A novel deletion in the GH1 gene including the IVS3 branch site responsible for autosomal dominant isolated growth hormone deficiency.

CONTEXT

The majority of mutations responsible for isolated GH type II deficiency (IGHD II) lead to dominant negative deleteriously increased levels of the GH1 exon 3 skipped transcripts.

OBJECTIVE

The aim of this study was the characterization of the molecular defect causing a familial case of IGHD II.

PATIENTS

A 2-yr-old child and her mother with severe growth failure at diagnosis (-5.8 and -6.9 sd score, respectively) and IGHD were investigated for the presence of GH1 mutations.

RESULTS

We identified a novel 22-bp deletion in IVS3 (IVS3 del+56-77) removing the putative branch point sequence (BPS). Analysis of patients' lymphocyte mRNA showed an excess exon 3 skipping. The mutated allele transfected into rat pituitary cells produced four differently spliced products: the exon 3 skipped mRNA as the main product and lower amounts of the full-length cDNA and of two novel mRNA aberrant isoforms, one with the first 86 bases of exon 4 deleted and the other lacking the entire exon 4. A mutagenized construct lacking exclusively the 7 bp of the BPS only generated the exon 4 skipped and the full-length isoforms. The presence of the full-length transcript in the absence of the canonical BPS points to an alternative BPS in IVS3.

CONCLUSION

The IVS3 del+56-77 mutation, causing IGHD II in this family, has two separate effects on mRNA processing: 1) exon 3 skipping, analogous to most described cases of IGHD II, an effect likely caused by the reduction in size of the IVS3, and 2) partial or total exon 4 skipping, as a result of the removal of the BPS.