Hereditary spastic paraplegia type 43 (SPG43) is caused by mutation in C19orf12

Abstract

We report here the genetic basis for a form of progressive hereditary spastic paraplegia (SPG43) previously described in two Malian sisters. Exome sequencing revealed a homozygous missense variant (c.187G>C; p.Ala63Pro) in C19orf12, a gene recently implicated in neurodegeneration with brain iron accumulation (NBIA). The same mutation was subsequently also found in a Brazilian family with features of NBIA, and we identified another NBIA patient with a three-nucleotide deletion (c.197_199del; p.Gly66del). Haplotype analysis revealed that the p.Ala63Pro mutations have a common origin, but MRI scans showed no brain iron deposition in the Malian SPG43 subjects. Heterologous expression of these SPG43 and NBIA variants resulted in similar alterations in the subcellular distribution of C19orf12. The SPG43 and NBIA variants reported here as well as the most common C19orf12 missense mutation reported in NBIA patients are found within a highly conserved, extended hydrophobic domain in C19orf12, underscoring the functional importance of this domain.

Keywords: C19orf12; NBIA; SPG43; hereditary spastic paraplegia.

Figure 1

MRI and genetic characterization of SPG43 and NBIA. (A) Brain MRI images of a SPG43 subject with the homozygous C19orf12 mutation c.187G>C, p.Ala63Pro. (B) MRI images of a NBIA patient with the homozygous C19orf12 deletion c.197_199GGG, p.Gly66del. The white arrow indicates iron deposition. (C) Schematic diagram of C19orf12, with the hydrophobic sequence in blue, and a protein sequence alignment of C19orf12 in various species (amino acid numbers refer to the human sequence). The SPG43 and NBIA mutations cause amino acid changes at Ala63 and Gly66, respectively, both highly conserved residues (in red, asterisks above).

Figure 2

C19orf12 localizes to the ER, and the SPG43 and NBIA mutations alter its subcellular distribution. (A) COS7 cells expressing HA-tagged wild-type (WT) or p.Ala63Pro mutant C19orf12 were co-stained for endogenous calreticulin, an ER protein (red). (B) Quantification of the subcellular distribution of C19orf12 in cells expressing Myc-tagged, wild-type C19orf12 or C19orf12 containing the SPG43 mutation p.Ala63Pro, the NBIA-associated mutants p.Gly69Arg and p.Gly66del, or two known polymorphisms (p.Val52Ile and p.Leu55Phe). Cells with C19orf12 clearly at the ER or mitochondria were counted (n=3 trials, with >100 cells per trial). (C and D) Cells expressing HA-tagged wild-type or p.Ala63Pro mutant C19orf12 (green) were co-immunostained for calreticulin (red) and DAPI (blue) (D) and their distributions quantitated (C) as in panel B. Data represent the means ± SEM of three independent, blinded experiments. C-term, C-terminal; N-term, N-terminal. ***P<0.001. Bars, 20 μm.