Supplementary Material for: The Novel Ser18del AVP Variant Causes Inherited Neurohypophyseal Diabetes Insipidus by Mechanisms Shared with Other Signal Peptide Variants

<p><b><i>Background/Aim:</i></b> Variability in the severity and age at onset of autosomal dominant familial neurohypophyseal diabetes insipidus (adFNDI) may be associated with certain types of variants in the arginine vasopressin (<i>AVP</i>) gene. In this study, we aimed to describe a large family with an apparent predominant female occurrence of polyuria and polydipsia and to determine the underlying cause. <b><i>Methods:</i></b> The family members reported their family demography and symptoms. Two subjects were diagnosed by fluid deprivation and dDAVP challenge tests. Eight subjects were tested genetically. The identified variant along with 3 previously identified variants in the <i>AVP</i> gene were investigated by heterologous expression in a human neuronal cell line (SH-SY5Y). <b><i>Results:</i></b> Both subjects investigated clinically had a partial neurohypophyseal diabetes insipidus phenotype. A g.276_278delTCC variant in the <i>AVP</i> gene causing a Ser18del deletion in the signal peptide (SP) of the AVP preprohormone was perfectly co-segregating with the disease. When expressed in SH-SY5Y cells, the Ser18del variant along with 3 other SP variants (g.227G>A, Ser17Phe, and Ala19Thr) resulted in reduced <i>AVP</i> mRNA, impaired AVP secretion, and partial AVP prohormone degradation and retention in the endoplasmic reticulum. Impaired SP cleavage was demonstrated directly in cells expressing the Ser18del, g.227G>A, and Ala19Thr variants, using state-of-the-art mass spectrometry. <b><i>Conclusion:</i></b> Variants affecting the SP of the AVP preprohormone cause adFNDI with variable phenotypes by a mechanism that may involve impaired SP cleavage combined with effects at the mRNA, protein, and cellular level.</p>