Mutations inSIPA1L3cause eye defects through disruption of cell polarity and cytoskeleton organization

Abstract

Correct morphogenesis and differentiation are critical in development and maintenance of the lens, which is a classic model system for epithelial development and disease. Through germline genomic analyses in patients with lens and eye abnormalities, we discovered functional mutations in the Signal Induced Proliferation Associated 1 Like 3 (SIPA1L3) gene, which encodes a previously uncharacterized member of the Signal Induced Proliferation Associated 1 (SIPA1 or SPA1) family, with a role in Rap1 signalling. Patient 1, with a de novo balanced translocation, 46,XY,t(2;19)(q37.3;q13.1), had lens and ocular anterior segment abnormalities. Breakpoint mapping revealed transection of SIPA1L3 at 19q13.1 and reduced SIPA1L3 expression in patient lymphoblasts. SIPA1L3 downregulation in 3D cell culture revealed morphogenetic and cell polarity abnormalities. Decreased expression of Sipa1l3 in zebrafish and mouse caused severe lens and eye abnormalities. Sipa1l3(-/-) mice showed disrupted epithelial cell organization and polarity and, notably, abnormal epithelial to mesenchymal transition in the lens. Patient 2 with cataracts was heterozygous for a missense variant in SIPA1L3, c.442G>T, p.Asp148Tyr. Examination of the p.Asp148Tyr mutation in an epithelial cell line showed abnormal clustering of actin stress fibres and decreased formation of adherens junctions. Our findings show that abnormalities of SIPA1L3 in human, zebrafish and mouse contribute to lens and eye defects, and we identify a critical role for SIPA1L3 in epithelial cell morphogenesis, polarity, adhesion and cytoskeletal organization.

A 16.7 kb deletion in Sipa1l3 is associated with juvenile cataract in mice

“…The mouse protein encoded by Sipa1l3 is predicted to include a Rap GTPase-activating protein (Rap-GAP) domain, a PDZ domain, and a Cterminal domain of the SPAR protein (Greenlees et al 2015). The Rap-GAP domain is specific for Rap1, which is a GTP-binding protein and suggested, like other small GTPases, to be involved in functions associated with lens development (Rao et al 1997).…”

“…The peptide encoded by this variant is truncated and contains the Rap-GAP and PDZ domains, but includes only a small fragment of the C-terminal domain of the SPAR protein. Recently, Greenlees et al (2015) found that dysfunction in Sipa1l3 in human, zebrafish, and mouse, affected lens and eye development. For example, a missense mutation (p.Asp148Tyr) and a balanced chromosomal translocation affecting the 5′UTR of Sipa1l3 were found in patients with congenital cataract and other eye abnormalities.…”

“…For example, a missense mutation (p.Asp148Tyr) and a balanced chromosomal translocation affecting the 5′UTR of Sipa1l3 were found in patients with congenital cataract and other eye abnormalities. The missense mutation affected a putative actinbinding site while the chromosomal translocation resulted in reduced Sipa1l3 expression (Greenlees et al 2015).…”

An Epha4/Sipa1l3/Wnt pathway regulates eye development and lens maturation

“…Recently, the SIPA1L3 gene has also been associated with congenital human cataracts (Evers et al, 2015;Greenlees et al, 2015). SIPA1L3 is a member of the signal-induced proliferationassociated [SIPA, also known as the spine-associated rap-gap (SPAR)] proteins.…”

“…Besides its expression in the rodent brain, Sipa1l3 is also localized in the developing eye in humans, mice, frog and zebrafish, especially within the lens (Evers et al, 2015;Greenlees et al, 2015;Lachke et al, 2012;Rothe et al, 2016). In 2015, two groups independently showed that mutations of the human SIPA1L3 gene are related to congenital cataracts.…”