Background: Two siblings of non-consanguineous parents presented with FGD, demonstrated by ACTH resistance with glucocorticoid but not mineralocorticoid deficiency. The proband presented at 21 months, unresponsive with hypoglycaemia (BGL 1.5 mmol/l). Endocrine evaluation subsequent to resuscitation indicated adrenal insufficiency with elevated ACTH. Hydrocortisone therapy was commenced. A sibling, 4 years younger than the proband had a short Synacthen test (SST) performed on day 4 of life: baseline cortisol 38 nmol/l, 60 min peak 380 nmol/l. The child was clinically well and remained under surveillance. Increased pigmentation was noted by her parents from 6 months and repeat SST was performed when she presented with gastrointestinal symptoms at 8 months: baseline 110 nmol/l, 60 min 130 nmol/l, consistent with FGD. Hydrocortisone therapy was commenced but no fludrocortisone was required for either child.
Objective and hypotheses: To discover the genetic aetiology of the siblings disease.
Method: Whole exome sequencing, cDNA analysis and sequencing of genomic DNA.
Results: Whole exome sequencing identified a novel, heterozygous variant (R71X) in the antioxidant defence gene Nicotinamide Nucleotide Transhydrogenase (NNT), in both affected individuals. Follow-up cDNA analysis detected the pseudoexon inclusion (p.P998_D999ins23) and sequencing of genomic DNA identified a 4 bp duplication responsible for its activation. Both affected siblings were compound heterozygotes for the NNT mutations and an unaffected sibling had inherited only the R71X variant. Neither variant has been annotated in SNP/mutation databases. Both will lead to premature truncation and presumably result in an inactive protein.
Conclusion: Aberrant pseudoexon inclusion is rarely recognised as a cause of human disease. Here, we report two novel, compound heterozygous mutations in NNT, including one which activates a pseudoexon, as the cause of FGD in two siblings. This case highlights the importance of cDNA analysis, particularly for recessive disorders where one defective allele has already been demonstrated, to investigate the possibility of non-coding variants contributing to disease.
20 - 22 Sep 2014
European Society for Paediatric Endocrinology