Growth hormone insensitivity (GHI) is characterized by severe short stature, normal to elevated serum levels of growth hormone (GH) and resistance to exogenous GH therapy. The aetiology of GHI is classically associated with mutations in the GH receptor (GHR) gene or with mutations affecting the signalling cascade of the GHR (OMIM 245590). Intracellular signalling molecules activated by GH belong to the Janus kinase-signal transducer and activator of transcription 5B (JAK2-STAT5B) pathway. Amongst others, this pathway activates the insulin-like growth factor (IGF1), which is implicated in the regulation of protein turnover and exerts potent mitogenic and differentiating effects on most cell types.
There are two genomic isoforms of the GHR gene in humans: a full-length isoform (GHRfl) and an isoform lacking exon 3 (GHRd3). The distribution of these genotypes differs among populations, with the frequency for GHRfl/GHRfl ranging from 35-53%, for GHRfl/GHRd3 between 33-58%, and for GHRd3/GHRd3 ranging from 7-26%. Growth hormone is used to increase height in short children who are not deficient in growth hormone receptor, but its efficacy varies widely between individuals. Dos Santos et al. (2004, Nat Genet.) found that the GHRd3 isoform was associated with 1.7 to 2 times more growth acceleration induced by growth hormone than the full-length isoform.
The GHR gene (10 exons) spans ~298 kb of genomic DNA and is located on chromosome 5p12, ~42 Mb from the p-telomere. The P262 probemix contains one probe for each GHR exon and an additional one for exon 10. Please note that one GHR probe is for exon 3, which will only be present in the GHRfl isoform.
The JAK2 gene (25 exons) spans ~143 kb of genomic DNA and is located on chromosome 9p24.1, ~5 Mb from the p-telomere. The P262 probemix contains 15 JAK2 probes for 15 different exons.
The IGF1 gene (5 exons) spans ~63 kb of genomic DNA and is located on chromosome 12q23.2, ~101 Mb from the p-telomere. The P262 probemix contains four IGF1 probes for different exons.
The STAT5B (19 exons) spans ~77 kb of genomic DNA and is located on chromosome 17q21.2, ~38 Mb from the p-telomere. The P262 probemix contains 11 STAT5B probes for 11 different exons.
In addition, nine reference probes are included in this probemix, detecting different autosomal chromosomal locations.
This SALSA® MLPA® probemix is designed to detect deletions/duplications of one or more sequences in the aforementioned genes in a DNA sample. Heterozygous deletions of recognition sequences should give a 35-50% reduced relative peak height of the amplification product of that probe. 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 these genes are expected to be small (point) mutations which will not be detected by this SALSA® MLPA® test.