Observations on this history
– In contrast to type 2 DM, type 1 isn’t usually strongly familial, which makes this case history stand out – although 10% of patients may have another family member with DM1, and some will have more than one.
– familial autoimmune diabetes can occur in autoimmune polyendrocrine syndrome (APS1), but then it occurs with multiple other endocrine autoimmunity/failure
– the mixture of multiple cases of type 1 diabetes, plus type 2 diabetes in one family might suggest MODY, Maturity Onset Diabetes of the Young. The suggestion would be even stronger if the relatives with type 2 diabetes were not obese.
– the history does not mention any episodes of ketoacidosis despite poor control (which goes against type 1 diabetes)

So could he have MODY? How would you prove it?
GAD antibodies and C-peptide levels would have been a useful test at the time of diagnosis. Most patients with type 1 DM have high GAD65 antibodies and low C-peptide levels, whereas the opposite is true in patients with type 2 DM
– Are GAD antibodies still useful this long after diagnosis? YES, they would usually be high in autoimmune diabetes
– C-peptide might be useful – levels low or zero in DM type 1
If it looks likely, you can now test for mutations in the commonly involved genes.

So did he have MODY?
Yes.
There are multiple types of MODY but HNF1a mutations are the most common, and they are important because they characteristically respond well to sulphonylureas.
He had that, and his diabetes was subsequently managed by gliclazide 40mg twice daily and his HbA1c fell from 12% to 7.3%.
The history is much more suggestive of HNF1A than some others common MODYs . Glucokinase mutations for example are more likely to present with ‘mild diabetes’ and rarely develops complications.

This case is based on Thethy et al, J R Coll Physicians Edinb 2011;41:119-21 [PMID: 21677915]
Further info about MODY at http://www.ex.ac.uk/diabetesgenes/mody