Biallelic Mutations in DNM1L are Associated with a Slowly Progressive Infantile Encephalopathy

Abstract

Mitochondria are highly dynamic organelles, undergoing continuous fission and fusion, and mitochondrial dynamics is important for several cellular functions. DNM1L is the most important mediator of mitochondrial fission, with a role also in peroxisome division. Few reports of patients with genetic defects in DNM1L have been published, most of them describing de novo dominant mutations. We identified compound heterozygous DNM1L variants in two brothers presenting with an infantile slowly progressive neurological impairment. One variant was a frame-shift mutation, the other was a missense change, the pathogenicity of which was validated in a yeast model. Fluorescence microscopy revealed abnormally elongated mitochondria and aberrant peroxisomes in mutant fibroblasts, indicating impaired fission of these organelles. In conclusion, we described a recessive disease caused by DNM1L mutations, with a clinical phenotype resembling mitochondrial disorders but without any biochemical features typical of these syndromes (lactic acidosis, respiratory chain complex deficiency) or indicating a peroxisomal disorder.

Keywords: DNM1L; mitochondrial disorders; mitochondrial dynamics; mitochondrial fission; peroxisome.

Figure 1

Molecular and protein studies in patients with DNM1L mutations. A: Pedigree of the family, with segregation of the identified DNM1L variants. Black symbols indicate affected subjects. B: Electropherograms of the regions containing the DNM1L mutations identified in genomic DNA from patient 1 (gDNA, upper panels) and in complementary DNA, retro‐transcribed by RNA obtained from patient's fibroblasts (cDNA, lower panels). C: Immunoblot analysis of total lysates from control (CT1, CT2, and CT3) and patient's (P1) fibroblasts using α‐DRP1, α‐HSP60, α‐VDAC1, and α‐GAPDH antibodies. The latter was used as loading control; VDAC1 and HSP60, mitochondrial proteins, are an index of mitochondrial content in each sample. D: mtDNA levels assessed by quantitative PCR in fibroblasts from P1 and three control subjects (CT). The mean value of the mtDNA/nDNA ratio obtained in the control subjects was set as 1. Two different mtDNA amplicons were used (blue and red bars). Error bars represent SDs.

Figure 2

Mitochondrial and peroxisomal morphology: analysis by fluorescence microscopy. A: Representative images of mitochondrial morphology, showing the filamentous mitochondrial network of fibroblasts from patient 1 (P1) and a control (CT), grown in DMEM‐glucose. In the inset, a digital magnification (3×) shows the peculiar “chain‐like” structure observed in P1 cells. Scale bar: 25 μm. B: Immunofluorescence staining with the anti‐PMP70 antibody of fibroblasts from P1 and CT. In the right panels, a digital magnification (3×) to show the different morphology of peroxisomes in P1 and CT cells. Scale bar: 25 μm.