By Lisa Munsie, PhD
1 – IDENTIFYING HD IN LOW INCOME COUNTRIES
The natural history of Huntington’s disease has largely focused on Caucasian people of European descent. Best practice treatments are largely reserved for populations in areas of economic stability, due to the costs associated with genetic testing and other treatment options. This also means that genetic treatments focused on allele-specific silencing are tailored to these populations. These issues are being acknowledged by scientists.
Correspondence in The Lancet brought up the problem of identifying HD in low/middle income countries like Bangladesh, where doctors will often misdiagnose HD due to lack of funding and availability of genetic testing.1 This hinders research on actual prevalence, and means patients in these areas are not getting the required resources. In Caucasian European populations, the mutant huntingtin gene (HTT) is associated with haplogroup A, which is the target of many allele-specific silencing efforts, whereas population data in other areas of the world suggests association with other haplotypes.
A family from Oman with several generations of HD were the subject of a recent study by the Hayden group. This family tree had eight generations and represented the largest known HD cluster in the Middle East.2 The study found the oldest African family ancestor had a C6xC9 haplotype associated with this cluster, supporting the hypothesis that mutant HTT has arisen spontaneously and independently in different geographical locations. These studies shed light on the initial spread of HD through different populations as well as indicate how therapies need to be targeted to serve different genetic backgrounds.
In order to expedite therapies, the FDA uses the Clinical Data Interchange Consortium (CDISC) standards. Clinical data standardization will lead to a faster path to regulatory approval for urgently needed new therapies for patients. Therapeutic Area User Guides (TAUG) describe CDISC standards and are used to document efficacy in clinical trials. A TAUG for HD has recently been established in conjunction with HD Regulatory Science Consortium.3 This will be used in validating treatments for HD and acting as an example for the development of TAUG for other rare and orphan diseases.
1 Islam, Z et.al. (2020) Huntington’s Disease in Bangladesh. Lancet Neurology 19(8):644-645
2 Squitieri, F et.al. (2020) Tracing the mutated HTT and haplotype of the African ancestor who spread Huntington Disease into the Middle East. Genet Med. 10.1038/s41436-020-0895-1
3 Mullin, A et.al (2020) Standardized data structures in rare disease: CDISC user guides for Duschenne Muscular Dystrophy and Huntington’s Disease. Clinical and Translational Science. Doi.org/10.1111/cts.12845
2 – UNDERSTANDING HD IN NEURODEVELOPMENT
Huntington’s disease is well-known to be neurodegenerative disorder, but it is also hypothesized to be a neurodevelopmental disorder. Understanding if and how mutant huntingtin (mtHTT) is involved in neurodevelopment will have consequences for treatment. Imaging studies can look at brain morphology related to the development process. An imaging study involving whole-brain anatomical MRI acquisition of adult early stage HD patients and looking at sulcal morphometry was recently performed.1 Sulcal length is related to development. Most of the degeneration found in this study could be related to atrophy, however the length of the Sylvian fissure was abnormal and strikingly lacking in symmetry between brain hemispheres. These changes are indicative of abnormal neocortical development as the Sylvian fissure appears in utero.
Another imaging study using resting state functional MRI (fMRI) looking at neurodevelopmental problems associated with mtHTT was performed on children between the age of 6-18.2 Asymptomaic youth at risk for carrying mHTT were consented for the study and their gene status was kept confidential. The findings from this study support the neurodevelopmental theory of neurodegeneration. The study showed hyperconnectivity of the striatum to cerebellum following the loss of connectivity over time, specific to the children carrying mHTT. These connectivity issues are noted years before predicted motor onset.
One additional study on the field of neurodevelopment in HD examined tissue from 13 week gestational fetuses that were carrying mHTT. Imaging studies showed mislocalization of mHTT in the ventricular zone which would disrupt the polarity of the neuroepithelium.3 This was recapitulated in HD mouse models. Using the mouse models, the study goes on to probe the mechanism of this neurodevelopmental hindrance and finds issues with endosome secretion, abnormal cell cycle and altered neurogenesis may contribute to these early defects. The understanding of HTT role in neurodevelopment may explain outcomes from HTT lowering and other therapeutic strategies.
1 Mangin, Jean-Francois et.al (2020) Neocortical morphometry in Huntington’s disease: Indication of the coexistence of abnormal neurodevelopmental and neurodegenerative processes. NeorImage:Clinical 26:102211
2 Tereshchenki, A.V et.al (2020) Abnormal development of cerebellar-striatal circuitry in Huntington disease. Neurology 94:e1908-e1915
3 Barnat, M et.al (2020) Huntington’s disease alters human neurodevelopment. Science 6506:787-793
3 – THE ROLE OF IMMUNE RESPONSE IN HD
The mechanism of cell selectivity in Huntington’s disease (HD) is often traced back to transcription changes leading to alterations in the immune response in the vulnerable cell populations. Discovering the mechanisms of these changes and the genes and proteins responsible could lead to therapeutic pathways.
A report in Neuron uses single nuclear RNA sequencing (snRNA-seq) and translating ribosome affinity purification (TRAP) to examine cell-type specific alterations to gene expression in HD.1 The group uses the TRAP assay to assess different striatal subpopulations in a series of HD mouse models. The group additionally used snRNA-seq on post-mortem brain tissue from HD patients to confirm their findings. Results showed a release of mitochondrial RNA specific to the spiny projection neurons, leading to an immune response, and a combination of non-celltype and cell-type specific responses contributing to the selective vulnerability of these cell types. Regulating transcription and suppressing the immune response could therefore be a target for HD therapy.
Immune activation is hallmarked by the release of cytokines. Interleukin-6 (IL-6) upregulation has been noted in HD but whether this upregulation is beneficial or pathological remains unknown. A recent study crosses the R6/2 HD model with a mouse model deficient in IL-6 and examines the phenotypic outcomes on mouse behaviour: rotarod, open field assay, rearing and climbing and grip strength. Knocking out IL-6 led to an increase in severity in almost all behaviours, indicating that the increase in IL-6 is likely beneficial. This increase in phenotypic severity seems to be a consequence of dysregulation of synaptic transmission and neurotrophin signaling caused by the IL-6 knockout, indicating that elevating IL-6 levels at certain timepoints in the course of HD may have beneficial effects.2 Further evidence to support inflammation as a cellular response in HD comes from another study that identifies cGMP-AMP synthase (cGAS), with known activity in regulating autophagy and inflammation, as upregulated in HD striatum.3 As with the other groups, evidence of this upregulation is found both in mouse models, where there is a high occupany of cGAS at the ribosome, and in human tissue, where an upregulation of cGAS regulated genes was noted. It is hypothesized that inhibiting cGAS during the course of HD may help alleviate symptoms associated with the inflammation response.
1 Lee, H et al (2020) Cell Type-Specific Transcriptomics Reveals that Mutant Huntingtin Leads to Mitochondrial RNA Release and Neuronal innate Immune Activation. Neuron 107: 1-18
2 Wertz, M.H. et al. (2020 Interleukin-6 deficiency exacerbates Huntington’s disease model phenotypes. Molecular Neurodegeneration. 15:29
3 Sharma, M et.al (2020) Cyclin GMP-AMP synthase promotes the inflammatory and autophagy responses in Huntington disease. PNAS 117(27) 15989-15999