Abstract Details

Title Decreased PI3,5P2 and PI5P in Fibroblasts from Patients with FIG4 Deficiency

Topic General Neurology

Presentation(s) P1 - Poster Session 1 (5:30 PM-6:30 PM)

Poster/Presentation
Number 4-031

Objective To quantify phosphatydilinositol phosphates (PIPs) in fibroblasts from patients with Charcot-Marie-Toot disease type-4J (CMT4J).

Background CMT4J is autosomal recessively inherited peripheral neuropathy caused by compound heterozygous mutations in FIG4 (also known as SAC3) gene, to result in severe loss/absence of the FIG4/SAC3 protein, which triggers neuronal degeneration, segmental demyelination, sensory disorder and limb muscle weakness. In primary mouse fibroblasts, the absence of FIG4 (PIP 5-phosphatase) leads to disassembly of PI3,5P₂-metabolizing machinery, composed of the PIKfyve kinase, the ArPIKfyve scaffold and the phosphatase, and reduction of specific PIs. Decreases of PI3,5P₂ over 70% is incompatible with life as shown in genetically modified mouse models deficient in either of the three genes. However, PIPs in humans with loss-of-function CMT4J mutations have never been quantitatively evaluated. How changes in PIPs relate to lysosomal phenotypes is also unclear.

Design/Methods De-identified fibroblasts were obtained as previously described (Hu et al Ann Neurol 2016). Fibroblasts were labeled with myo-[2-³H]inositol to equilibrium. PIPs were extracted and quantified by HPLC.

Results In comparison to normal human control fibroblasts, both PI3,5P₂ and PI5P levels were significantly decreased in CMT4J fibroblasts (by 41.9±4.9% and 36.5±7.8%, respectively, mean±SEM; n=6; p<0.005). In contrast, PI3P levels were unchanged. Unlike abundant endolysosomal vacuoles and an increase of PI3P levels in fibroblasts from Fig4^-/- mice, human CMT4J fibroblasts showed no or minimal vacuoles similar to those in normal controls.

Conclusions 1). Our study identifies for the first time PIP deficiency in humans with CMT4J. 2). The phenotypes in CMT4J patients may not be solely due to reduction of PI3,5P₂, but also to that of PI5P, known to be involved in non-canonical autophagy. 3). Lack of vacuoles in human CMT4J fibroblasts may relate to unaltered PI3P levels, usually increased in mouse cells with complete elimination of FIG4.

Authors/Disclosures
PRESENTER	No disclosure on file
	No disclosure on file
	No disclosure on file
Jun Li, MD, PhD, FAAN (Harris Methodist Hospital)	The institution of Dr. Li has received personal compensation in the range of $500-$4,999 for serving as a Consultant for FDA. The institution of Dr. Li has received research support from NIH. Dr. Li has a non-compensated relationship as a Associate Editor of ACTN journal with ANA that is relevant to AAN interests or activities.

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