Midfacial toddler excoriation syndrome (MiTES): case series, diagnostic criteria and evidence for a pathogenic mechanism

Abstract

Background: PRDM12 polyalanine tract expansions cause two different disorders: midfacial toddler excoriation syndrome (MiTES; itch with normal pain sensation associated with 18 homozygous alanines (18A); and congenital insensitivity to pain (CIP) with normal itch associated with 19 homozygous alanines (19A). Knowledge of the phenotype, genotype and disease mechanism of MiTES is incomplete. Why 18A vs. 19A PRDM12 can cause almost opposite phenotypes is unknown; no other polyalanine or polyglutamine tract expansion disease causes two such disparate phenotypes.

Objectives: To assess the genotype and phenotype of nine new, nine atypical and six previously reported patients diagnosed with MiTES.

Methods: Using cell lines with homozygous PR domain zinc finger protein 12 (PRDM12) containing 12 alanines (12A; normal), 18A (MiTES) and 19A (CIP), we examined PRDM12 aggregation and subcellular localization by image-separation confocal microscopy and subcellular fractionation Western blotting.

Results: MiTES presents in the first year of life; in all cases the condition regresses over the first decade, leaving scarring. The MiTES phenotype is highly distinctive. Features overlapping with PRDM12 CIP are rarely found. The genotype-phenotype study of the PRDM12 polyalanine tract shows that having 7-15 alanines is normal; 16-18 alanines is associated with MiTES; 19 alanines leads to CIP; and no clinically atypical cases of MiTES had a polyalanine tract expansion. PRDM12 aggregation and subcellular localization differed significantly between 18A and normal 12A cell lines and between 18A and 19A cell lines. MiTES is a new protein-aggregation disease.

Conclusions: We provide diagnostic criteria for MiTES and improved longitudinal data. MiTES and CIP are distinct phenotypes, despite their genotypes varying by a single alanine in the PRDM12 polyalanine tract. We found clear distinctions between the cellular phenotypes of normal, MiTES and CIP cells. We hypothesize that the developmental environment of the trigeminal ganglion is unique and critically sensitive to pre- and postnatal levels of PRDM12.

Figure 1

Facial features of mid-facial toddler excoriation syndrome (MiTES). (a) New patients with MiTES: N1 at 6 months and 13 months; N2 at 1 year 6 months and 4 years; N3 at 1 year 10 months and 4 years; N4 at 2 years and 3 years; N6 at 13 years; N7 at 10 years and 13 years; N8 at 7 years and 10 years; N9 at 2 years old. (b) Natural history of MiTES by review of cases: R1 at 4 years and 9 years; R2 at 1 year and 6 years; R3 at 2 years and 7 years; R4 at 5 years and 8 years; and R5 at 3 years and 7 years old. (c) Atypical cases where the facial features did meet MiTES diagnostic criteria but the PRDM12 genotype was normal: AT3 at 3 years and 7 years old. See Table S2 for clinical details.

Figure 2

Facial areas affected by mid-facial toddler excoriation syndrome (MiTES) and electrophoretograms of an unaffected MiTES-carrier parent, a patient with MiTES and a patient with hereditary autonomic and sensory neuropathy 8 (HSAN8) congenital insensitivity to pain (CIP). (a) The affected facial regions in patients with MiTES are highlighted in orange. (b) Chromatogram traces serve as examples to illustrate the GC-rich nature of the polyalanine tract in exon 5 of PRDM12. (b1) Chromatogram trace of homozygous wild-type 12-alanine (12A) PRDM12. (b2) Chromatogram trace of heterozygous 13-alanine/18-alanine (13A/18A) PRDM12 from a carrier parent of an affected child. (b3) Chromatogram trace of homozygous PRDM12 18A from a patient with MiTES. (b4) Chromatogram trace of homozygous PRDM12 19A from a patient with HSAN8/CIP. Reading the genetic sequence is relatively easy when the polyalanine tract length is homozygous. However, in cases where a frameshift occurs due to differences in the polyalanine tract between two alleles, such as those from an affected parent, the precise assignment of nucleotides at each site is essential.

Figure 3

Cellular phenotypes and subcellular localizations of wild-type (WT) 12-alanine (12A) PR domain zinc finger protein 12 (PRDM12) and mutant 18- (18A) and 19-alanine (19A) proteins. (a) WT PRDM12 12A was expressed diffusely in the nucleus (red channel, upper panel) and co-localized with 4ʹ,6-diamidino-2-phenylindole (DAPI; blue channel). The PRDM12 18A form representing the mid-face toddler excoriation syndrome (MiTES) condition produced small but bright nuclear aggregates in addition to diffuse nuclear staining (red channel, second row). Cytoplasmic aggregates were also observed. The PRDM12 19A form representing HSAN8/CIP formed larger plaques in the nucleus and the cytoplasm with less diffuse expression of PRDM12 in the nucleoplasm, as though more protein had been drawn up into aggregates (red channel, third row). (b) The percentage of cells exhibiting aggregated PRDM12 was calculated for each image. Cells transfected with PRDM12 18A or PRDM12 19A displayed a notably higher percentage of cells exhibiting aggregated PRDM12 than those transfected with PRDM12 12A. In particular, PRDM12 19A exhibited a higher degree of aggregation than PRDM12 18A. (c) The percentage of cells exhibiting diffuse PRDM12 was lower in cells transfected with mutant PRDM12 18A or 19A than those transfected with the WT. (d) The percentage of cells expressing both PRDM12 18A and 19A forms in the cytoplasm was higher than that of WT PRDM12 12A, with PRDM12 19A showing a significantly higher percentage of expressing cells than PRDM12 18A. All graphs display minimum to maximum whiskers and each datapoint represents the percentage of cells from one image from three independent experiments (12A, n = 31; 18A, n = 32; 19A, n = 39). The line in the box indicates the median. ns, nonsignificant, one-way Anova with Tukey’s post-hoc test. *P < 0.05 ; ****P < 0.0001.

Figure 4

Graphic illustration of human PRDM12, PRDM12 protein, polyalanine tract (Poly A) and the human phenotype of biallelic polyalanine expansion mutations. The figure shows the PRDM12 locus on chromosome 9, the gene and the protein architecture. Amino acid numbering is shown along the top of the PRDM12 protein schematic representation. Depending on the number of alanines in the polyalanine tract, the phenotype might be different (7–13 alanines: no sensory dysfunction; 18 alanines: mid-facial toddler excoriation syndrome; 19 alanines: congenital insensitivity to pain). PR/SET, PR domain; WT, wild type; ZF, zinc finger motif.