Mental retardation (MR) is defined as a significant impairment of cognitive and adaptive functioning, with onset before age 18 years, and it is estimated to occur in about 1-3% of the population (Chelly and Mandel, 2001, Nat Rev Genet.). Among mentally retarded patients, an excess of males over females has long been noted, which is usually explained by the presence of many genes responsible for MR on the X chromosome.
X-linked mental retardation (XLMR) is usually divided into syndromic and non-syndromic or non-specific forms. In syndromic forms (MRXS), MR is present in association with a specific pattern of physical, neurological, and/or metabolic abnormalities. The term non-specific or non-syndromic X-linked mental retardation (MRX) was introduced to indicate a condition segregating in an X-linked manner in which male patients have no consistent phenotypic manifestations other than MR. Many different genes responsible for MRX have been identified.
This P106-C1 MRX MLPA probemix can be used to detect copy number changes of several genes on the X-chromosome that have been implicated in (non-specific) X-linked mental retardation. The MLPA P106-C1 MRX probemix includes probes for 16 different MRX genes: RPS6KA3, ARX, IL1RAPL1, TSPAN7, PQBP1, HUWE1, OPHN1, ACSL4, PAK3, DCX, AGTR2, ARHGEF6, FMR1, AFF2 (FMR2), SLC6A8 and GDI1. For most genes, probes are present for only some of the exons.
This SALSA® MLPA® probemix is designed to detect deletions/duplications of one or more sequences in the aforementioned genes in a DNA sample. In males, deletions of a probe’s recognition sequence on the X-chromosome will lead to a complete absence of the corresponding probe amplification product, whereas female heterozygotes are recognisable by a 35-50% reduction in relative peak height. Note that a mutation or polymorphism in the sequence detected by a probe can also cause a reduction in relative peak height, even when not located exactly on the ligation site! In addition, some probe signals are more sensitive to sample purity and small changes in experimental conditions. Therefore, deletions and duplications detected by MLPA should always be confirmed by other methods. Not all deletions and duplications detected by MLPA will be pathogenic; users should always verify the latest scientific literature when interpreting their findings. We have no information on what percentage of defects in these genes is caused by deletions/duplications of complete exons. Finally, note that most defects in this gene are expected to be small (point) mutations which will not be detected by this SALSA® MLPA® test.