The N25 probe is 63kb, labelled in red and covers a region including the D22S75 marker and the centromeric end of the CLTCL1 gene. The N85A3 (44kb) probe, labelled in green, is located within 22q13.3 band and covers the telomeric end of the SHANK3 gene allowing for identification of the most distal 22q13.3 deletions. The two unique sequences provide control probes for each other and allow identification of chromosome 22.
DiGeorge syndrome1, and a variety of congenital malformation syndromes including Velocardiofacial syndrome (VCFS)2, share the deletion of chromosome 22 at 22q11.22,3,4,5.These chromosome 22 deletions are collectively coined CATCH22, a mnemonic that covers the clinical findings of Cardiac abnormality, Abnormal facies, Thymic aplasia, Cleft palate and Hypocalaemia/Hyperthyroidism due to a chromosome 22 deletion.
In addition, around 29% of nonsyndromic patients with isolated conotruncal defects have been shown to have a 22q11.2 microdeletion6. The incidence of these anomalies is estimated to be 1:4000 to 1:9700 live births7 and the deletion of 22q11.2 therefore represents one of the most common genetic defects.
A region of approximately 2Mb, referred to as the DiGeorge Critical Region (DGCR), is the most commonly deleted region and occurs in up to 90% of patients5,8,9. Within the DGCR, a minimal critical region of 300-480kb has been described10,11, containing several genes, including TUPLE1 (HIRA), TBX1, SLC25A1 (CTP) and CLTD.
22q13.3 Deletion Syndrome
The 22q13.3 deletion syndrome presents a recognisable phenotype characterised by hypotonia, delay or absence of expressive speech, global developmental delay, normal to accelerated growth and mild dysmorphic features12,13.
Some deletions of the terminal region of chromosome 22q are cytogenetically visible. However, a few cases of cryptic deletions have been reported12,14, suggesting that the actual incidence of 22q telomere deletion may be higher than previously thought.
Several observations of patients with 22q13.3 deletion showed that the SHANK3 (ProSAP2)20 gene, encoding a structural protein of the postsynaptic density of excitation synapses and expressed in the cortex and cerebellum of the brain15, was disrupted15,16,17 or deleted18, making it a candidate causative gene for this syndrome. The deletion varies dramatically in size from 130kb to 9Mb18,19,20. The use of 22q subtelomeric probes, distal to the ARSA gene, have therefore been recommended for examining all 22q13.3 deletions20,21.
Running our PETS protocol was taking upwards of 5 hours to complete based on the previous SOP. After the technical training visit from CytoCell, we were able to make some tweaks to reduce the protocol time down to just 1 hour and 15 minutes, with the same or better results.
Assistant Genetic Technologist, Leicestershire Genetics Centre, UK