I have an extremely rare mutation

[nandixon]

@sue1234

It might help in your research on rs8103451 to know that this SNP or allele is referred to as the "XbaI" (that's a Roman numeral 1) or "Xba1" polymorphism on the GYS1 gene. It can also be written as XbaIC>T. [In the older literature, the risk allele, rather than being referred to as "T" (= "A" in 23andMe), is referred to as "A2," with the wild type/normal allele referred to as "A1."]

It was first identified in 1993:
Association between Polymorphism of the Glycogen Synthase Gene and Non-Insulin-Dependent Diabetes Mellitus

And has been looked at in a number of studies, including this one from 2001:
The Stimulation-Induced Increase in Skeletal Muscle Glycogen Synthase Content Is Impaired in Carriers of the Glycogen Synthase XbaI Gene Polymorphism

There are a number of other studies as well, some of which can be found using, e.g., the following search string:
http://www.ncbi.nlm.nih.gov/pubmed/...+synthase")+(xbai+OR+xba1+OR+xbal+OR+"xba-i")

A commentary from 2008 includes this synopsis on XbaI (rs8103451) by the original discoverer of the SNP:

New Insights into Impaired Muscle Glycogen Synthesis

A common variant in the GYS1 gene has been associated with type 2 diabetes and insulin resistance, as well as increased risk of cardiovascular morbidity and mortality [5,6]. Carriers of the variant allele of this polymorphism are not able to increase their GS protein in response to exercise [7]. Interestingly, there seems to be an interaction between this polymorphism and exercise in the prevention of cardiovascular disease, i.e., risk genotype carriers do not experience the protective effect of exercise on cardiovascular risk [8].

Note that this intronic SNP may be acting as a marker/proxy for a functional SNP located either elsewhere on GYS1 or on a different gene on chromosome 19, although I doubt it can be ruled out completely that rs8103451 itself isn't being affected by some transcription factor that might affect expression of GYS1.