Periventricular Calcification, Abnormal Pterins and Dry Thickened Skin: Expanding the Clinical Spectrum of RMND1?

Abstract

Background: We report a consanguineous Sudanese family whose two affected sons presented with a lethal disorder characterised by severe neonatal lactic acidosis, hypertonia, microcephaly and intractable seizures. One child had additional unique features of periventricular calcification, abnormal pterins and dry thickened skin.

Methods: Exome enrichment was performed on pooled genomic libraries from the two affected children and sequenced on an Illumina HiSeq2000. After quality control and variant identification, rare homozygous variants were prioritised. Respiratory chain complex activities were measured and normalised to citrate synthase activity in cultured patient fibroblasts. RMND1 protein levels were analysed by standard Western blotting.

Results: Exome sequencing identified a previously reported homozygous missense variant in RMND1 (c.1250G>A; p.Arg417Gln), the gene associated with combined oxidation phosphorylation deficiency 11 (COXPD11), as the most likely cause of this disorder. This finding suggests the presence of a mutation hotspot at cDNA position 1250. Patient fibroblasts showed a severe decrease in mitochondrial respiratory chain complex I, III and IV activities and protein expression, albeit with normal RMND1 levels, supporting a generalised disorder of mitochondrial translation caused by loss of function.

Conclusions: The current study implicates RMND1 in the development of calcification and dermatological abnormalities, likely due to defective ATP-dependent processes in vascular smooth muscle cells and skin. Review of reported patients with RMND1 mutations shows intra-familial variability and evidence of an evolving phenotype, which may account for the clinical variability. We suggest that COXPD11 should be considered in the differential for patients with calcification and evidence of a mitochondrial disorder.

Fig. 1

Clinical, genetic and proteomic analyses. (a) Pedigree of a consanguineous Sudanese family who have had two affected sons and one healthy daughter. DNA was not available for II:3. (b) Photograph of patient II:1 showing stiffness in both arms, fisting of the hands and flexion of the toes. (c) Photograph of patient II:1 showing dry thickened skin with a pigmented skin rash. (d) Axial T1 MR image at day 7 of life shows posterior predominant ventricular dilation with bilateral symmetric white matter volume reduction. The periventricular calcifications detected by ultrasound are evident here as bilateral punctate hyperintensities. A focal area of hyperintensity in the left parietal parenchyma is in keeping with an acute haemorrhagic infarction. It was associated with restricted diffusion. (e) A CT scan at 4 weeks of age confirms the periventricular calcification bilaterally. Moderate ventricular dilation was unchanged. (f) The RMND1 NM_017909.3 c.1250G>A variant was validated by Sanger sequence analysis. Traces are shown for parent I:1 and affected child II:1. The inverted triangle indicates the position of the mutated G base which changes Arg (R) to Gln (Q) at residue 417. (g) Assessment of individual respiratory chain enzyme activities in fibroblasts identified severe OXPHOS deficiencies involving complexes I, III and IV with sparing of complex II activity in patient II:1 (blue bars) compared to controls (red bars). Mean enzyme activities shown for fibroblast controls (n = 10) are set at 100%. (h) Western blot analysis of cell lysates from control (C1 and C2) and patient II:1 (P) fibroblasts. Antibodies against RMND1, NDUFB8 (complex I), SDHA (complex II), UQCRC2 (complex III), COXI (complex IV), COXII (complex IV), ATP5A (complex V) and VDAC1 (mitochondrial loading control) were used