Functional Defects From Endocrine Disease–Associated Mutations in HLXB9 and Its Interacting Partner, NONO

The insulin-secreting pancreatic neuroendocrine tumors, insulinomas, characterized by increased pancreatic islet β-cell proliferation, express the phosphorylated isoform of the β-cell differentiation factor HLXB9 that interacts with NONO/p54NRB, a survival factor. Interestingly, two different homozygous germline mutations in HLXB9, p.F248L and p.F272L, were reported in neonatal diabetes, a condition with functional β-cell deficiency. Also, two somatic heterozygous NONO mutations were found in endocrine-related tumors, p.H146R (parathyroid) and p.R293H (small intestine neuroendocrine tumor). However, the biological consequence of the mutations, and the role of HLXB9-NONO interaction in normal or abnormal β cells, is not known. Expression, localization, and functional analysis of the clinically relevant HLXB9 and NONO mutants showed that HLXB9/p.F248L mutant localized in the nucleus but lacked phosphorylation, and NONO/p.R293H mutant was structurally impaired. The HLXB9 and NONO mutants retained the ability to interact, and overexpression of wild-type or mutant HXLB9 in MIN6 cells suppressed cell proliferation. To further understand the biological consequence of the HLXB9-NONO interaction, we mapped the NONO-interacting region in HLXB9. An 80-amino acid conserved region of HLXB9 could compete with full-length HLXB9 to interact with NONO; however, in functional assays, nuclear expression of this HLXB9-conserved region in MIN6 cells did not interfere with cell proliferation. Overall, our results highlight the importance of HLXB9 in conditions of β-cell excess (insulinomas) and in conditions of β-cell loss or dysfunction (diabetes). Our studies implicate therapeutic strategies for either reducing β-cell proliferation in insulinomas or alleviating normal β-cell deficiency in diabetes through the modulation of HLXB9 phosphorylation.