Assessing non-Mendelian inheritance in inherited axonopathies

PURPOSE: Inherited axonopathies (IA) are rare, clinically and genetically heterogeneous diseases that lead to length-dependent degeneration of the long axons in central (hereditary spastic paraplegia [HSP]) and peripheral (Charcot–Marie–Tooth type 2 [CMT2]) nervous systems. Mendelian high-penetrance alleles in over 100 different genes have been shown to cause IA; however, about 50% of IA cases do not receive a genetic diagnosis. A more comprehensive spectrum of causative genes and alleles is warranted, including causative and risk alleles, as well as oligogenic multilocus inheritance. METHODS: Through international collaboration, IA exome studies are beginning to be sufficiently powered to perform a pilot rare variant burden analysis. After extensive quality control, our cohort contained 343 CMT cases, 515 HSP cases, and 935 non-neurological controls. We assessed the cumulative mutational burden across disease genes, explored the evidence for multilocus inheritance, and performed an exome-wide rare variant burden analysis. RESULTS: We replicated the previously described mutational burden in a much larger cohort of CMT cases, and observed the same effect in HSP cases. We identified a preliminary risk allele for CMT in the EXOC4 gene (p value= 6.9 × 10-6, odds ratio [OR] = 2.1) and explored the possibility of multilocus inheritance in IA. CONCLUSION: Our results support the continuing emergence of complex inheritance mechanisms in historically Mendelian disorders

Interplay between Nitric Oxide and Vasoactive Intestinal Polypeptide in Inducing Fluid Secretion in Rat Jejunum

Nitric oxide (NO) and vasoactive intestinal polypeptide (VIP) interact in the regulation of neuromuscular function in the gut. They are also potent intestinal secretogogues that coexist in the enteric nervous system. The aims of this study were: (1) to investigate the interaction between NO and VIP in inducing fluid secretion in the rat jejunum, and (2) to determine whether the NO effect on intestinal fluid movement is neurally mediated. The single pass perfusion technique was used to study fluid movement in a 25 cm segment of rat jejunum in vivo. A solution containing 20 mml-arginine, a NO precursor, was perfused into the segment. The effect of the NO synthase inhibitors (l-NAME and l-nitroindazole (l-NI)) and the VIP antagonist ([4Cl-D-Phe6,Leu17]VIP (VIPa)) on l-arginine-induced changes in fluid movement, expressed as μl min−1 (g dry intestinal weight)−1, was determined. In addition, the effect of neuronal blockade by tetrodotoxin (TTX) and ablation of the myenteric plexus by benzalkonium chloride (BAC) was studied. In parallel groups of rats, the effect of l-NAME and l-NI on VIP-induced intestinal fluid secretion was also examined. Basal fluid absorption in control rats was (median (interquartile range)) 65 (45–78). l-Arginine induced a significant fluid secretion (−14 (−20 to −5); P < 0.01). This effect was reversed completely by l-NAME (60 (36–65); P < 0.01) and l-NI (46 (39–75); P < 0.01) and partially by VIPa (37 (14–47); P < 0.01). TTX and BAC partially inhibited the effect of l-arginine (22 (15–32) and 15 (10–26), respectively; P < 0.05). The effect of VIP on fluid movement (−23 (−26 to −14)) was partially reversed by l-NAME (24 (8.4–35.5); P < 0.01) and l-NI (29 (4–44); P < 0.01). The inhibition of VIP or NO synthase prevented l-arginine- and VIP-induced intestinal fluid secretion through a neural mechanism. The data suggest that NO enhances the release of VIP from nerve terminals and vice versa. Subsequently, each potentiates the other's effect in inducing intestinal fluid secretion