This study observes the I278T mutation in 7 of 11 patients responsive to pyridoxine treatment and in 0 of 27 pyridoxine-nonresponsive patients.
In 15 Dutch patients with CBS deficiency, all presumably homozygous or compound heterozygous for CBS mutations, 50% of the alleles were I278T (called 844T->C here). In a control group of 111 subjects only one heterozygote was detected.
They report on a common 68 base insertion that appears to create an alternate splicing site that causes the I278T variant to be untranslated.
This paper observes that a 68 base pair insertion cosegregates with I278T variants and appears to have a neutralizing affect, presumably due to an effect on splicing reported on by Tsai et al.
Mice homozygous for the I278T mutation were bred; liver, kidney and colon, and liver extracts have only 2-3% of the CBS enzyme activity found in wild-type mice.