mapped and a substantial portion of it cloned in preparation for the isolation of genes in this region.