Defining the clinical, molecular and imaging spectrum of adaptor protein complex 4-associated hereditary spastic paraplegia

Abstract

Bi-allelic loss-of-function variants in genes that encode subunits of the adaptor protein complex 4 (AP-4) lead to prototypical yet poorly understood forms of childhood-onset and complex hereditary spastic paraplegia: SPG47 (AP4B1), SPG50 (AP4M1), SPG51 (AP4E1) and SPG52 (AP4S1). Here, we report a detailed cross-sectional analysis of clinical, imaging and molecular data of 156 patients from 101 families. Enrolled patients were of diverse ethnic backgrounds and covered a wide age range (1.0-49.3 years). While the mean age at symptom onset was 0.8 ± 0.6 years [standard deviation (SD), range 0.2-5.0], the mean age at diagnosis was 10.2 ± 8.5 years (SD, range 0.1-46.3). We define a set of core features: early-onset developmental delay with delayed motor milestones and significant speech delay (50% non-verbal); intellectual disability in the moderate to severe range; mild hypotonia in infancy followed by spastic diplegia (mean age: 8.4 ± 5.1 years, SD) and later tetraplegia (mean age: 16.1 ± 9.8 years, SD); postnatal microcephaly (83%); foot deformities (69%); and epilepsy (66%) that is intractable in a subset. At last follow-up, 36% ambulated with assistance (mean age: 8.9 ± 6.4 years, SD) and 54% were wheelchair-dependent (mean age: 13.4 ± 9.8 years, SD). Episodes of stereotypic laughing, possibly consistent with a pseudobulbar affect, were found in 56% of patients. Key features on neuroimaging include a thin corpus callosum (90%), ventriculomegaly (65%) often with colpocephaly, and periventricular white-matter signal abnormalities (68%). Iron deposition and polymicrogyria were found in a subset of patients. AP4B1-associated SPG47 and AP4M1-associated SPG50 accounted for the majority of cases. About two-thirds of patients were born to consanguineous parents, and 82% carried homozygous variants. Over 70 unique variants were present, the majority of which are frameshift or nonsense mutations. To track disease progression across the age spectrum, we defined the relationship between disease severity as measured by several rating scales and disease duration. We found that the presence of epilepsy, which manifested before the age of 3 years in the majority of patients, was associated with worse motor outcomes. Exploring genotype-phenotype correlations, we found that disease severity and major phenotypes were equally distributed among the four subtypes, establishing that SPG47, SPG50, SPG51 and SPG52 share a common phenotype, an 'AP-4 deficiency syndrome'. By delineating the core clinical, imaging, and molecular features of AP-4-associated hereditary spastic paraplegia across the age spectrum our results will facilitate early diagnosis, enable counselling and anticipatory guidance of affected families and help define endpoints for future interventional trials.

Keywords: SPG47; SPG50; SPG51; SPG52; neurodegeneration.

Figure 1

Demographic data of the study cohort. (A) A total of 156 patients were enrolled in the study, the majority carrying bi-allelic variants in AP4B1 and AP4M1. (B) Most patients were under the age of 18 years at the last follow-up with only a few adult patients reported. (C and D) Enrolled patients cover all major ethnicities with the majority being Caucasian or of Arab descent. Most families are located in Europe, the Middle East, North America and North Africa.

Figure 2

Key clinical features in AP-4-HSP. (A) Distribution of height, weight and head circumference according to sex- and age-appropriate CDC growth charts. A subset of patients shows significant growth failure with a height and/or weight under the −2 SD mark (hash symbol indicates two patients with a weight above +3 SD). Postnatal microcephaly is found in the majority of patients, most commonly in a moderate range between −2 SD and −4 SD. (B) Foot deformities may include macrodactyly (top, macrodactyly of the right foot shown before and after surgical debulking), and pes equinovarus as well as pes planus. (C) Developmental delay is a universal feature and motor milestones are often prominently delayed. The bar graph shows the percentage of patients who achieved a given motor milestone. The average age at which each milestone was achieved is shown above the graph. White numbers on bars indicate the number of individuals for whom information was available. (D) Motor symptoms evolve from mild hypotonia in infancy to spastic diplegia and later tetraplegia. Progressive spasticity is a hallmark feature of AP-4-HSP and accounts for a large part of the morbidity. (E and F) Extrapyramidal movement disorders and cerebellar signs are found in a subset of patients, often with advanced disease. (G and H) Seizures in AP-4-HSP include frequent seizures in the setting of fever as well unprovoked seizures leading to a diagnosis of epilepsy in about two-thirds of patients. (I) About 40% of all patients present with at least one episode of status epilepticus, often with their first seizure. White numbers on bars indicate the number of individuals for whom information was available.

Figure 3

Key imaging features in AP-4-HSP. Brain MR images of AP-4-HSP patients [mean age: 7.4 ± 7.3 years (SD)]. Representative images of key imaging features are shown. Top left: Thinning of the corpus callosum, shown here on a sagittal T₁-weighted image, is found in 90% (94/104) of patients and is prominent in the posterior parts. Non-specific white matter abnormalities are noted in 68% (69/102). Top right: Ventriculomegaly, shown here on an axial T₂-weighted image, is common (65%, 67/103) and often presents as asymmetric colpocephaly. Middle left: Cortical atrophy, shown here on an axial T₂-weighted image, is more commonly found in patients with advanced disease but can be present in young patients as well. Cerebral atrophy is overall found in 37% of cases (38/102). Middle right: Cerebellar atrophy, shown here on a sagittal T₁-weighted image, is found in a subset of patients (23%, 20/89). Bottom left: Bilateral perisylvian polymicrogryria (arrows), shown on an axial T₂-weighted image. Bottom right: Bilateral symmetric hypointensity of the globus pallidus on axial susceptibility-weighted imaging is suggestive of iron accumulation in this previously reported patient with AP4M1-associated HSP (Roubertie et al., 2018).

Figure 4

The molecular spectrum of AP-4-HSP. Schematic representation showing the four genes encoding subunits of the AP-4 (AP4B1, AP4M1, AP4E1, AP4S1) and the distribution of the reported variants. Parentheses indicate the number of alleles with that variant. Reference sequences: AP4B1: NM_001253852.1; AP4M1: NM_004722.3; AP4E1: NM_007347.4; AP4S1: NM_007077.4. NC = non-coding.

Figure 5

Clinical rating scales and disease progression in AP-4-HSP. Standardized assessment of disease severity using the (A) SPRS, (B) SPRS spasticity subscore (items 7–10 of the SPRS), (C) Modified Ashworth Scale (MAS), (D) SPATAX EUROSPA disability score (0 = no functional handicap, 1 = no functional handicap but signs at examination, 2 = mild, able to run, walking unlimited, 3 = moderate, unable to run, limited walking without aid, 4 = severe, walking with one stick, 5 = walking with two sticks or four-wheel walker, 6 = unable to walk, requiring wheelchair, 7 = confined to bed), (E) Four Stage Functional Mobility score (1 = mild symptoms walking without an aid; 2 = walking without aid but unable to run; 3 = walking with aid; and 4 = wheelchair dependent). The number of individuals with data available is indicated above each graph. Assessing disease severity across the age spectrum, we found a moderate but significant linear correlation between the (F) SPRS total score (n = 37) and (G) SPRS spasticity subscore (n = 36) with age as determined by Pearson correlation index for linear regression. The red line indicates a linear regression curve while the blue line indicated a non-linear (exponential) curve. (H) Longitudinal assessment of SPATAX-EUROPA disability stages in patients who achieved ambulation (n = 39). Each patient is shown in a different colour with lines between different time points. A general trend towards higher scores, indicating greater motor disability, with increasing age becomes apparent. (I and J) A subanalysis by genotype showed no significant difference in disease severity by (I) SPRS total score and (J) SPRS spasticity subscore. (K) Significantly higher SPRS scores are present in individuals with epilepsy (unpaired t-test, P = 0.04).