Huntington disease (HD; OMIM 143100), a progressive neurodegenerative disorder, 
is caused by an expanded trinucleotide CAG (polyQ) motif in the HTT gene. 
Cardiovascular symptoms, often present in early stage HD patients, are, in 
general, ascribed to dysautonomia. However, cardio-specific expression of polyQ 
peptides caused pathological response in murine models, suggesting the presence 
of a nervous system-independent heart phenotype in HD patients. A positive 
correlation between the CAG repeat size and severity of symptoms observed in HD 
patients has also been observed in in vitro HD cellular models. Here, we test 
the suitability of human embryonic stem cell (hESC) lines carrying HD-specific 
mutation as in vitro models for understanding molecular mechanisms of cardiac 
pathology seen in HD patients. We have differentiated three HD-hESC lines into 
cardiomyocytes and investigated CAG stability up to 60 days after starting 
differentiation. To assess CAG stability in other tissues, the lines were also 
subjected to in vivo differentiation into teratomas for 10 weeks. Neither 
directed differentiation into cardiomyocytes in vitro nor in vivo 
differentiation into teratomas, rich in immature neuronal tissue, led to an 
increase in the number of CAG repeats. Although the CAG stability might be cell 
line-dependent, induced pluripotent stem cells generated from patients with 
larger numbers of CAG repeats could have an advantage as a research tool for 
understanding cardiac symptoms of HD patients.