Huntington’s Disease Pathology Explained

Huntington’s disease pathology starts with an inherited CAG repeat expansion in the HTT gene and ends in loss of striatal medium spiny neurons, with the caudate nucleus and putamen losing volume. The genotype explains the histology.

Picture a pathology report that mentions striatal atrophy, huntingtin inclusions and a repeat count. A reader might take these for three unrelated findings. They are one chain read from DNA to tissue, and each link is easy to misread. The overview of types of neurodegenerative diseases places Huntington disease among related disorders, while this page follows the chain from repeat to slide.

How the CAG repeat leads to mutant huntingtin

Huntington disease is caused by a CAG trinucleotide repeat expansion in the HTT gene on chromosome 4. A review of the cellular and molecular basis of HD pathology describes it as an autosomal dominant condition. Three points keep the genetics straight when you read a report.

• The gene is universal; the expansion is not. Everyone has the huntingtin gene, but only people who inherit the expanded form develop HD, as the Huntington’s Disease Society of America explains.
• Repeat counts separate typical from affected. Most people have around 20 CAG repeats, whereas people with HD have around 40 or more, according to HDSA. Protein-level literature frames the cut-off differently, as a polyglutamine expansion of more than 36 glutamine repeats in the huntingtin protein. Quote a threshold together with how it is framed, because one figure contrasts typical with affected counts and the other describes the expansion in the protein.
• Extra repeats change the protein. The additional CAG repeats make huntingtin extra-long and difficult to maintain. That altered protein is the step connecting the inherited change to the cell damage described below.

Common misunderstandings at a glance

Most errors in reading HD pathology come from treating one finding as the whole disease. The table sets four frequent assumptions against what the evidence supports.

The repeat counts and the striatal and cortical findings come from the HDSA overview and the Rare Disease Advisor pathophysiology review.

What the brain looks like in Huntington disease

The striatum is the part of the brain most affected in HD, according to HDSA. Within it, the caudate nucleus and putamen show particular volume loss. This is why the caudate and putamen are the anchor structures when a pathologist or radiologist describes the disease.

Regions beyond the striatum

The striatum is the anchor finding but not the only one. Atrophy occurs in the external segment of the globus pallidus, and gray matter loss occurs in the occipital, frontal, insular and parietal cortices, as the pathophysiology review reports. Frontal lobe cortical volume loss correlates with lower performance on assessments of executive function. A pathologist reading a case should therefore expect a distributed picture with the striatum at its centre.

Striatal medium spiny neuron loss

The cells most closely tied to striatal atrophy are the medium-sized spiny GABAergic projection neurons. Degeneration of these neurons within the basal ganglia’s striatum begins before the onset of motor symptoms. The neuron loss therefore comes first, and the visible shrinkage of the caudate and putamen follows from it.

Loss of these cells has a chemical signature. Levels of gamma-aminobutyric acid (GABA) and substance P decrease, and the loss of inhibitory striatal regulation leads to increased excitatory input. The same review that gives the repeat threshold calls dysfunction of the corticostriatal system one of the main pathological events. That links the striatal cell loss to the cortical changes in the previous section and explains why the disease is not confined to one structure.

Huntingtin inclusions and their role

Mutant huntingtin forms clumps in brain cells and causes them to become damaged and die over many years, according to HDSA. The aggregates localize in medium-sized spiny GABAergic striatal neurons and in cortical neurons. That distribution fits the striatal and cortical changes described above.

What the inclusions do is described cautiously. The pathophysiology review says the aggregates “somehow” disrupt cellular processes, naming protein recycling, axonal transport, transcription and energy metabolism. The practical reading is that an inclusion shows where mutant huntingtin has accumulated and which cell types are involved. It does not prove that each individual clump kills its neuron.

Reading the chain from genotype to slide

Put together, the findings form a sequence that runs from the inherited change to what is visible in tissue.

1. Inherited change. An expanded CAG repeat in HTT is passed on in the family.
2. Altered protein. Huntingtin becomes extra-long and hard to maintain.
3. Clumps and cellular stress. Mutant huntingtin aggregates in striatal and cortical neurons and disrupts essential cell processes.
4. Selective cell loss. Medium spiny neurons degenerate, and GABA and substance P fall.
5. Visible volume loss. The caudate nucleus and putamen lose volume, with further changes in the globus pallidus and cortex.

Because neuron loss begins before motor symptoms, the tissue at autopsy reflects a process that has run for a long time. Atrophy in the caudate and putamen is a record of that process, and it is the feature a severity scale would build on.

Other disorders show their own lesion distributions. For contrast, see the notes on multiple system atrophy pathology and chronic traumatic encephalopathy pathology.

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