Mutation screen reveals novel variants and expands the phenotypes associated with DYNC1H1

Abstract

Dynein, cytoplasmic 1, heavy chain 1 (DYNC1H1) encodes a necessary subunit of the cytoplasmic dynein complex, which traffics cargo along microtubules. Dominant DYNC1H1 mutations are implicated in neural diseases, including spinal muscular atrophy with lower extremity dominance (SMA-LED), intellectual disability with neuronal migration defects, malformations of cortical development, and Charcot-Marie-Tooth disease, type 2O. We hypothesized that additional variants could be found in these and novel motoneuron and related diseases. Therefore, we analyzed our database of 1024 whole exome sequencing samples of motoneuron and related diseases for novel single nucleotide variations. We filtered these results for significant variants, which were further screened using segregation analysis in available family members. Analysis revealed six novel, rare, and highly conserved variants. Three of these are likely pathogenic and encompass a broad phenotypic spectrum with distinct disease clusters. Our findings suggest that DYNC1H1 variants can cause not only lower, but also upper motor neuron disease. It thus adds DYNC1H1 to the growing list of spastic paraplegia related genes in microtubule-dependent motor protein pathways.

Keywords: DYNC1H1; Epilepsy; Gastric volvulus; Peripheral neuropathy; Spastic paraplegia; Spinal muscular atrophy.

Figure 1. Pedigree, clinical presentation, and MRI from SMA-LED family IHG20108

A. IHG20108 family pedigree. The pedigree structure has been altered to preserve the confidentiality of the patients and the family; some siblings have been omitted, and sibling order has been changed. Women are represented by circles, men by squares, and individuals with the gender not shown are represented by diamonds. The shapes of affected individuals are filled, while those of unaffected individuals are unfilled, and members with unknown diagnoses contain a question mark. Below each sequenced individual, the corresponding Sanger sequencing results are shown. The mutation location (c.10078) is highlighted in each chromatogram with an arrow. The heterozygous A>G change can be seen in the index patient III/1, the affected sibling III/4, and in parent II/2. B. Multiple-sequence alignment showing strong evolutionary conservation of the amino acid residue p.Ser3360. C, D. Clinical presentation of patient III/1 at age 6 months (C) and at age 12 (D). Note foot deformity, distal and proximal wasting of lower limb muscles and lumbar hyperlordosis. E. T1-weighted MRI in patient III/1 in the coronal (a) and axial plane (b) of the thigh and of the lower leg (c, d).

Figure 2. Pedigree and muscle biopsy from SMA-LED family IHG75979

A. IHG75979 family pedigree. Genders and disease presentation are represented as described in Figure 1. The mutation location (c.1792) is shown in the middle of each chromatogram with an arrow. The heterozygous C>T change can be seen in the index patient III/2. B. A quadriceps muscle biopsy from patient III/2 shows severe atrophy of some entire muscle fascicles (highlighted by dashed line oval) in the paraffin-embedded tissue (H&E, B1). Angulated atrophic fibers in small groups were numerous in the frozen tissue (H&E, B2 and immunoperoxidase-stained sections B3, B4). There is a marked predominance of type I fibers: all muscle fibers express slow myosin heavy chain (B3), while only rare fibers co-express fast myosin heavy chain (B4). These fibers are hybrid fibers (marked by lower case letter h). Scale bar is 50 μm for panels B1 and B2; 100 μm for B3 and B4.

Figure 3. Pedigree and brain MRI from cHSP family IHG26107

A. IHG26107 family pedigree. Genders and disease presentation are represented as described in Figure 1. A double line indicates consanguinity. The mutation location (c.3185) is shown in the middle of each chromatogram with an arrow. The heterozygous C>T change can be seen in the index patient II/1. B–E. Sagittal T1 MRI images of the index patient (top row, B, C) reveal bilateral perisylvian polymicrogyria-like cortical malformations (arrows, B), a thin corpus callosum (*, C) and an enlarged horizontal fissure, indicating beginning cerebellar atrophy (arrow head, B). For better illustration of these alterations in the index patient, MRI images of an age-matched control are presented in the bottom row (D, E).

Figure 4. Vertebrate mutation locations in DYNC1H1

Dominant mutations cluster in and around the stalk and homodimerization domains. Novel segregating and de novo mutations are annotated and marked with stars. Mutation locations in mouse models of peripheral neuropathy are also shown, including point mutations in Legs at odd angles (Loa) and Cramping (Cra), and a three-amino acid deletion in Sprawling (Swl) (31,32). The two zebrafish mutations include the photoreceptor-degenerating point mutation in cannonball (cnb) and the unnamed insertion mutant which shows PNS and CNS myelination defects (33,34). int. – dynein intermediate chain binding domain; li. int. – dynein light intermediate chain binding domain; MB – microtubule binding domain; AAA – ATPase associated with diverse cellular activities domain.