SPG21 and Alzheimer disease: Numerous HSP proteins influence the BMP signaling pathway; these include Spastin (causing SPG4, the commonest AD HSP) in addition to Magnesium transporter NIPA1 (SPG6, AD), Maspardin (SPG20, AR), Spastizin (SPG15, AR), Atlastin (SPG3A, AD/AR), and PNPLA6 (SPG39), influence the BMP signaling pathway (Charron and Tessier-Lavigne, 2007; Wang et al., 2007, 2014; Wen et al., 2007; Tsang et al., 2009; Fassier et al., 2010; Bayat et al., 2011; Hegarty et al., 2013; Nahm et al., 2013; Song et al., 2013).