Clinical characteristics.

Spinocerebellar ataxia type 11 (SCA11) is characterized by progressive cerebellar ataxia and abnormal eye signs (jerky pursuit, horizontal and vertical nystagmus). Pyramidal features are seen on occasion. Peripheral neuropathy and dystonia are rare. Six families have been reported to date, one each from the UK, Pakistan, France, Germany, Denmark, and China. Age of onset ranged from early childhood to the mid-40s. Life span is thought to be normal.

Diagnosis/testing.

The diagnosis of spinocerebellar ataxia type 11 (SCA11) is established in a proband with a heterozygous pathogenic variant in TTBK2 identified by molecular genetic testing.

Management.

Treatment of manifestations: Management is supportive; there are no known disease-modifying treatments to date. Physiotherapy and assessment for assistive devices for ambulation; occupational therapy, including home adaptations; speech and language therapy for dysarthria and dysphagia; ankle-foot orthotics if required and good foot care for those with neuropathy; treatment per ophthalmologist for vision issues; prism glasses may be helpful for diplopia.

Surveillance: Annual neurologic evaluation; evaluations with physiotherapist, occupational therapist, speech and language therapist, and ophthalmologist as indicated.

Genetic counseling.

SCA11 is inherited in an autosomal dominant manner. The proportion of SCA11 caused by de novo mutation is unknown. Each child of an individual with SCA11 has a 50% chance of inheriting the pathogenic variant. Prenatal testing for an at-risk pregnancy is possible if the diagnosis has been established by molecular genetic testing in an affected family member.

Suggestive Findings

Spinocerebellar ataxia type 11 (SCA11) should be considered in individuals with the following clinical features:

• Progressive cerebellar ataxia
• Abnormal eye signs (jerky pursuit, horizontal and vertical nystagmus)
• Dysarthria
• Pyramidal features (mild-to-moderate lower-extremity hyperreflexia; in very rare cases, a positive Babinski sign or other pyramidal features)
• Swallowing difficulties

Rare findings in SCA11:

• Peripheral neuropathy
• Dystonia

Establishing the Diagnosis

The diagnosis of spinocerebellar ataxia type 11 (SCA11) is established in a proband with a heterozygous pathogenic variant in TTBK2 identified by molecular genetic testing (see Table 1).

Because the phenotype of SCA11 is indistinguishable from many other inherited disorders with ataxia, recommended molecular genetic testing approaches include use of a multigene panel or comprehensive genomic testing.

Note: Single-gene testing (sequence analysis of TTBK2, followed by gene-targeted deletion/duplication analysis) is rarely useful and typically NOT recommended.

• An ataxia multigene panel that includes TTBK2 and other genes of interest (see Differential Diagnosis) is most likely to identify the genetic cause of the condition while limiting identification of variants of uncertain significance and pathogenic variants in genes that do not explain the underlying phenotype. Note: (1) The genes included in the panel and the diagnostic sensitivity of the testing used for each gene vary by laboratory and are likely to change over time. (2) Some multigene panels may include genes not associated with the condition discussed in this GeneReview. Of note, given the rarity of SCA11, some panels for ataxia may not include this gene. (3) In some laboratories, panel options may include a custom laboratory-designed panel and/or custom phenotype-focused exome analysis that includes genes specified by the clinician. (4) Methods used in a panel may include sequence analysis, deletion/duplication analysis, and/or other non-sequencing-based tests.
  For an introduction to multigene panels click here. More detailed information for clinicians ordering genetic tests can be found here.
• Comprehensive genomic testing (which does not require the clinician to determine which gene[s] are likely involved) is another good option. Exome sequencing is most commonly used; genome sequencing is also possible.
  For an introduction to comprehensive genomic testing click here. More detailed information for clinicians ordering genomic testing can be found here.

Clinical Description

To date, 28 individuals from six families have been identified with a pathogenic variant in TTBK2 [Houlden et al 2007, Bauer et al 2010, Lindquist et al 2017, Deng et al 2020]. The following description of the phenotypic features associated with this condition is based on these reports.

Onset. In the six families described with spinocerebellar ataxia type 11 (SCA11), age of onset ranged from age nine years in the family of Danish origin to age 40-50 years in the families from France, Germany, and China. Most individuals present with a pure ataxia phenotype, with few additional features. Abnormal eye findings were identified in a third of individuals, a small proportion of whom presented with diplopia at onset.

Ataxia. The cerebellar ataxia was clinically similar in all six families. All individuals presented with an ataxia-predominant disorder and difficulty walking due to unsteadiness and maintaining balance. In approximately a third of individuals, limb ataxia was also present. Ataxia was usually slowly progressive. For example, in the British family described, the mean disease duration was 26.8 years [Houlden et al 2007].

Abnormal eye findings include jerky pursuit and horizontal and vertical nystagmus. All of the individuals with SCA11 from Devon had abnormal eye movements at presentation, with jerky pursuit and vertical nystagmus more prevalent than horizontal nystagmus [Houlden et al 2007]. Half of the individuals with vertical nystagmus had an upbeat nystagmus [Giunti et al 2012]. Only a very small proportion were found to be symptomatic with ophthalmoplegia and diplopia. No members of the French family had abnormal eye findings. One individual in the German family had oculomotor disturbances with jerky pursuit, gaze-evoked nystagmus, dysmetric saccades, and impaired optokinetic nystagmus on presentation, nine years after symptom onset [Bauer et al 2010]. In the Danish family, one individual presented with diplopia and nystagmus at age nine years [Lindquist et al 2017]. A sib presented at age four years with ataxia and was found to have nystagmus at age nine years [Lindquist et al 2017]. Three individuals of Chinese descent had nystagmus at the time of presentation [Deng et al 2020]. It is unclear if abnormal eye findings progress, but ocular symptoms were the only presenting feature for one individual out of 28 affected. Although abnormal eye findings may be seen at the time of presentation, they are rarely symptomatic.

Pyramidal features exist in varying degrees across the different families. In the British family, mild limb hyperreflexia more prevalent in the upper than the lower limbs (with negative Babinski sign) was found in all but one affected individual [Houlden et al 2007]. In the family from Pakistan, mild-to-moderate hyperreflexia was observed in only two of five affected individuals [Houlden et al 2007]. Only one of the three individuals affected from the Danish family had hyperreflexia [Lindquist et al 2017]. Two of the five individuals of Chinese descent had hyperreflexia [Deng et al 2020]. Reflexes and tone were, however, normal in the German and French families described [Bauer et al 2010].

No other pyramidal signs apart from hyperreflexia were observed in the 28 individuals apart from one individual with upgoing plantar reflexes. This individual from Devon presented with both extrapyramidal and pyramidal signs including spastic gait, hyperreflexia with upgoing plantar reflexes, "no-no" head tremor, and upper-limb tremor with laterocollis [Giunti et al 2012]. No extrapyramidal signs have been described in other individuals.

Bulbar symptoms. Dysarthria and swallowing difficulties are common in individuals with SCA11. Dysarthria as a result of cerebellar dysfunction was moderate to severe in almost all individuals in the families of British and Pakistani origin but was not present at diagnosis [Houlden et al 2007]. In the French family, dysarthria was an early feature [Bauer et al 2010]. Dysarthria was reported to be progressive in individuals in the family of Chinese origin [Deng et al 2020]. Liquid dysphagia was also noted in individuals with SCA11, especially in the families from Devon and China, but was not common at presentation.

Peripheral neuropathy is not a common feature of SCA11. In the British cohort, nerve conduction studies (NCS) and electromyography (EMG) were normal in eight affected subjects [Houlden et al 2007]. One other affected subject had slightly small sensory nerve action potentials at the age of 61 (disease duration 43 years) but without clinically manifesting neuropathy. In the Pakistani, Danish, French, and German families, neuropathy was not seen. In the Chinese family, EMG of the proband showed extensive neurogenic damage. Somatosensory evoked potentials of the lower limbs were abnormal. Generalized neurogenic damage was seen on NCS and EMG of two other affected Chinese individuals, but it is not apparent whether these individuals also presented with neuropathy symptoms clinically.

Other. One individual from Devon presented with laterocollis [Giunti et al 2012]. Dystonia has not been described in other individuals.

Prognosis. SCA11 is slowly progressive with severity ranging from very mild balance problems at disease onset, to severe speech and swallowing problems and ataxia requiring the use of a wheelchair. In affected individuals from the British and Pakistani families, eight of 17 persons required a wheelchair 20 to 30 years after onset. The same was reported in the French and German families, where progression of disease is slow; individuals remained active many years and required a wheelchair decades after onset [Bauer et al 2010]. Life span in individuals with SCA11 is normal; many affected individuals live beyond age 75 years. In nine individuals from the British and Pakistani families, death occurred between ages 55 and 88 years.

Neuroimaging. Brain MRI examination shows mild-to-severe atrophy in both cerebellar hemispheres and the vermis. The brain stem and cerebrum were normal in most individuals [Giunti et al 2012]. Occasionally, atrophy has also been described in the medulla but this was not associated with disease severity [Giunti et al 2012, Deng et al 2020]. In a Danish proband, an 18Ffluorodeoxyglucose positron emission tomography scan showed reduced metabolic activity in the cerebellum and pons, and repeat brain MRI four years later showed worsening cerebellar atrophy with olivopontine atrophy [Lindquist et al 2017].

Neuropathology. Neuropathologic examination of the brain of one affected individual showed marked cerebellar and brain stem loss with Purkinje cell degeneration and abnormal tau deposition in the brain stem and cortex [Houlden et al 2007].

Genotype-Phenotype Correlations

No genotype-phenotype correlations have been identified.

Penetrance

The TTBK2 pathogenic variants in the six families described to date appear to be fully penetrant, although a number of at-risk relatives are younger than the typical age of onset. To date, no non-penetrant pathogenic variants have been identified in older individuals.

Prevalence

Prevalence is unknown but SCA11 is a rare cause of pure spinocerebellar ataxia. It accounts for less than 1% of autosomal dominant ataxia in Europe [Bauer et al 2010]. Pathogenic variants in TTBK2 were identified in six of 238 families with spinocerebellar ataxia [Houlden et al 2007; Bauer et al 2010; Author, personal observation]. A study in Germany of 49 individuals with a family history of ataxia did not identify pathogenic variants in TTBK2 [Edener et al 2009]. A study in China of 68 unrelated probands with autosomal dominant ataxia also did not identify pathogenic variants in TTBK2 [Xu et al 2010].

The six families described with SCA11 are from Devon (UK), Pakistan, France, Germany, Denmark, and China.

Evaluations Following Initial Diagnosis

To establish the extent of disease and needs in an individual diagnosed with spinocerebellar ataxia type 11 (SCA11), the evaluations summarized in Table 4 (if not performed as part of the evaluation that led to the diagnosis) are recommended.

Treatment of Manifestations

Management is supportive; no disease-modifying treatments are known to date.

Surveillance

Evaluation of Relatives at Risk

See Genetic Counseling for issues related to testing of at-risk relatives for genetic counseling purposes.

Therapies Under Investigation

Search ClinicalTrials.gov in the US and EU Clinical Trials Register in Europe for access to information on clinical studies for a wide range of diseases and conditions. Note: There may not be clinical trials for this disorder.

Mode of Inheritance

Spinocerebellar ataxia type 11 (SCA11) is inherited in an autosomal dominant manner.

Risk to Family Members

Parents of a proband

• Twenty-seven of 28 of individuals diagnosed with SCA11 have an affected parent.
• A proband with SCA11 may have the disorder as the result of a de novo TTBK2 pathogenic variant. The proportion of cases caused by de novo variants is unknown but likely small. To date, de novo SCA11 has been seen in one individual, from the family of Danish origin [Lindquist et al 2017].
• Molecular genetic testing is recommended for the parents of a proband with an apparent de novo pathogenic variant.
• If the pathogenic variant found in the proband cannot be detected in the leukocyte DNA of either parent, possible explanations include a de novo pathogenic variant in the proband or germline mosaicism in a parent (though theoretically possible, no instances of germline mosaicism have been reported).
• The family history of some individuals diagnosed with SCA11 may appear to be negative because of failure to recognize the disorder in family members with a milder phenotypic presentation, early death of the parent before the onset of symptoms, or late onset of the disease in the affected parent. Therefore, an apparently negative family history cannot be confirmed unless appropriate molecular genetic testing has been performed on the parents of the proband.

Sibs of a proband. The risk to the sibs of a proband depends on the genetic status of the proband's parents:

• If a parent of the proband is affected and/or is known to have the pathogenic variant identified in the proband, the risk to the sibs is 50%. Age of onset may vary within a family.
• If the proband has a known SCA11-related pathogenic variant that cannot be detected in the leukocyte DNA of either parent, the recurrence risk to sibs is estimated to be 1% because of the theoretic possibility of parental germline mosaicism [Rahbari et al 2016].
• If the parents have not been tested for the TTBK2 pathogenic variant but are clinically unaffected, sibs are still presumed to be at increased risk for SCA11 because of the possibility of age-related penetrance in a heterozygous parent or the theoretic possibility of parental germline mosaicism.

Offspring of a proband. Each child of an individual with SCA11 has a 50% chance of inheriting the TTBK2 pathogenic variant.

Other family members. The risk to other family members depends on the status of the proband's parents: if a parent has the pathogenic variant, his or her family members may be at risk.

Related Genetic Counseling Issues

Predictive testing (i.e., testing of asymptomatic at-risk individuals)

• Predictive testing for at-risk relatives is possible once the TTBK2 pathogenic variant has been identified in an affected family member. Such testing is not useful in accurately predicting age of onset, severity, type of symptoms, or rate of progression in asymptomatic individuals.
• Potential consequences of such testing (including but not limited to socioeconomic changes and the need for long-term follow up and evaluation arrangements for individuals with a positive test result) as well as the capabilities and limitations of predictive testing should be discussed in the context of formal genetic counseling prior to testing.

Predictive testing in minors (i.e., testing of asymptomatic at-risk individuals age <18 years)

• For asymptomatic minors at risk for adult-onset conditions for which early treatment would have no beneficial effect on disease morbidity and mortality, predictive genetic testing is considered inappropriate, primarily because it negates the autonomy of the child with no compelling benefit. Further, concern exists regarding the potential unhealthy adverse effects that such information may have on family dynamics, the risk of discrimination and stigmatization in the future, and the anxiety that such information may cause.
• For more information, see the National Society of Genetic Counselors position statement on genetic testing of minors for adult-onset conditions and the American Academy of Pediatrics and American College of Medical Genetics and Genomics policy statement: ethical and policy issues in genetic testing and screening of children.

In a family with an established diagnosis of SCA11, it is appropriate to consider testing of symptomatic individuals regardless of age.

Considerations in families with an apparent de novo pathogenic variant. When neither parent of a proband with an autosomal dominant condition has the pathogenic variant identified in the proband or clinical evidence of the disorder, the pathogenic variant is likely de novo. However, non-medical explanations including alternate paternity or maternity (e.g., with assisted reproduction) and undisclosed adoption could also be explored.

Family planning

• The optimal time for determination of genetic risk and discussion of the availability of prenatal/preimplantation genetic testing is before pregnancy.
• It is appropriate to offer genetic counseling (including discussion of potential risks to offspring and reproductive options) to young adults who are affected or at risk.

Prenatal Testing and Preimplantation Genetic Testing

Once the TTBK2 pathogenic variant has been identified in an affected family member, prenatal testing for a pregnancy at increased risk and preimplantation genetic testing are possible.

Differences in perspective may exist among medical professionals and within families regarding the use of prenatal testing, particularly if the testing is being considered for the purpose of pregnancy termination rather than early diagnosis. While most centers would consider use of prenatal testing to be a personal decision, discussion of these issues may be helpful.

Molecular Pathogenesis

TTBK2 encodes tau-tubulin kinase 2 (TTBK2), which modifies targets such as tau and tubulin, to initiate ciliogenesis [Goetz et al 2012]. TTBK2 phosphorylates the tau protein to exacerbate tau toxicity as well as TDP43, driving neurodegeneration [Taylor et al 2018]. Abnormal tau deposition has been described in SCA11, further implicating the direct link between TTBK2 and tauopathic disease [Houlden et al 2007].

Non-ciliary functions of TTBK2 within the brain include phosphorylation of synaptic vesicle protein 2A (SV2A), which is important for synaptic vesicle trafficking and regulation of neurotransmitter release [Zhang et al 2015]. TTBK2 has also been implicated in regulation of sodium-coupled transporters [Alesutan et al 2012].

Loss of TTBK2 function affects normal phosphorylation of tau, which leads to tau deposition and impaired ciliogenesis. There is evidence that TTBK2 may interact with the inositol/IP3 pathway and stabilize cells (in particular, Purkinje cells) against calcium-induced cell death [Houlden et al 2007] (see bioRxiv).

Mechanism of disease causation. Reported TTBK2 pathogenic variants are predicted to result in premature stop codons, and individuals with SCA11-associated TTBK2 variants were reported to have mRNA levels reduced by 50% when compared to unaffected individuals [Houlden et al 2007]. Further, treating lymphoblasts from affected individuals with cycloheximide, a known inhibitor of nonsense-mediated decay (NMD), resulted in increased total TTBK2 mRNA and a selective increase in the abundance of the mutated mRNA, suggesting that a proportion of the abnormal mRNA escapes NMD [Houlden et al 2007]. This means that a truncated protein could be produced, causing a dominant-negative effect on the normal allele. A recent study showed that the abnormal truncated protein can disrupt function of the residual normal protein in cilium assembly, stability, and signaling, supporting a dominant-negative effect [Bowie et al 2018].

Further supporting the dominant-negative effect is the presence of other reported heterozygous frameshift variants clustered in exon 11, leading to truncation of the TTBK2 protein C-terminal to the kinase domain at amino acid 450 [Bauer et al 2010] and truncation at amino acid 402 [Lindquist et al 2017] – suggesting a dominant-negative effect exerted by the truncated protein. One missense variant has recently been described in the Chinese population but the mechanism for this is unclear.

No gene-dosage alterations have been detected in TTBK2.

Literature Cited

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Acknowledgments

We are grateful to the Medical Research Council (MRC) for their support: HH holds an MRC clinician scientist fellowship. This work was also supported by the NIHR UCL/UCLH biomedical research centre (BRC). ZC was supported by a fellowship from the Leonard Wolfson Foundation. We thank the families involved and the organization Ataxia UK for their continued support and assistance with our work.

Revision History

• 31 October 2019 (sw) Comprehensive update posted live
• 7 March 2013 (me) Comprehensive update posted live
• 22 July 2008 (me) Review posted live
• 9 June 2008 (hh) Original submission