Dmitri Borgmann has informed us in the February 1974 and May 1985 Word Ways that the longest English-language isograms (words without repeated letters) are UNCOPYRIGHTABLE and DERMATOGLYPHICS, each iwth fifteen letters. Although the former word uses all six vowels, the latter does not, as it lacks U. An affixed form of either the word or its adjectival cousin, the 14-letter DERMATOGLYPHIC, seemed scientifically and linguistically plausible, employing this unused vowel.
Wolpow, Edward R.
"Subdermatoglyphic: A New Isogram,"
Word Ways: Vol. 24
, Article 4.
Available at: http://digitalcommons.butler.edu/wordways/vol24/iss1/4