Explor­ing the genet­ics of neuro­de­gen­er­at­ive dis­orders means under­stand­ing dis­eases and devel­op­ing bet­ter treat­ments. In Huntington’s dis­ease, CAG repeats, and frameshift muta­tions have already been stud­ied by using anti­bod­ies to detect mutant poly­alanines. Read on to learn how frameshifts affect tox­ic inclu­sions and how our pro­tein arrays can help you in your own research of neuro­de­gen­er­at­ive dis­ease and bey­ond.

Huntington’s Disease: Genetic Basis and Clinical Consequences

Huntington’s dis­ease (HD) is an incur­able neuro­de­gen­er­at­ive dis­order caused by a defect­ive hunt­ingt­in pro­tein. A muta­tion in the HTT gene res­ults in an abnor­mally long pro­tein, prone to cleav­age and aggreg­a­tion.
In HD patients, there is an expan­sion of the cysteine-aden­osine-guan­ine (CAG) repeat. This, in turn, leads to a longer glutam­ine repeat at the N‑terminus of the pro­tein product. (Schilling et al., 2007)

Cas­pases cleave the mutant hunt­ingt­in, res­ult­ing in N‑terminal pep­tide frag­ments, which are likely to mis­fold, aggreg­ate and form inclu­sion bod­ies. Over time, the tox­ic aggreg­ates inter­fere with nor­mal neur­on­al func­tion, bring­ing about the clas­sic signs of the dis­ease.

The Hunt­ingt­in pathophen­o­type con­sists of:

• Motor dys­func­tion, includ­ing pro­gress­ive chorea, rigid­ity, psy­cho­mo­tor impair­ment.
• Cog­nit­ive dis­turb­ances, declin­ing into demen­tia.
• Psy­chi­at­ric symp­toms, begin­ning as subtle per­son­al­ity changes and poten­tially pro­gress­ing into anxi­ety, depres­sion, emo­tion­al blunt­ing, etc.

Frameshift Mutation and Neurodegenerative Disease

Huntington’s dis­ease is the most pre­val­ent and best-known mem­ber of the poly­glutam­ine dis­ease group. But you will also see a poly‑G expan­sion due to CAG repeats in:

• Spino­cere­bel­lar ataxi­as (SCA) type 1, 2, 3, 6, 7 and 17
• Spinob­ul­bar mus­cu­lar atrophy
• Dent­ator­ubal-pal­lid­oluysi­anatrophy

In these dis­orders, the mutant gene encodes aggreg­ate-prone pro­teins with long poly­glutam­ine tracts.

How­ever, a dinuc­leotide dele­tion or single nuc­le­otide inser­tion with­in the CAG tract shifts the cod­ing frame by +1 to GCA. GCA encodes alan­ine, lead­ing to poly­alan­ine expan­sion in poly­glutam­ine dis­eases.
A study by Dav­ies and Rubin­sztein con­firmed both +1 and +2 frameshift-mutated pro­tein in Huntington’s dis­ease patients. The authors sug­ges­ted this can be rel­ev­ant to dis­ease patho­gen­es­is, con­sid­er­ing pre­vi­ous find­ings that poly­s­er­ine and poly­alan­ine-con­tain­ing pro­tein modi­fy mutant hunt­ingt­in tox­icity. (Ber­ger et al., 2006)

How did they go about this? By search­ing for anti­bod­ies rather than dir­ectly look­ing for the muta­tion.

Finding the Frameshift Mutation: How Antisera Made It Happen

To find the mutated pro­tein, Dav­ies and Rubin­sztein star­ted by mod­el­ling pos­sible frameshift products of the HTT exon 1. Then, they pre­dicted the mutant epi­topes and raised poly­clonal anti­bod­ies against them.

These anti­bod­ies were a fast and easy way to find +1 and +2 frameshift products. The anti­sera were used dir­ectly on post­mortem patient brain and trans­gen­ic mouse brain samples — and here is what they found:

Polyalanine Mutant Protein in Polyglutamine Diseases: Findings & Consequences

Dav­ies and Rubin­sztein only found frameshifts 4% of pro­tein aggreg­ated . This sup­ports the the­ory of low-level, time-depend­ent frameshift product occur­rence.

Why Polyalanines Appear Along With Long Polyglutamines

The frameshift that causes poly­alanines to appear hap­pens more fre­quently in expan­ded CAG tracts. The longer the repeat sequence is, the more likely it is for a frameshift muta­tion to hap­pen — and for poly­alanines to appear.
Toulouse et al. have hypo­thes­ised that this hap­pens because of ribosomal slip­page, which occurs with long CAG tracts. (Toulouse et al., 2005)

They also state that poly­alan­ine products increase poly‑G tox­icity, con­trib­ut­ing to the patho­gen­es­is of neuro­de­gen­er­at­ive dis­ease. If we admin­is­ter ani­so­my­cin, which inter­acts with ribosomes, the tox­icity drops - and this seems to con­firm the the­ory.

Drugs that act on the ribosomes could be a prom­ising new ther­apy for Huntington’s patients. There is still more to explore. Ani­so­my­cin reduces pro­tein syn­thes­is as a whole. Thus, the pro­tect­ive effect might be due to a lower­ing of the poly‑G pep­tide and not because the drug pre­vents frameshift­ing.

But there is more:
The poly­alan­ine frameshift products have actu­ally been shown to pro­tect against poly­glutam­ine tox­icity (Dav­ies et al., 2006). While high amounts of long poly­alanines are tox­ic, these pro­teins have a pos­it­ive effect at a lower level.

How Polyalanines Reduce Toxicity

One of the hypo­theses tested was through sequest­ra­tion of the poly­glutam­ine pro­tein. How­ever, tests failed to show any dir­ect inter­ac­tion between the mutant repeats. Instead, the pro­tect­ive effect of poly­alanines was remin­is­cent of heat-shock pro­tein inter­ac­tion with ataxin‑1. Poly­alan­ine repeats would reduce the pro­por­tion of cells with inclu­sions, but they’d increase the num­ber of inclu­sions per cell.

Could it be that long poly­alanines induced a heat-shock response, even at sub­tox­ic doses? It appears so. In par­tic­u­lar, poly­alanines were found to:

• Be the spe­cif­ic reas­on behind heat-shock response induc­tion and poly­glutam­ate tox­icity pro­tec­tion — oth­er aggreg­ate-prone pro­tein did not have the same prop­erty.
• Induce hsp70 (70 kilo­dalton heat shock pro­tein), which is known to pro­tect from apop­tos­is.
• Cause the induc­tion through HSF‑1 driv­en tran­scrip­tion.