A number sign (#) is used with this entry because of evidence that spinocerebellar ataxia-27A (SCA27A) is caused by heterozygous mutation in the FGF14 gene (601515) on chromosome 13q33. Some patients have heterozygous deletions of chromosome 13q33 affecting the FGF14 and ITGBL1 genes (604234), which may thus be considered a contiguous gene deletion syndrome.

See also SCA27B (620174), an adult-onset form of spinocerebellar ataxia caused by an expanded GAA trinucleotide repeat in the FGF14 gene (601515.0006).

Spinocerebellar ataxia-27A (SCA27A) is an autosomal dominant neurologic disorder characterized by general cerebellar dysfunction manifest as gait disturbances, ataxia, tremor, dysarthria, and gaze-evoked nystagmus. The age at onset is highly variable: some patients present in infancy with nystagmus or delayed motor development, whereas others present as adults with tremor or gait difficulties. The disorder is slowly progressive, and ataxia may be very subtle or even absent. Cerebellar atrophy may or may not be observed on brain imaging. Individuals with SCA27A often show mild developmental delay with variably impaired intellectual development. Many patients report an exacerbation of symptoms with fever, emotional stress, or exercise, which can be reminiscent of episodic ataxia or be associated with outbursts, depression, or other behavioral and psychiatric disturbances. There is significant inter- and intrafamilial variability and patients show various combinations of neurologic features (summary by Tucker et al., 2013; Piarroux et al., 2020; Ceroni et al., 2023).

For a general discussion of autosomal dominant spinocerebellar ataxia, see SCA1 (164400).

Harris et al. (1993) reported an unusual eye movement abnormality in 10 members of an English Caucasian family. The proband was a 7-year-old boy whose mother had been diagnosed with neurofibromatosis I (NF1; 162200) and who himself had cutaneous signs of the disorder and Lisch nodules of the irides. In addition, however, he had gaze-paretic nystagmus, rebound nystagmus, and saccadic pursuit. Despite these vestibulocerebellar ocular motor signs, he had no ataxia and magnetic resonance imaging of the brain showed no abnormality. Similar eye movement abnormalities but no stigmata of NF1 were found in the father, a younger sister, and 7 other paternal relatives, whereas the mother had normal eye movements. Harris et al. (1993) concluded that the boy inherited NF1 from his mother and the eye movement abnormality, which was unrelated, from the father. The onset of the disorder was in early childhood. Seven patients who were examined showed no tremor, dizziness, consistent ataxia, or other cerebellar signs that are often associated with these oculomotor deficits, and, apart from strabismus, the patients were asymptomatic. After childhood, there appeared to be no progression; the oldest affected member was 40 years of age. Two members had been prone to falling in childhood, and 1 admitted to dizziness when tired. Harris et al. (1993) concluded that the disorder represented a benign and previously undescribed condition. The pedigree contained one instance of male-to-male transmission, suggesting autosomal dominant inheritance.

Ragge et al. (2003) provided follow-up on the family reported by Harris et al. (1993). There were 14 affected members over 3 generations. An eye movement disorder began within the first 2 years of life and was characterized by poor or absent smooth pursuit, gaze-evoked nystagmus, upbeat nystagmus, and poor vestibuloocular reflex. Nine patients had strabismus, but vision was generally well preserved. The condition was nonprogressive, and only 4 affected members showed mild balance problems. The authors suggested a cerebellar origin of the defect, likely in the flocculus. Ceroni et al. (2023) provided follow-up of this family, noting that there were 17 affected individuals with eye movement abnormalities, mainly nystagmus and poor smooth pursuit. Several patients also had ataxia or balance problems.

Van Swieten et al. (2003) reported a large 3-generation Dutch family in which 14 members had cerebellar ataxia inherited in an autosomal dominant pattern. Since childhood, all patients had trembling of both hands, which was exacerbated by emotional stress and physical exercise. Mild unsteadiness and ataxia of the upper limbs, especially under unusual circumstances, began at age 15 to 20 years. Six patients did not complete primary education, and only 4 of the 14 attended secondary school. Aggressive outbursts were mentioned in 5 patients and depression in 3. Neurologic examination showed dysmetric saccades, disrupted ocular pursuit movements, gaze-evoked nystagmus, cerebellar dysarthria, and a high-frequency, small-amplitude tremor in both hands in most of the patients. Six patients showed head tremor, and subtle orofacial dyskinesias were seen in 8. Severe limb and gait ataxia were present in the 3 eldest patients. Two patients showed cerebellar atrophy on MRI. The inability to complete primary education, low cognitive performances, and aggressive outbursts in several of the patients may reflect changes in the development and survival of neuronal populations in the cerebral cortex, amygdala, and basal ganglia.

Dalski et al. (2005) reported an ataxia patient with mild intellectual disability (IQ of 70), inborn strabismus, and red-green colorblindness. His motor development was normal until age 12 years, when he developed a slowly progressive gait disturbance, memory loss, and depressed mood. Clinical examination at age 13 years showed truncal and gait ataxia, small-amplitude tremor in both hands, gaze-evoked nystagmus, and pes cavus. Nerve conduction studies revealed mild axonal sensory neuropathy. The patient's father, who was deceased, reportedly had gait disturbance, memory loss, and pes cavus.

Tucker et al. (2013) reported a 5-year-old boy with a history of developmental delay from infancy who presented in early childhood with delayed motor skills, awkward gait, tremor, speech delay with stuttering, and occasional drooling. Composite IQ score was 77 and he attended a special school. He also had mild dysmorphic features, including acrocephaly, cupped ears with creases, epicanthal folds, smooth philtrum with thin upper lip, and broad forehead. Family history was unremarkable. Genetic analysis identified a de novo heterozygous 97-kb microdeletion of chromosome 13q33.1 including a significant portion of the FGF14 gene; no other genes appeared to be involved. Functional studies of the variant and studies of patient cells were not performed.

Coebergh et al. (2014) reported a 6-year-old boy who presented at 2 years of age with delayed walking and later showed poor axial balance, dysdiadochokinesis, and dysmetria. He had dysmetric saccadic eye movements, intrusive square wave jerks, and horizontal and vertical gaze-evoked nystagmus. He showed neurologic deterioration during fever-related episodes, and there was an episodic ataxia syndrome with interictal nystagmus and postural imbalance. IQ was normal. The patient's mother had the same abnormal eye movements and difficulties walking, and the maternal grandmother had adult-onset head tremor and mild cerebellar ataxia leading to a diagnosis of essential tremor. Brain imaging was normal in all 3 individuals. Microarray analysis identified a heterozygous 201.8-kb deletion on chromosome 13q33.1 that included the last 4 exons of ITGBL1 (604234) and the first 4 exons of FGF14 that segregated with the disorder in the family. Functional studies of the variant and studies of patient cells were not performed. Additional family members were reportedly affected, but they were not available for study.

Choquet et al. (2015) reported a French Canadian family in which 3 brothers and their mother presented with a neurologic disorder with features of SCA and episodic ataxia. The proband was a 31-year-old man who had a 5-year history of episodes characterized by incoordination, unsteady gait, vertical oscillopsy, dysarthria, headaches, and dystonic posturing. These attacks were followed by a progressive and permanent cerebellar ataxia with dysarthria, tremor, and episodes of aggravation. Other features included horizontal nystagmus, mild limb weakness, postural tremor, dysmetria, ataxia, and dysdiadochokinesis. Brain imaging was normal. He had 2 brothers with milder symptoms; 1 was noted to have permanent nystagmus. Their mother had migraine headaches and rare episodes of vertigo and incoordination; she also had sustained horizontal nystagmus.

Miura et al. (2019) reported a 62-year-old Japanese man with onset of gait disturbance at age 47 years and dysarthria age 54. On examination, he showed pathologic saccadic dysmetria, saccadic intrusions into smooth pursuit eye movements, dysarthria, bilateral hand tremor with low amplitude, and limb and truncal ataxia. He had a wide-based gait. Muscle tone and strength in his limbs were normal. Deep tendon reflexes were normal or slightly reduced, and pathologic reflexes were absent. Vibration sense in the lower limbs was mildly impaired. Brain MRI showed cerebellar atrophy without brainstem involvement. The patient reported that his father had spinocerebellar ataxia, but the father was not available for study.

Piarroux et al. (2020) reported a large multigenerational family (family A) in which 7 individuals, ranging from 3 to 45 years of age, had genetically confirmed SCA27A. There was significant intrafamilial variability. Three patients presented between 2 and 8 years of age with acute episodic ataxia associated with a febrile illness or stress. Symptoms included ataxia, vertigo, and dizziness; one patient had breath-holding spells. The other 4 affected family members presented in early childhood with tremor without frank episodic ataxia, although 1 had brief episodes of limb hypertonicity. All patients developed permanent symptoms, including gross and fine motor difficulties, postural tremor of the upper limb, and coordination problems. Four patients had nystagmus and 3 had learning difficulties. An unrelated girl (family B) presented in infancy with vertical nystagmus and upper limb tremor, followed by episodic ataxia triggered by fever. She had coordination difficulties and delayed development with mild learning disabilities and language delay.

Paucar et al. (2020) reported a multigenerational Swedish family in which 9 individuals had SCA27A. Five patients described in detail had tremor and ataxia with a variable age at onset (range infancy to 30 years). Cognitive profiles ranged from low to intellectual disability; all required special assistance at school. Additional features included dysmetria, gaze-evoked nystagmus, and impaired optokinetic nystagmus. Psychiatric disturbances such as angry outbursts and ADHD were common. The most severely affected individual was a woman with hallucinations, self-harming behavior, psychosis, and personality disorder. Three patients were obese. Four patients examined had variable cerebellar atrophy, and a few had cortical atrophy on brain imaging. The index case, who was younger (age 18 years), had normal brain imaging. Three patients examined demonstrated hypometabolism in the prefrontal cortex, temporal cortex, and cerebellum on PET scan.

Ceroni et al. (2023) reported a father and son (family 1) with SCA27A. The son presented with nystagmus at age 4 years. He also had mild developmental delay with walking after age 2 years. Additional features included intention tremor, dysmetria, dysdiadochokinesis, walking difficulties, and behavioral issues, including mood disorder and aggression. Brain imaging was normal. His father had similar features, including poor balance, tremor, nystagmus, and mood disorder.

The transmission pattern of SCA27A in the families reported by Harris et al. (1993), van Swieten et al. (2003), and Paucar et al. (2020) was consistent with autosomal dominant inheritance with variable expressivity.

Misceo et al. (2009) reported a mother and daughter with SCA27A resulting from a translocation, t(5;13)(q31.2;q33.1), that disrupted the FGF14 gene. Both patients showed signs of SCA, although the 5.5-year-old daughter was more severely affected with early onset cerebellar ataxia, microcephaly, and severe mental retardation. In the first year of life, the daughter showed truncal unsteadiness, which progressed to gait ataxia, axial tremor with titubation of head and neck, action and intention tremor, and dysarthria. She occasionally displayed dyskinetic jerky movements in the neck and arms. Other features included a short neck, fifth finger clinodactyly, and high-arched feet. Both mother and daughter also showed upper neuron involvement, with hyperreflexia and possible extensor plantar responses. The translocation breakpoint on der(13) was located between exons 1 and 2, encoding isoform 1b of FGF14. Isoform 1a of FGF14 was unaffected. The findings indicated that haploinsufficiency of FGF14 can cause SCA27A.

In affected members of a multigenerational Swedish family with SCA27A, Paucar et al. (2020) identified a heterozygous 599-kb deletion on chromosome 13q33 that segregated with the disorder in the family. The deletion affected the entire coding regions of the FGF14 and ITGBL1 genes. Neuropathologic studies of parvalbumin (PV)-positive interneurons in the prefrontal cortex of Fgf14-null mice showed reduced levels of VGAT (616440), a GABAergic transporter. Loss of GABAergic signaling in the PV interneurons has been associated with neuropsychiatric disorders, including schizophrenia. The authors concluded that these findings supported the clinical data of psychiatric traits observed in many patients with SCA27A.

In 12 affected individuals from a large multigenerational family (family 2) with SCA27A originally reported by Harris et al. (1993), Ceroni et al. (2023) identified a heterozygous 161-kb deletion of chromosome 13q13.1 that encompassed 2 exons of the FGF14 gene and 4 or 5 exons of the ITGBL1 gene (depending on isoform). The deletion segregated with the disorder in the family. A father and son from another family (family 1) with the disorder carried a heterozygous partial duplication of FGF14 that was predicted to lead to a frameshift in isoform 1B, resulting in haploinsufficiency. Functional studies of the variants and studies of patient cells were not performed.

By linkage analysis of the family reported by Harris et al. (1993), Ragge et al. (2003) identified a locus within a 13.8-cM region on chromosome 13q31-q33 (lod score of 6.3 at marker D13S159).

In affected members of a large Dutch family with early-onset tremor, dyskinesia, and slowly progressive cerebellar ataxia, van Swieten et al. (2003) identified a heterozygous missense mutation in the FGF14 gene (F145S; 601515.0001). Other SCA loci were excluded, including that for SCA8 (608768), which maps to chromosome 13q21. The disorder was consistent with the occurrence of ataxia and paroxysmal dyskinesia in Fgf14 knockout mice (Wang et al., 2002) (see ANIMAL MODEL).

By molecular analysis of the FGF14 gene in an 18-year-old man with early-onset ataxia and mild mental retardation, Dalski et al. (2005) identified a heterozygous frameshift mutation in the FGF14 gene (c.487delA; 601515.0002).

By whole-exome sequencing of genes known to cause SCA in a 62-year-old affected Japanese man, Miura et al. (2019) identified a heterozygous nonsense mutation in the FGF14 gene (K177X; 601515.0003). The patient's father was reportedly affected, but neither detailed clinical information nor DNA was available for him. The variant was not found in public databases or in 502 Japanese controls.

In a mother and her 3 sons, of French Canadian origin, with variable manifestations of SCA27A, Choquet et al. (2015) identified a heterozygous frameshift mutation in the FGF14 gene (601515.0004). The mutation, which was found by whole-exome sequencing and confirmed by Sanger sequencing, segregated with the disorder in the family. Functional studies of the variant and studies of patient cells were not reported, but the authors postulated haploinsufficiency and suggested that SCA27A may be a type of channelopathy characterized by impaired function of voltage-gated ion channels due to mutation in the FGF14 gene.

In 7 affected members of a large multigenerational family (family A) with variable manifestations of SCA27A, Piarroux et al. (2020) identified a heterozygous nonsense mutation in the FGF14 gene (E147X; 601515.0005). An unrelated girl with SCA27A was found to carry a different mutation in the FGF14 gene (Y162X). The mutations, which were found by direct Sanger sequencing of a limited gene panel, segregated with the disorder in family A. Functional studies of the variants and studies of patient cells were not performed.

To study the role of Fgf14 in CNS development and adult brain function, Wang et al. (2002) generated Fgf14-deficient mice that expressed an FGF14/beta-galactosidase fusion protein in place of the normal Fgf14 protein. The Fgf14-deficient mice were viable, fertile, and anatomically normal, but developed ataxia and a paroxysmal hyperkinetic movement disorder. The authors noted that the motor abnormalities are associated with dysfunction of the basal ganglia system and resemble several human dystonia syndromes. Using neuropharmacologic studies, Wang et al. (2002) showed that the Fgf14-deficient mice had reduced responses to dopamine agonists. They suggested a function for Fgf14 in neuronal signaling, axonal trafficking, and synaptosomal function.