A Restricted Repertoire of De Novo Mutations in ITPR1 Cause Gillespie Syndrome with Evidence for Dominant-Negative Effect

Abstract

Gillespie syndrome (GS) is characterized by bilateral iris hypoplasia, congenital hypotonia, non-progressive ataxia, and progressive cerebellar atrophy. Trio-based exome sequencing identified de novo mutations in ITPR1 in three unrelated individuals with GS recruited to the Deciphering Developmental Disorders study. Whole-exome or targeted sequence analysis identified plausible disease-causing ITPR1 mutations in 10/10 additional GS-affected individuals. These ultra-rare protein-altering variants affected only three residues in ITPR1: Glu2094 missense (one de novo, one co-segregating), Gly2539 missense (five de novo, one inheritance uncertain), and Lys2596 in-frame deletion (four de novo). No clinical or radiological differences were evident between individuals with different mutations. ITPR1 encodes an inositol 1,4,5-triphosphate-responsive calcium channel. The homo-tetrameric structure has been solved by cryoelectron microscopy. Using estimations of the degree of structural change induced by known recessive- and dominant-negative mutations in other disease-associated multimeric channels, we developed a generalizable computational approach to indicate the likely mutational mechanism. This analysis supports a dominant-negative mechanism for GS variants in ITPR1. In GS-derived lymphoblastoid cell lines (LCLs), the proportion of ITPR1-positive cells using immunofluorescence was significantly higher in mutant than control LCLs, consistent with an abnormality of nuclear calcium signaling feedback control. Super-resolution imaging supports the existence of an ITPR1-lined nucleoplasmic reticulum. Mice with Itpr1 heterozygous null mutations showed no major iris defects. Purkinje cells of the cerebellum appear to be the most sensitive to impaired ITPR1 function in humans. Iris hypoplasia is likely to result from either complete loss of ITPR1 activity or structure-specific disruption of multimeric interactions.

Inositol 1,4,5-Trisphosphate Receptor Mutations Associated with Human Disease

“…Patients with GS exhibit symptoms similar to those observed in SCA29 including early-onset, non-progressive ataxia, developmental delay, hypotonia, variable intellectual disability, and progressive cerebellar atrophy (McEntagart et al, 2016). However, patients with GS are distinguished from those with SCA29 based on the presence of aniridia (McEntagart et al, 2016, Hingorani et al, 2012).…”

“…However, patients with GS are distinguished from those with SCA29 based on the presence of aniridia (McEntagart et al, 2016, Hingorani et al, 2012). Aniridia is a rare, inborn developmental error where the patient exhibits hypoplasia or complete loss of the iris of the eye 14 .…”

“…The majority of GS-associated missense mutations in IP 3 R1 are located in the C-terminal domain, however two missense mutations have been identified in the regulatory domain. Both mutations are found in residue E2061; however, one is inherited (E2061G), while the other is a de novo mutation in an individual (E2061Q) (McEntagart et al, 2016). McEntagart et al theorize that these dominant negative mutations resulting in iris hypoplasia are due disruption of functional interactions that are critical to the formation and/or maintenance of the sphincter pupillae muscle or a lower level of residual IP 3 R function compared to that in SCA29 (McEntagart et al, 2016).…”

“…For example, mutation of residue G2506 (G2506R) results in multiple de novo cases of GS (McEntagart et al, 2016) and SCA29, both de novo and inherited (Zambonin et al, 2017). Located in the selectivity filter ( G GGVG) mutation of this residue from a small Gly to a large, positively changed Arg is predicted to be strongly destabilizing.…”

The clinical evaluation of infantile nystagmus: what to do first, and why

“…Other signs of a primarily neurologic disorder may be unusual nystagmus such as see-saw or spasmus nutans (shimmering and asymmetric) (4,5), co-occurrence of ocular motor apraxia (6), ataxia, pale or small optic nerves, or swollen optic nerves. Chorioretinal colobomas may also be associated with midline brain defects, while iris-only colobomas, when in an unusual position away from the inferonasal region, may signify Gillespie syndrome due to mutations in PAX6 or other genes (7,8). Small optic nerves may be due to primary optic nerve hypoplasia (ONH), with or without abnormalities of the septum pellucidum, pituitary gland and posterior pituitary bright spot.…”