According to Scientific Style and Format: The CBE Manual for Authors, Editors, and Publishers (C B E Style Manual),
Even if the editor chooses a style that calls for initial capitalization of every term, some terms should retain an initial lowercase letter. [...] 3 A standard symbol or abbreviation that begins with a lowercase letter (pH, pK', mRNA).
Also, according to apsstylemanual.org,
An abbreviation that begins with a lowercase letter, or a term that must remain lowercase should not be changed to all caps when it begins a sentence; it should not be expanded. The word following should be is lowercase unless it is a proper noun or another acronym. If possible, reword the sentence so that it does not begin with the lowercase term
I don't see anything (other than the one regarding brand names) in the Chicago Manual of Style.