Mutations in BICD2 cause dominant congenital spinal muscular atrophy and hereditary spastic paraplegia

Abstract

Dominant congenital spinal muscular atrophy (DCSMA) is a disorder of developing anterior horn cells and shows lower-limb predominance and clinical overlap with hereditary spastic paraplegia (HSP), a lower-limb-predominant disorder of corticospinal motor neurons. We have identified four mutations in bicaudal D homolog 2 (Drosophila) (BICD2) in six kindreds affected by DCSMA, DCSMA with upper motor neuron features, or HSP. BICD2 encodes BICD2, a key adaptor protein that interacts with the dynein-dynactin motor complex, which facilitates trafficking of cellular cargos that are critical to motor neuron development and maintenance. We demonstrate that mutations resulting in amino acid substitutions in two binding regions of BICD2 increase its binding affinity for the cytoplasmic dynein-dynactin complex, which might result in the perturbation of BICD2-dynein-dynactin-mediated trafficking, and impair neurite outgrowth. These findings provide insight into the mechanism underlying both the static and the slowly progressive clinical features and the motor neuron pathology that characterize BICD2-associated diseases, and underscore the importance of the dynein-dynactin transport pathway in the development and survival of both lower and upper motor neurons.

Figure 1

Pedigrees of Kindreds Affected by DCSMA, DCSMA+UMN, or HSP A pattern consistent with autosomal-dominant inheritance is shown in five of the six kindreds. The sixth kindred, UK2, consists of a simplex case, confirmed to be de novo by segregation analysis. Although there is no documented male-to-male transmission in AUS1, linkage data had previously excluded X-linked loci in this kindred. The three pedigrees on the top row of the figure (AUS1, AUSTRIA1, and USA1) share the same N-terminal p.Ser107Leu substitution; haplotype analysis suggests that AUS1 and AUSTRIA1 share a common ancestry. USA1 also has European ancestry. The other three kindreds have kindred-specific mutations. The three c.320C>T (p.Ser107Leu) kindreds are affected by a classical DCSMA phenotype, and limited UMN features are present only in USA1. Affected individuals in UK1 and UK2 have more marked UMN features. Germany 1 segregates with an HSP phenotype.

Figure 2

Clinical Features in Kindreds Affected by DCSMA Due to BICD2 Mutations (A) A severely affected 4-year-old member of AUS1 (IV.5) shows marked wasting of the buttocks and entire lower limb, pes planus, and calcaneovalgus foot deformities but preserved truncal and upper-limb muscle bulk. This child has a wide-based waddling gait and hyperlordotic posture and uses a wheelchair to travel distances of more than a few hundred meters. (B) AUS1 IV.5, now aged 7 years, shows stable wasting of the lower limbs and stable preservation of upper-limb muscle bulk. His sister (IV.6) shares similar features. Both siblings have required noninvasive nocturnal ventilation since 4 years of age for obstructive sleep-disordered breathing. (C) MRI of the 32-year-old mother of siblings IV.5 and IV.6 shows a specific pattern of muscle involvement and sparing with or without relative hypertrophy in the upper thigh and hallmark preservation of medial adductors and semitendinosus. (D) The MRI features of the mother in (C) are identical to those of a 24-year-old male AUSTRIA1 member, who shares the same c.320C>T (p.Ser107Leu) mutation. Members of UK1 and UK2 have more severe muscle involvement, but sparing of the same subset of muscles (e.g., medial adductors and semitendinosus in the upper thigh) is still evident. The lower-limb MRI features are almost identical to those seen in individuals with DYNC1H1-associated DCSMA.

Figure 3

Functional Characterization of BICD2 Mutations (A) A schematic diagram demonstrating the three BICD2 regions composed of five coiled-coil domains, the four identified alterations, and known interacting proteins. (B) Coimmunoprecipitation studies of HEK293 cells transfected with BICD2 C terminally tagged with FLAG-Myc show a higher interaction between the p.Arg501Pro and p.Ser107Leu altered proteins and DIC and the p150Glued subunit of dynactin (band shown with ^∗). The control represents a nontransfected cell lysate. For the FLAG-BICD2 pulldown, the pulled down DIC is highlighted by an arrow (the upper band is present in the nontransfected control). Reverse immunoprecipitation with DIC antibody confirmed the increased interaction between the p.Arg501Pro and p.Ser107Leu variants and DIC. (C and D) Optical densitometry for DIC and dynactin p150Glued pulldown for a single immunoprecipitation performed in triplicate. ANOVA p = 0.003 in (C), and ANOVA p = 0.004 in (D). Error bars represent the SEM. (E) Immunocytochemistry of SH-SY5Y cells transfected with constructs encoding p.Arg501Pro and p.Ser107Leu and stained for Myc and RAB6 show the presence of a BICD2 p.Arg501Pro-RAB6 ring structure.