Spinocerebellar Ataxias

Slide 1 · Spinocerebellar Ataxias

Inherited neurodegeneration • mutation classes • molecular pathology

Pathology lecture

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning outcomes

Define the autosomal dominant spinocerebellar ataxias within the broader spinocerebellar degenerations.

Distinguish coding-repeat, noncoding-repeat, and conventional-mutation disease classes.

Explain established and proposed molecular consequences of polyglutamine expansion.

Relate selected SCA names and molecular categories to meaningful differential diagnoses.

Slide 3 · Spinocerebellar degenerations: Affected systems

Cerebellum

Neuronal loss and dysfunction impair the machinery of coordinated movement.

Spinal cord

Degeneration can extend beyond cerebellar structures.

Peripheral nerves

Peripheral nerve involvement contributes to the multi-system category.

The shared lesion distribution at the family level—not an identical distribution in every subtype.

Slide 4 · One category, many diseases

The spinocerebellar degenerations differ in genetics, age at onset, and presenting manifestations.

The shared neuroanatomic theme does not imply a single causative gene or pathway.

Genetic analysis has improved classification more than it has resolved disease pathogenesis.

A mechanistic classification is therefore more informative than the generic label “ataxia.”

Slide 5 · Where SCAs sit in the disease hierarchy

Spinocerebellar degenerations

Autosomal dominant SCAs

Multiple molecular SCA subtypes

Friedreich ataxia and ataxia-telangiectasia belong to the broader degeneration discussion but are autosomal recessive.

This inheritance distinction is a core boundary of the topic as presented.

Slide 6 · What the term SCA means here

Spinocerebellar ataxia (SCA) denotes a group of autosomal dominant inherited degenerations.

“Ataxia” denotes the clinical coordination problem; “spinocerebellar” signals a wider neuroanatomic context.

Genetic, clinical, and pathologic findings all contribute to subtype assignment.

A family diagnosis is not equivalent to identifying the responsible molecular lesion.

Slide 7 · Inheritance is a classification boundary

Group

Inheritance in Robbins

Teaching distinction

Spinocerebellar ataxias

Autosomal dominant

Several molecular subclasses

Friedreich ataxia

Autosomal recessive

Separate disease; FXN repeat expansion

Ataxia-telangiectasia

Autosomal recessive

Separate disease; ATM pathway

All three can involve spinocerebellar degeneration; They are not synonymous diagnoses.

Slide 8 · The classification problem

More than 40 distinct SCA subtypes in classifications integrating several evidence streams.

The disease family is too diverse for one universal histologic or genetic signature.

Subtype names remain useful only when paired with a molecular or clinicopathologic explanation.

Different mutation classes require different mechanistic reasoning.

Slide 9 · Three ways to classify the same phenotype

Genetic

What type of pathogenic variant is present?

Clinical

Which manifestations accompany the ataxia?

Pathologic

Which neuronal populations and cellular abnormalities are implicated?

These classifications overlap but are not interchangeable.

Slide 10 · Three mutation classes organize the SCAs

Coding CAG repeats

Repeat translation produces expanded polyglutamine proteins.

Noncoding expansions

The disease-linked repeat lies outside the translated coding tract.

Conventional variants

Sequence and copy-number changes can alter signaling proteins.

Parallel genetic classes—not a single stepwise pathway.

Slide 11 · Why a mutation-based approach helps

An inherited ataxia phenotype is a downstream consequence—not a molecular diagnosis.

Expanded coding repeats act through altered proteins; Noncoding expansions may act differently.

Conventional mutations span several sequence and copy-number variant classes.

Start with the variant mechanism before inferring pathogenesis.

Slide 12 · Trinucleotide repeats: A genetic substrate

Repeat motif

A short three-base DNA sequence is arranged in tandem copies.

Expanded allele

The number of consecutive copies is increased.

Disease context

Coding position versus noncoding position changes possible consequences.

The same “repeat expansion” label can conceal fundamentally different molecular mechanisms.

Slide 13 · The coding CAG repeat pathway

Coding CAG expansion

Longer glutamine tract

Altered protein properties

Neuronal dysfunction

CAG specifies glutamine; An expanded CAG tract produces polyglutamine.

This is a class-level schematic: The particular affected protein differs by subtype.

Slide 14 · A codon is not a protein inclusion

CAG is a DNA/RNA coding triplet, whereas polyglutamine is an amino-acid segment of a protein.

Translation links the expanded coding repeat to the longer glutamine-containing protein tract.

Subsequent protein misfolding and accumulation are separate downstream events.

Distinguish the genetic event from the microscopic finding.

Slide 15 · Genomic location predicts the first mechanistic question

Location

Direct consequence

What to investigate

Coding CAG repeat

Expanded polyglutamine protein

Protein folding and toxicity

Noncoding repeat

Repeat outside translated coding sequence

RNA or other regulatory consequences

Conventional mutation

Sequence or copy-number alteration

Function of the affected protein

This table is a mechanistic framework, not a definitive mechanism for every subtype.

Slide 16 · Polyglutamine: A protein-level lesion

The chapter identifies eight SCA-related forms with CAG-driven polyglutamine expansion.

The expanded tract modifies the product of the disease-associated gene.

Affected proteins differ, but accumulation in nuclear inclusions is described across this group.

Shared inclusion morphology does not prove identical neuronal vulnerability.

Slide 17 · Polyglutamine: Proposed injury sequence

Expanded protein

Misfolding / accumulation

Cellular stress

Neuronal dysfunction

Plausible toxic pathways; The precise causal contributions remain unresolved.

An arrow in this diagram indicates a proposed relationship, not a demonstrated universal sequence.

Slide 18 · Toxic gain of function

Expanded polyglutamine proteins can gain activities harmful to cells.

Proposed actions include interfering with the normal protein or acquiring a novel toxic activity.

This differs conceptually from simple absence of a gene product.

The chapter does not identify a single universal downstream toxic process.

Slide 19 · Two models of altered protein function

Interference

Mutant protein impairs the normal protein or its interactions.

Novel activity

The expanded protein acquires a pathogenic property.

Net consequence

Neuronal dysfunction arises despite protein production.

Both within the toxic gain-of-function framework.

Slide 20 · How abnormal proteins appear in nuclei

Expanded polyQ protein

Misfolded molecular species

Aggregation

Nuclear inclusions

Large intranuclear aggregates are a morphologic hallmark of many polyglutamine diseases.

The inclusion is an observed correlate, not automatic proof of direct toxicity.

Slide 21 · Intranuclear inclusions: What they establish

An inclusion indicates accumulation of abnormal protein material within a neuronal nucleus.

Nuclear inclusions in the CAG-polyglutamine SCA group.

This is a cellular pathology clue to disturbed protein homeostasis.

The presence of aggregates alone cannot resolve the precise neuronal injury mechanism.

Slide 22 · Aggregation: Two competing interpretations

Potential harm

Protein assemblies may disrupt transcription or cellular processes.

Potential protection

Sequestration into larger aggregates may limit freely toxic species.

Robbins explicitly cautions that a direct toxic role of visible inclusions is not universal.

Slide 23 · Transcriptional dysregulation is a candidate pathway

Aggregated or misfolded polyglutamine proteins may affect transcription of other genes.

This is one proposed toxic consequence, not a proven explanation for every SCA.

Distinguish a general disease-class mechanism from an established subtype-specific mechanism.

Slide 24 · Candidate mitochondrial injury pathway

Abnormal polyQ protein

Mitochondrial dysfunction

Reduced cell resilience

Mitochondrial dysfunction as a possible effect of protein aggregation.

The schematic does not assert that all polyglutamine SCAs share this single mechanism.

Slide 25 · Proteostasis stress and apoptosis

Misfolded protein burden

Unfolded protein response

Cellular injury response

Possible apoptosis

The unfolded-protein response and programmed cell death are proposed downstream processes.

Not every histologic inclusion represents a cell currently undergoing apoptosis.

Slide 26 · Proteolytic fragments: Another hypothesis

Robbins also considers downstream toxicity mediated by fragments of expanded polyglutamine proteins.

This concept differs from assigning toxicity solely to a large visible aggregate.

Fragment-mediated injury remains mechanistic context rather than a definitive subtype criterion.

Slide 27 · Observed feature versus pathogenic hypothesis

Observation

Interpretation

Evidence boundary

CAG expansion

Translated polyglutamine tract

Coding context required

Nuclear inclusion

Abnormal protein accumulation

Not necessarily the toxic agent

Cell stress

Possible downstream mechanism

Not universal for all SCAs

Neuronal dysfunction

Clinical/pathologic consequence

Subtype distribution varies

Slide 28 · What neuropathology can and cannot tell us

The broad degeneration category includes neuronal loss and dysfunction in several nervous-system regions.

Polyglutamine SCAs can show neuronal intranuclear inclusion pathology.

A shared microscopic feature does not identify which CAG-expanded gene is responsible.

Subtype-level interpretation requires the clinical and genetic context.

Slide 29 · The eight polyglutamine-associated entities named

Named forms

Textbook distinction

SCA1, SCA2

Included in coding-CAG class

SCA3, SCA6

SCA3 is also Machado–Joseph disease

SCA7, SCA8

SCA7 has visual impairment as a distinguishing clue

SCA17, DRPLA

Listed with the polyglutamine diseases

This reproduces the source’s grouping; Its description of SCA8 is not independently reclassified here.

Slide 30 · SCA1: Gene–repeat association

SCA1 with the ATXN1 gene.

The reported expansion is a coding CAG repeat, placing it in the polyglutamine model.

The molecular name identifies the subtype more precisely than the generic phenotype.

Locus identification is more specific than shared ataxia or aggregate morphology.

Slide 31 · SCA2: Another distinct ataxin locus

The source identifies ATXN2 as the gene associated with SCA2.

SCA2 belongs to the coding-CAG/polyglutamine category of the SCA family.

Identical repeat motifs at different genes do not create identical genetic diseases.

An SCA label must remain linked to the specific locus under investigation.

Slide 32 · SCA3: Machado–Joseph disease

SCA3 is explicitly identified as Machado–Joseph disease.

This entity to the ATXN3 gene and a coding CAG expansion.

The alternate eponym and genetic designation refer to the same subtype.

The eponym by itself does not explain why its expanded protein is neurotoxic.

Slide 33 · SCA6: A channel-related protein

Robbins groups SCA6 with the coding-CAG polyglutamine disorders.

The textbook associates its protein with a voltage-dependent calcium-channel subunit.

This links a named SCA protein to membrane excitability machinery at the molecular level.

The supplied chapter does not specify an entire SCA6 injury cascade.

Slide 34 · SCA7: A clinically useful association

SCA7 belongs to the textbook’s CAG/polyglutamine-associated group.

The text singles out visual impairment as a relatively unusual accompanying feature.

The associated gene is ATXN7.

Visual involvement supports targeted consideration of SCA7; It is not a stand-alone molecular test.

Slide 35 · Less detailed members of the listed group

SCA8

Listed polyglutamine group.

SCA17

Listed group; Phenotype details are not established in this discussion.

DRPLA

Dentatorubral–pallidoluysian atrophy is listed with these diseases.

The Robbins section does not supply enough detail to create phenotype-specific claims for these entities.

Slide 36 · Genetically mapped coding-repeat examples

Subtype

Gene/protein association

Mechanistic class

SCA1

ATXN1 / ataxin-1

CAG / polyglutamine

SCA2

ATXN2 / ataxin-2

CAG / polyglutamine

SCA3

ATXN3 / ataxin-3

CAG / polyglutamine

SCA6

Voltage-dependent Ca²⁺-channel α1A subunit

CAG / polyglutamine

SCA7

ATXN7 / ataxin-7

CAG / polyglutamine

Gene/protein names are transcribed only where supported by the source table; Numerical repeat cutoffs are intentionally omitted.

Slide 37 · Why one repeat motif yields different diseases

The disease-linked locus determines which protein acquires the expanded glutamine tract.

Polyglutamine proteins differ in their normal biological roles.

Heterogeneity across genetic and clinical features despite shared molecular themes.

A shared mutation class is not a sufficient diagnostic description.

Slide 38 · Applied interpretation: Ataxia plus visual impairment

Clinical vignette — hypothetical

A hypothetical multigenerational family has progressive ataxia; Several affected members also report worsening vision.

Which named SCA is specifically highlighted by this combination in the source?

SCA7 should be considered because visual impairment is a distinguishing association. A molecular diagnosis remains necessary.

Slide 39 · Huntington disease is a mechanism comparator

Both Huntington disease and several SCAs belong to the repeat-expansion/polyglutamine disease group.

Huntington disease classically targets the striatum, whereas the SCA family has spinocerebellar involvement.

The common inclusion/proteostasis model should not erase the major clinical-anatomic differences.

Slide 40 · Shared mechanism does not mean shared anatomy

Feature

Huntington disease

PolyQ SCA group

Inheritance

Autosomal dominant

Autosomal dominant SCA family

Relevant repeat

Coding CAG

Coding CAG for this subset

Principal disease emphasis

Striatal degeneration

Spinocerebellar degeneration

Protein pathology

Intranuclear inclusions

Intranuclear inclusions described

Slide 41 · Noncoding expansions begin differently

Repeat outside coding sequence

Expanded repeat RNA or local genomic effects

Disease mechanism unresolved

Unlike coding-CAG disease, a noncoding expansion does not directly create an elongated polyglutamine tract by standard translation.

Robbins specifically states that the mechanism of SCA noncoding-repeat disease remains unknown.

Slide 42 · Six noncoding-repeat SCAs in the chapter

SCA10, SCA12, and SCA27B are identified as noncoding-repeat forms.

SCA31, SCA36, and SCA37 complete the six named examples.

In most, the underlying disease-linked locus has been defined.

The common pathogenic sequence from noncoding expansion to neuronal injury is not established.

Slide 43 · Source-defined noncoding-repeat classification

Noncoding-repeat subtype

What the Robbins excerpt establishes

SCA10

Class membership

SCA12

Class membership

SCA27B

Class membership

SCA31

Class membership

SCA36

Class membership

SCA37

Class membership

The excerpt supplies a list, not six validated subtype-specific molecular cascades.

Slide 44 · Coding and noncoding are not synonyms

Coding CAG

Directly changes amino-acid repeat length of the protein.

Noncoding repeat

Does not necessarily change an encoded polyglutamine tract.

Unresolved pathway

Mechanism requires evidence beyond identifying the expanded repeat.

A noncoding SCA should not inherit the entire polyglutamine schematic by analogy alone.

Slide 45 · The myotonic dystrophy comparison has limits

A mechanistic resemblance between noncoding-repeat SCAs and myotonic dystrophy.

The analogy identifies an expansion in noncoding sequence, not a proven identical toxic pathway.

The textbook explicitly calls the mechanism of these noncoding SCAs unknown.

Avoid depicting RNA toxicity as conclusively demonstrated for every subtype.

Slide 46 · Molecular identification ≠ pathogenetic explanation

Locating an expanded repeat can establish a strong molecular association.

Understanding how that repeat injures neurons is a distinct research question.

The Robbins account acknowledges that locus discovery has outpaced mechanistic understanding.

Classification should preserve this difference between association and explanation.

Slide 47 · Applied interpretation: A noncoding expansion

Clinical vignette — hypothetical

A hypothetical laboratory report identifies a pathogenic repeat expansion outside the coding sequence of an SCA-associated locus.

Is an expanded polyglutamine protein necessarily produced?

No. The direct coding-CAG-to-polyglutamine inference does not follow from a noncoding expansion. Mechanism must be evaluated separately.

Slide 48 · Conventional SCA mutations: Several variant types

Copy-number changes

Alter dosage of a chromosomal segment or gene.

Deletions / insertions

Remove or add DNA sequence.

Point mutations

Change one nucleotide at a specific site.

All are represented among the non-repeat forms.

Slide 49 · “Conventional” is not a single molecular pathway

The category includes diverse sequence and structural variants.

Affected genes are likewise diverse rather than members of one protein family.

Proteins functioning in cellular signal transduction.

Interpretation therefore depends on the gene product and specific variant.

Slide 50 · Signal-transduction proteins: The common theme

Receptor or ion channel or kinase

Altered signaling function

Disordered neuronal physiology

Robbins gives protein categories, not a universal receptor-to-degeneration chain.

This is a conceptual framework for variants affecting signaling proteins.

Slide 51 · Conventional variant class versus protein class

Variant form

Potential protein target

Source-supported conclusion

Point mutation

Receptor / channel / kinase

Amino-acid or regulation may change

Deletion / insertion

Many possible proteins

Function depends on site and size

Copy-number variation

Many possible proteins

Gene dosage can be altered

Rows illustrate how the two axes differ; They are not confirmed subtype–gene pairings.

Slide 52 · Receptors: A mechanistic category, not a named SCA gene

Receptors participate in transmitting extracellular or synaptic information to cells.

Receptor proteins among SCA-relevant signal-transduction targets.

It does not claim that all receptor changes cause the same cellular response.

Do not assign a specific receptor mutation without a supported genotype.

Slide 53 · Ion channels: Another affected protein class

Ion channels regulate ionic flux and contribute to neuronal excitability.

The SCA chapter identifies channel proteins as one signal-transduction class.

Channel dysfunction is a plausible functional bridge to impaired neuronal signaling.

A specific electrophysiologic defect is not established for every conventional SCA.

Slide 54 · Kinases: Regulation through phosphorylation

Kinases modify signaling proteins through phosphorylation reactions.

Kinases in the broad set of SCA-associated proteins.

A kinase lesion can influence cellular communication or regulation.

The named excerpt does not provide a kinase-specific SCA pathway.

Slide 55 · Three mechanistic classes at a glance

Class

Variant substrate

Level of explanation

Coding CAG repeats

Protein with expanded polyglutamine

Misfolding/inclusions; toxicity debated

Noncoding repeats

Repeat expansion outside coding tract

Pathogenic mechanism unknown

Conventional variants

Copy-number or sequence changes

Diverse signaling-protein effects

The boundaries matter more than memorizing a single molecular cartoon.

Slide 56 · From mutation to clinicopathologic phenotype

Different inherited lesions converge on dysfunction in cerebellar and related circuits.

Neuronal loss and dysfunction explain why a molecularly diverse family shares a neurologic label.

Distribution of injury and extra-cerebellar involvement help determine clinical patterns.

The source does not support a uniform anatomic map for all individual SCA types.

Slide 57 · Anatomic distribution: Shared category, variable involvement

Cerebellum

Coordination-centered dysfunction

Spinal cord

Possible extension beyond cerebellum

Peripheral nerves

Possible contribution to broader syndrome

The diagram depicts the family-level distribution—not the expected lesion burden in a given genotype.

Slide 58 · Ataxia as a presenting label has limited specificity

Several distinct degenerative entities can produce a cerebellar ataxia phenotype.

Clinical phenotype must be paired with inherited versus sporadic context.

Mode of inheritance is informative but does not identify a precise SCA subtype.

A molecular diagnosis cannot be deduced from the word “ataxia” alone.

Slide 59 · Dominant SCAs versus Friedreich ataxia

Characteristic

SCA family

Friedreich ataxia

Inheritance

Autosomal dominant

Autosomal recessive

Disease classification

Multiple mutation classes

FXN GAA repeat expansion

Additional findings in text

Subtype-variable

Neuropathy, cardiomyopathy, other systemic effects

Teaching value

Heterogeneous family

Distinct recessive diagnosis

Do not conflate the noncoding GAA expansion in FXN with a named autosomal dominant SCA subtype.

Slide 60 · Dominant SCAs versus ataxia-telangiectasia

Characteristic

SCA family

Ataxia-telangiectasia

Inheritance

Autosomal dominant

Autosomal recessive

Molecular focus

Multiple variant classes

ATM-dependent DNA damage response

Accompanying clues

Subtype-dependent

Recurrent infections and lymphoid neoplasms

Disease grouping

Dominant SCAs

Separate recessive degeneration

Slide 61 · Inherited SCAs versus multiple system atrophy

Characteristic

Inherited SCA family

Multiple system atrophy

Disease setting

Autosomal dominant

Sporadic disorder

Protein theme

Several classes; polyQ subset

α-synuclein inclusions in oligodendrocytes

Anatomic emphasis

Spinocerebellar circuits variable

Cerebellar, extrapyramidal, autonomic systems

Diagnostic caution

Genetic heterogeneity

Different proteinopathy and distribution

Slide 62 · A clinical phenotype cannot resolve repeat location

Ataxia does not distinguish a coding repeat from a noncoding repeat.

Even among polyglutamine SCAs, different genes can yield overlapping disease-family features.

Genetic testing or documented familial molecular findings are necessary to define a specific molecular class.

The clinical workup must respect this underdetermination.

Slide 63 · Applied interpretation: Autosomal dominant ataxia

Clinical vignette — hypothetical

A hypothetical pedigree shows progressive ataxia in several successive generations, affecting both sexes.

What family-level classification is most consistent with the source, and what remains unknown?

An autosomal dominant SCA is compatible. The pathogenic gene, mutation class, and subtype cannot be inferred from the pedigree alone.

Slide 64 · Applied interpretation: Nuclear inclusions

Clinical vignette — hypothetical

A hypothetical neuropathology description reports neuronal nuclear aggregates in a patient with progressive inherited ataxia.

Do these aggregates prove which SCA subtype is present or that inclusions themselves are toxic?

Neither. Polyglutamine disease is a possible mechanistic category; Locus and causation require independent evidence.

Slide 65 · Applied interpretation: Variant versus mechanism

Clinical vignette — hypothetical

A hypothetical sequencing result identifies a pathogenic point mutation affecting a neuronal signaling protein.

Which SCA mechanistic class applies, and what should not be inferred?

This fits the conventional-mutation class. It does not establish a coding-CAG expansion or polyglutamine nuclear-inclusion mechanism.

Slide 66 · Three errors to avoid in SCA teaching

Do not treat all autosomal dominant SCAs as one molecular disease.

Do not infer polyglutamine protein expansion from a noncoding-repeat allele.

Do not claim visible inclusions are necessarily the direct toxic species.

Do not include recessive Friedreich ataxia or ataxia-telangiectasia as dominant SCA subtypes.

Slide 67 · A defensible molecular interpretation sequence

Define family context

Identify variant class

Map to protein / noncoding locus

State mechanism and uncertainty

Report clearly what Robbins establishes and where it explicitly leaves causality unresolved.

Keep clinical phenotype, observed morphology, and proposed mechanism in separate evidentiary categories.

Slide 68 · Summary: The stable conceptual framework

Autosomal dominant SCAs are genetically and clinicopathologically heterogeneous.

Coding CAG expansions link a subset to polyglutamine-associated neuronal protein accumulation.

Noncoding expansions and conventional mutations create different mechanistic questions.

Shared ataxia does not replace molecular classification or eliminate uncertainty.

Slide 69 · Selected source readings

Robbins, Cotran & Kumar. Pathologic Basis of Disease. 11th ed. (2026): Spinocerebellar Ataxias.

Robbins, Cotran & Kumar. Diseases Caused by Trinucleotide-Repeat Expansions; repeat-disease table.

Supporting textbook sections: Friedreich Ataxia; Ataxia-Telangiectasia; Huntington Disease; Multiple System Atrophy.