dbSNP ID: rs35037984. Data from exome sequencing project on dbSNP reports this variant has an allele frequency of 1.4% in 3362 sample chromosomes.
Two out of 30 chromosomes in the cases reported by Santoro et al. does not have a statistically significant difference from the 1.4% R928C allele frequency reported in exome sequencing data listed in dbSNP. With this variant frequency, we would expect cases to be significantly enriched for carrying this variant — and so it is highly unlikely that this variant is causal. It is possible R414L or some other unknown cis variant was causal in these cases.
To clarify the significance of the same allele frequencies: If this variant were causing disease, we might attribute it to causing disease in 10% cases caused by UNC13D (which accounts for ~25% of all cases) — with a disease frequency of 1 out of 50,000, this implies an allele frequency of around .002 * .1 * .25 ~= 5 * 10^-5. This differs from the exome sequencing data allele frequency (1.4%) with extremely high statistical significance.