Clinical and pathologic phenotype of a large family with heterozygous STUB1 mutation

Discussion

We present a multigenerational family with late-onset ataxia associated with a novel heterozygous STUB1 mutation, fitting the diagnosis of SCA48. Although the various initial diagnoses of the patients illustrate the clinical heterogeneity of this new SCA subtype, there are some strongly overlapping features. The clinical presentation is dominated by cognitive decline and profound behavioral changes, combined with cerebellar ataxia and movement disorders.

Cognitive impairment has been found in other SCA subtypes, but is usually preceded by cerebellar symptoms.²⁰ Here, it was present in all affected family members and occasionally as the first manifestation. One case was initially diagnosed as FTD, although frontal and temporal atrophy were lacking. Other published SCA48 pedigrees with mutations in close proximity to the current mutation showed a broad range of cognitive symptoms.^9,–,12 Another interesting observation is the presence of other movement disorders besides gait ataxia. Chorea or uncontrolled motor activity was frequently reported, showing resemblance with the chorea described in several other families.^10,–,12 Parkinsonism was also present in 4 of 11 described Italian patients with SCA48.¹¹ Atypical parkinsonism with features resembling PSP was observed in 1 patient. The combination of cognitive and movement disorders in SCA48 also resembles SCA17 and dentatorubral-pallidoluysian atrophy. In both disorders, dementia and chorea are part of the clinical features.^1,21

The clinical presentation of SCA48 shows overlap with SCAR16. The latter is more complex with a myriad of phenotypes and seems to constitute a multisystemic disorder usually presenting at an earlier age.^7,22 Endocrine abnormalities have been described as a major feature of SCAR16, and hypogonadism was present in patients with SCA48.¹¹ Although not formally tested, endocrine abnormalities were not reported in the medical records of the current pedigree.

The homozygous and compound heterozygous STUB1 mutations in SCAR16, and the recent identification of heterozygous STUB1 mutations in adult-onset familial ataxia, support STUB1 as the most likely candidate gene in this family. The identified frameshift variant is predicted to result in a premature stop codon and is located in the highly conserved U-box domain, similar to previously reported variants causing SCAR16 and SCA48.^8,9,11,23 The protein encoded by STUB1, CHIP, regulates several proteins involved in neurodegenerative disorders.⁶ Through its tetratricopeptide repeat domain, CHIP interacts with chaperones (Hsp70 and Hsp90) and a broad range of co-chaperones. The ubiquitin ligase region (U-box) is responsible for the ubiquitination of misfolded proteins destined for elimination.²⁴ Missense mutations in the U-box domain of CHIP impair the intracellular degradation of ataxin-3 microaggregates by preventing their ubiquitination, thereby accelerating disease progression.²⁵ We hypothesize that a similar pathophysiologic mechanism is likely in SCA48 due to impaired function of CHIP.

Earlier work has shown that mutations in the U-box domain causing SCAR16 destabilize CHIP, supporting a loss-of-function mechanism.²⁶ CHIP null mice and animal models with homozygous missense mutation (p.T246M) exhibited striking similarities with characteristics seen in SCAR16 patients, including neuronal degeneration and ataxic motor disturbances.^7,27 Patients with recessive STUB1 mutations had decreased CHIP protein levels in isolated fibroblasts in varying degrees, dependent on specific mutations.²⁷ The U-box domain is especially important, as mutations in this domain have a stronger effect on the overall loss of CHIP function and strongly associate with cognitive dysfunction.²⁸ Indeed, most frameshift mutations identified so far in SCA48 are located in the U-box domain (table e-1, links.lww.com/NXG/A251).^9,11 Whether this results in nonsense mediated RNA decay or a truncated protein (with a possible dominant negative effect) remains to be elucidated. Additional functional studies on different mutations in STUB1 are essential to provide more insight why some mutations cause SCA48 in heterozygous state and others SCAR16 in a recessive trait. The co-occurrence of recessive and dominant mutations in the same gene has been described before. For example, mutations in HTRA1 are responsible for cerebral autosomal recessive arteriopathy with subcortical infarcts and leukoencephalopathy (CARASIL), while heterozygous mutations causing a late-onset form of small vessel disease have been reported.²⁹ Likewise, truncating heterozygous progranulin mutations cause FTD, whereas homozygous mutations lead to neuronal ceroid lipofuscinosis.³⁰

In the current study, 2 carriers of the STUB1 mutation did not develop symptoms: 1 unaffected carrier died over age 75 years, and another (obligate) carrier died at age 64 years without neurologic disease. This could indicate incomplete penetrance, or it could reflect the diversity in phenotypical expression, given the large variety in ages at onset in the present family (50–72 years). Therefore, it cannot be excluded that parents of some patients with SCAR16 are at risk of developing SCA48.

It would also be of interest to investigate other genetic factors that could influence disease penetrance and progression. Previous studies have shown that expanded ataxin-2 repeats predispose to other neurodegenerative disorders besides SCA2.³¹ In line with this, it would be interesting to explore whether repeat length variation in ataxin-3, a known interactor of CHIP, might interfere with the function of CHIP and influence the clinical variability in SCA48. Although we have not found toxic exposures in patients (data not shown), it is worthwhile to study more systematically whether environmental factors can trigger the unfolded protein response and may therefore have an effect on disease onset or progression in STUB1-related disorders.

Neuropathologic changes were present in cerebellar but also neocortical areas, suggesting diffuse involvement of the brain. The almost complete loss of Purkinje cells in the present 3 cases resembles that of most other SCAs.² Neuropathologic studies of other STUB1 cases are scarce, with a single neuropathologic report of an SCAR16 patient.³² This case showed similar severe Purkinje cell loss and p62-positive NIIs, and CHIP staining revealed nuclear staining in neurons of the deep layers of the frontal cortex. We also observed diffuse staining of neurons with CHIP antibody, but not different from controls (data not shown).

NIIs have been considered the hallmark of disorders with repeat expansions.³³ However, several studies have demonstrated the presence of NIIs in the absence of pathologically expanded polyglutamine repeats.³⁴ Moreover, NIIs in the present cases did not show 1C2 positivity, which is typically present in SCA caused by polyglutamine repeat expansion.² Instead, we did observe a pattern of DNS with 1C2. Granular cytoplasmic and DNS of 1C2 as an earlier stage in aggregate formation have been described in both SCA3 and Huntington disease.³⁵ Other studies have demonstrated that 1C2 recognizes nonexpanded ataxin-3 present in NIIs.^36,37 It is possible that recruitment of ataxin-3 into the nucleus is involved in the pathogenesis of STUB1 disease.³⁸ Finally, 1C2 immunoreactivity might be less specific, as it has also been detected in nonrepeat disorder, such as a SCA6 case with 22–24 polyglutamine stretches³⁹ and in healthy controls.⁴⁰ Future investigations are needed to clarify the exact meaning of the different types of staining patterns on the cellular (dys)function.

Intriguingly, extensive tau pathology was found in 1 patient with clinical similarities to PSP (III-10). This has up till now only been seen in SCA11, a pure progressive cerebellar ataxia.² Several studies highlight the role of CHIP in tau pathology. It attenuates the pathologic changes associated with tau, playing a primary role in tau ubiquitination and degradation.⁴¹ CHIP levels were found to be elevated in AD, suggesting a role in tangle maturation,⁴² and CHIP depletion in mice was associated with accumulation of hyperphosphorylated tau.⁴³ The discovery of different pathologies within 1 kindred is known to occur also in other (genetic) neurodegenerative diseases.⁴⁴

The present pedigree along with the recently described families underscores the association between heterozygous STUB1 mutations and SCA48. The combination of familial late-onset gait ataxia, cognitive decline with behavioral changes, and other movement disorders (parkinsonism, chorea, and stereotypic movements) is indicative of SCA48. We show that the pathology of SCA48 is dominated by Purkinje cell loss, p62-positive intranuclear inclusions, and may also include tau pathology. Additional functional studies and reports of newly identified SCA48 and SCAR16 families will provide more insight into the heterogeneous spectrum of STUB1-related disorders.