Familial dysautonomia (FD) is a devastating developmental peripheral autonomic and sensory neuropathy caused by a mutation in the inhibitory kappa B kinase complex-associated protein (IKBKAP) gene. It is characterized by tachycardia, blood pressure instability, autonomic vomiting “crisis”, and decreased sensation of pain and temperature. FD is progressive, and affected individuals commonly die during early adulthood. To identify the cellular and molecular mechanisms that cause FD, we generated a mouse model for the disease in which Ikbkap expression is knocked out in the neural crest lineage. This study is a mechanistic analysis of cellular events that go awry in peripheral nervous system development in FD and identifies the essential functions of the IKAP protein in the peripheral nervous system.
Familial dysautonomia (FD) is a devastating developmental and progressive peripheral neuropathy caused by a mutation in the inhibitory kappa B kinase complex-associated protein (IKBKAP) gene. To identify the cellular and molecular mechanisms causing FD, we generated mice in which Ikbkap expression is knocked out in the peripheral nervous system and identified steps in peripheral nervous system development that are dependent on Ikbkap. We show that Ikbkap is not required for neural crest stem migration or pathfinding, nor for formation of the dorsal root or sympathetic ganglia, or the adrenal medulla. Instead, Ikbkap is essential for the second wave of neurogenesis during which most tropomyosin-related kinase A (TrkA+) nociceptors and thermoreceptors arise. In its absence, approximately half of the normal complement of TrkA+ neurons is lost, which we show is due in part to p53-mediated premature differentiation and death of mitotically active progenitors that express the Pax3 paired-box gene and give rise to to most TrkA+ neurons At the end of sensory development, the number of TrkC neurons increases significantly, which may result from an increase in Runx3+ cells. Furthermore, our data demonstrate that TrkA+ (but not TrkC+) sensory and sympathetic neurons experience exacerbated Caspase 3-mediated programmed cell death in the absence of Ikbkap and that this death is not due to a reduction in nerve growth factor synthesis. In summary, these data suggest that FD does not result from a failure of stem neural crest migration, but from a critical role for Ikbkap in TrkA progenitors and TrkA+ neurons.
Hereditary sensory and autonomic neuropathies (HSAN) are a group of five phenotypically diverse but overlapping disorders of the peripheral nervous system (PNS) that result from mutations in 12 distinct genes (1). HSAN type 3, or familial dysautonomia (FD) (also called Riley-Day syndrome), results from an intronic mutation (IVS20 + 6T > C; 99.5% of patients) in a gene called inhibitor of kappa complex-associated protein B kinase or IKBKAP, causing missplicing and subsequent tissue-specific reductions in IKAP protein (2, 3). FD is characterized by tachycardia, labile blood pressure, autonomic vomiting “crisis”, decreased sensation of pain and temperature, and usually death during early adulthood (4). The role of IKAP in the nervous system is unclear, nor is it understood why deletions in this widely expressed gene primarily devastate the PNS. The first pathological study, performed on a 2-year-old boy with FD, showed that approximately 90% of the cells in the dorsal root and sympathetic ganglia (SG) were missing (5). To identify the role of IKAP in the developing PNS, we must first establish the steps in which it is essential.
The vertebrate PNS is derived primarily from the neural crest, a multipotent and heterogeneous cell population that sheds from the neural tube and migrates throughout the embryo (6). Those neural crest cells that stop lateral to the neural tube give rise to the sensory dorsal root ganglia (DRG) chain, while those that migrate more ventrally give rise to the SG vertebral chain. Within the DRG, neural crest cells generate heterogeneous neuronal subpopulations that include nociceptors, thermoreceptors, mechanoreceptors, and proprioceptors. With the completion of neural crest migration, multiple steps occur that are essential for normal PNS development, including the proliferation of discrete sets of neural progenitor cells that are derived from different waves of migrating neural crest cells, the neuron differentiation