Clinical characteristics.

Spinocerebellar ataxia type 38 (SCA38) is characterized as a pure cerebellar ataxia with symptoms typically manifesting in the fourth decade of life. The most common presenting features are nystagmus and slowly progressive gait ataxia. As the disease progresses, cerebellar symptoms (limb ataxia, dysarthria, dysphagia, diplopia on the horizontal line) may emerge, and affected individuals may experience sensory loss. In the later stages of the condition, ophthalmoparesis followed by ophthalmoplegia may occur. Features that distinguish SCA38 from other spinocerebellar ataxias include pes cavus without paresis, hyposmia, hearing loss, and anxiety disorder. Dementia and extrapyramidal signs are not common features of SCA38. Brain imaging typically demonstrates cerebellar atrophy mainly affecting the vermis without atrophy of the cerebral cortex and a normal appearance of the brain stem. With disease progression, nerve conduction velocities and electromyography demonstrate a sensory and motor axonal polyneuropathy in all four extremities. Life span is apparently not decreased.

Diagnosis/testing.

The diagnosis of SCA38 is established in a proband with progressive gait ataxia and a heterozygous pathogenic variant in ELOVL5 identified by molecular genetic testing.

Management.

Treatment of manifestations: Management remains supportive and may include physical therapy to ameliorate coordination difficulties; crutches and walkers; home adaptations to avoid falls or accommodate motorized chairs; speech-language therapy; weighted utensils; hearing aids for those with hearing loss; and standard therapy for anxiety.

Prevention of secondary complications: Weight control, as obesity can exacerbate difficulties with ambulation and mobility; vitamin supplements for those with reduced caloric intake.

Surveillance: Annual examination by a physician who is experienced in movement disorders and ataxia.

Agents/circumstances to avoid: Alcohol and medications known to affect cerebellar function.

Therapies under investigation: Docosahexaenoic acid (DHA) supplementation (600 mg/day) may improve clinical symptoms and cerebellar hypometabolism; no data on the effectiveness of DHA in postponing signs and symptoms of SCA38 in asymptomatic individuals with a heterozygous pathogenic variant in ELOVL5 have been published.

Genetic counseling.

Spinocerebellar ataxia type 38 (SCA38) is inherited in an autosomal dominant manner. All reported individuals diagnosed with SCA38 have an affected parent. The offspring of an affected individual are at 50% risk of inheriting the ELOVL5 pathogenic variant. Prenatal testing for pregnancies at increased risk is possible if the pathogenic variant in the family is known; however, requests for prenatal diagnosis of adult-onset diseases are uncommon and require careful genetic counseling.

Suggestive Findings

Spinocerebellar ataxia type 38 (SCA38) should be suspected in individuals with the following clinical and brain MRI findings.

Clinical findings

• Slowly progressive gait ataxia with onset in adulthood (3rd-5th decade)
• Nystagmus in the lateral and vertical gaze
• Pes cavus
• Hyposmia

Brain imaging findings

• Cerebellar atrophy (sometimes referred to as cerebellar hypometabolism when visualized on PET scan) mainly affecting the vermis without atrophy of the cerebral cortex
• Normal appearance of the brain stem

Establishing the Diagnosis

The diagnosis of SCA38 is established in a proband with progressive gait ataxia and a heterozygous pathogenic (or likely pathogenic) variant in ELOVL5 identified by molecular genetic testing (see Table 1).

Note: (1) Per ACMG/AMP variant interpretation guidelines, the terms "pathogenic variant" and "likely pathogenic variant" are synonymous in a clinical setting, meaning that both are considered diagnostic and can be used for clinical decision making [Richards et al 2015]. Reference to "pathogenic variants" in this GeneReview is understood to include likely pathogenic variants. (2) Identification of a heterozygous ELOVL5 variant of uncertain significance does not establish or rule out the diagnosis.

Because the phenotype of SCA38 is indistinguishable from many other inherited disorders with SCA, recommended molecular genetic testing approaches include use of a multigene panel or comprehensive genomic testing.

Note: Single-gene testing (sequence analysis of ELOVL5) is rarely useful and typically NOT recommended unless there is a known familial pathogenic variant.

• A spinocerebellar ataxia multigene panel including ELOVL5 and other genes of interest (see Differential Diagnosis) is most likely to identify the genetic cause of the condition. This approach limits the identification of variants of uncertain significance and pathogenic variants in genes not related to 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. (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. Exome array (when clinically available) may be considered if exome sequencing is not diagnostic, particularly when evidence supports autosomal dominant inheritance.
  For an introduction to comprehensive genomic testing click here. More detailed information for clinicians ordering genomic testing can be found here.

Clinical Description

Spinocerebellar ataxia type 38 (SCA38) is characterized as a pure cerebellar ataxia with symptoms typically becoming apparent in the fourth decade of life (age range 26-50 years) [Borroni et al 2016].

• The most common presenting features are nystagmus (21/21; 100%) and slowly progressive gait ataxia (20/21; 95%).
• With disease progression, cerebellar symptoms such as limb ataxia, dysarthria, dysphagia, and diplopia on the horizontal line may develop.
• In the later stages, ophthalmoparesis followed by ophthalmoplegia may become apparent.
  However, visual evoked potentials performed in eight affected individuals were unremarkable [Borroni et al 2016].
• Peripheral nervous system involvement is present in the last phase of disease with sensory loss.
• Dementia or extrapyramidal signs are not detected.

Significant loss of abilities of daily living is reported only after 20 years of disease. In a study by Borroni et al [2016] of 21 affected individuals from three families, the following was reported:

• All affected individuals were able to participate in activities of daily living during the first decade after symptom onset.
• During the second decade of symptoms, of 13 affected individuals:
  • Nine (70%) had difficulties with ambulation that required the use of a cane.
  • Four (30%) required assistance with washing and dressing.
• During the third decade of symptoms, of eight affected individuals:
  • Seven (87%) required the use of a wheelchair and required assistance with activities of daily living.
  • Four (50%) required assistance with feeding.
  • Four (50%) developed dysphagia.
  • One (12%) developed incontinence.

Features that help distinguish SCA38 from other spinocerebellar ataxia conditions are pes cavus without paresis (14/17; 82%) and hyposmia (13/17; 76%).

Other signs and symptoms may include the following:

• Hearing loss (7/21; 33%), the basis of which has not been systematically studied
  Auditory evoked potentials were abnormal in 11/12 (92%) of individuals assessed.
• Anxiety disorder (7/21; 33%)

Brain imaging usually documents cerebellar atrophy with sparing of the cerebral cortex and no white matter disease.

Electrophysiology. With disease progression, nerve conduction studies and electromyography typically demonstrate a sensory and motor axonal polyneuropathy in all four extremities [Borroni et al 2016].

Prognosis. Data on life expectancy are limited. In three affected individuals, the mean time from symptom onset to death was 41 years (range 20-52 years), with the youngest person succumbing at age 70 years [Borroni et al 2016].

Genotype-Phenotype Correlations

No genotype-phenotype correlations are known.

Penetrance

Disease penetrance appears to be 100% in both males and females but is age dependent. In individuals with the c.689G>T (p.Gly230Val) pathogenic variant, SCA38 was fully penetrant by age 50 years [Borroni et al 2016].

Prevalence

This disease is likely extremely rare. A study of 346 individuals with spinocerebellar ataxia in China found no individuals with a pathogenic variant in ELOVL5 [Liu et al 2015]. No further cases beyond those described by Di Gregorio et al [2014] have been reported to date.

Anticipation

Anticipation has not been observed in families with SCA38.

Evaluations Following Initial Diagnosis

To establish the extent of disease and needs in an individual diagnosed with spinocerebellar ataxia type 38 (SCA38), the evaluations in Table 2 are recommended if they have not already been completed.

Treatment of Manifestations

Management of individuals remains supportive, as no known therapy to delay or halt the progression of the disease exists.

The American Academy of Neurology has developed guidelines for the treatment of motor dysfunction in individuals with ataxia [Zesiewicz et al 2018].

Prevention of Secondary Complications

Weight control is important because obesity can exacerbate difficulties with ambulation and mobility.

Vitamin supplements may be recommended, particularly if caloric intake is reduced.

Surveillance

Affected individuals should be examined at least annually by a physician experienced in movement disorders and ataxia.

Agents/Circumstances to Avoid

Alcohol and medications known to affect cerebellar function should be avoided.

Evaluation of Relatives at Risk

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

Therapies Under Investigation

A double-blind, randomized, placebo-controlled study followed by an open-label extension phase demonstrated the usefulness of docosahexaenoic acid (DHA) supplementation (600 mg/day) as a safe and effective treatment for SCA38, showing an improvement of clinical symptoms and cerebellar hypometabolism [Manes et al 2017]. The long-term safety and efficacy of 600 mg/day oral DHA in individuals with SCA38 was further supported by a two-year open-label extension study [Manes et al 2019].

No data on the effectiveness of DHA in postponing the signs and symptoms of SCA38 in asymptomatic individuals who have a heterozygous pathogenic variant in ELOVL5 are available.

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.

Mode of Inheritance

Spinocerebellar ataxia type 38 (SCA38) is inherited in an autosomal dominant manner.

Risk to Family Members

Parents of a proband

• All reported individuals diagnosed with SCA38 have an affected parent.
• SCA38 resulting from a de novo ELOVL5 pathogenic variant has not been reported to date.
• Symptom onset is age dependent (see Penetrance). 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 parental germline mosaicism have been reported).
• The family history of some individuals diagnosed with SCA38 may appear to be negative because of failure to recognize the disorder in family members, 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 molecular genetic testing has been performed on the parents of the proband.

Sibs of a proband. The risk to the sibs of the 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 ELOVL5 pathogenic variant identified in the proband, the risk to the sibs is 50%. In reported families, penetrance is 100% by age 50 years with minimal intrafamilial clinical variability.
• If the ELOVL5 pathogenic variant found in the proband 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].

Offspring of a proband. Each child of an individual with SCA38 has a 50% chance of inheriting the ELOVL5 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 ELOVL5 pathogenic variant, the parent's 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 ELOVL5 pathogenic variant has been identified in an affected family member.
• 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 SCA38, 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 ELOVL5 pathogenic variant has been identified in an affected family member, prenatal 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

ELOVL5 is a protein that is involved in the elongation of long-chain polyunsaturated fatty acids. It is a membrane protein located in the endoplasmic reticulum and expressed in human cerebellum.

Mechanism of disease causation. Altered function with possible gain of function of the enzyme [Manes et al 2017]; the molecular mechanism of SCA38 is still uncertain. A combination of loss-of-function and gain-of-function mechanisms may be responsible for the disease [Di Gregorio et al 2014, Hoxha et al 2017]. The p.Gly230Val pathogenic change has been shown to cause protein accumulation in the perinuclear area of transfected cells [Di Gregorio et al 2014].

ELOVL5-specific laboratory technical considerations. The chromosome 6:53152683 G>A change (rs150583340) (NP_001229757.1:p.Gln102Ter) is located on an alternative exon of the gene. This alternative transcript was not validated by experiments in several human tissues [Authors, personal communication]. This variant (47/21544 allele in ExAC database) is located in an intronic region of the main transcript and is likely to have no effect on the protein.

Gene structure. The main transcript is NM_021814.4, ELOVL fatty acid elongase 5, transcript variant 1 (7 exons).

Author Notes

A Brusco and E Di Gregorio are involved in the study of the genetic bases of spinocerebellar ataxia and the molecular pathogenesis of SCA28 and SCA38.

B Borroni is involved in the study of ataxias and dementia.

All the authors recently worked on the development of a therapy for SCA38 based on DHA administration.

Acknowledgments

Work supported by Telethon Foundation grant GGP14225.

Revision History

• 11 July 2019 (ma) Review posted live
• 31 December 2017 (ab) Original submission

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