The SALSA MLPA
Probemix P324 22q11 is a research use only (RUO)
assay for the detection of deletions or duplications in the 22q11 chromosomal region. Microdeletions/-duplications in the 22q11.2 region cause a variety of disorders, including the 22q11.2 distal deletion syndrome (MIM 611867), the 22q11.2 microduplication syndrome (MIM 608363), DiGeorge syndrome (DGS; MIM 188400), velocardiofacial syndrome (VCFS; MIM 192430) and cat eye syndrome (CES; MIM 115470). We recommend the use of the SALSA MLPA Probemix P250 DiGeorge as a primary screening for DiGeorge syndrome.
The high frequency of 22q11.2 copy number changes is due to the presence of several low copy repeats (LCRs), labelled A-H, which facilitate misalignment. The overall birth prevalence of 22q11.2 deletions appears to be approximately 1 in 4,000. The 22q11.2 deletion syndrome is inherited in an autosomal dominant manner, with about 93% of probands having a de novo
deletion, whereas the remaining 7% of probands have inherited the 22q11.2 deletion from a parent. The 22q11.2 deletion syndrome, which includes DGS and VCFS, most often (85%) results from a 3 Mb deletion on the 22q11.2 region. This 3 Mb deleted region is flanked by LCR-A and LCR-D, and includes the TBX1
gene. The remaining 15% of affected individuals have smaller, atypical deletions, which can include LCR A-B, B-D or C-D deletions. Haploinsufficiency of the TBX1
gene in particular is responsible for most of the physical malformations. Point mutations in this gene have also been observed in individuals with DGS. The phenotype of individuals with 22q11.2 deletion syndrome is characterised by palatal anomalies, congenital heart problems and distinct facial features. More information is available at https://www.ncbi.nlm.nih.gov/books/NBK1523/
The 22q11.2 microduplication syndrome is inherited in an autosomal dominant manner. Most individuals with a 22q11.2 duplication have inherited the duplication from a parent, but de novo
22q11.2 duplications also occur. The penetrance of 22q11.2 duplication is incomplete. The phenotype is generally mild and highly variable. It is characterised by intellectual disability, delayed psychomotor development, growth retardation and/or hypotonia. More information is available at https://www.ncbi.nlm.nih.gov/books/NBK3823/
CES has a large clinical variability, ranging from marginally affected individuals to those with the full pattern of malformations and a lethal outcome. The eyes are predominantly affected. CES is caused by a small, dicentric, supernumerary chromosome 22 representing an inv dup (22)(q11). In many cases this chromosomal abnormality is mosaic.
The SALSA MLPA Probemix P324-B1 22q11 contains 40 MLPA probes with amplification products between 130 and 463 nucleotides (nt). This includes 28 probes for the 22q11 chromosomal region, among which seven probes for the TBX1
gene, and one probe for the 22q12.1 chromosomal region. In addition, 11 reference probes are included, detecting several different autosomal chromosomal locations. Complete probe sequences and the identity of the genes detected by the reference probes is available online (www.mlpa.com
This Probemix contains nine quality control fragments generating amplification products between 64 and 121 nt: four DNA Quantity Fragments (Q-fragments), two DNA Denaturation Fragments (D-fragments), one Benchmark Fragment, and one chromosome X and one chromosome Y-specific fragment. More information on how to interpret observations on these control fragments can be found in the MLPA General Protocol and online at www.mlpa.com