For the first time, the fatty acid composition and desaturation indices are presented in three RADs compared with obese Controls. We hypothesized that fatty acid composition may be different if the SAT developed in a person with a RAD, with the fatty acid profile reflecting changes in desaturase activity. Fatty acid DIs reflect the proportion of MUFA product generated from the SFA precursor and is known to be upregulated in models of obesity, indicating enhanced lipogenesis [2, 7, 28]. We demonstrate in this study that the vaccenic/stearic DI in SAT from participants with DD, was lower than that of obese Controls, suggesting that the SAT in DD is a biochemically distinct subtype of SAT.
A contributing factor to the differences in the DIs was a trend to decreased vaccenic acid percent of total in the DD group compared to Controls, and a significant decrease in comparison to the FML group. FML is characterized by abnormal accumulation of SAT but not pain. It remains unclear whether the demonstrated differences are related to presence or absence of pain.
Although the palmitoleic/palmitic acid DI was not significantly decreased in DD subjects, the vaccenic/stearic acid DI was decreased. Vaccenic acid is actually derived through chain elongation of palmitoleic acid and is not made by direct desaturation of stearate . The ratio of (palmitoleic + vaccenic)/palmitic acid was determined to better represent the product/precursor relationships of this pathway, and there was a decrease in this ratio in DD subjects compared to FML, and a trend toward a decrease in DD compared to obese Controls. This decrease suggests a dysregulation in desaturation in DD, in which the palmitic to palmitoleic acid desaturation pathway is relatively suppressed, leading to decreased elongation to vaccenic acid. These findings appear consistent with a previous report on two DD subjects that demonstrated decreased acetate-1-14 C incorporation during MUFA synthesis in affected adipocytes compared to unaffected adipocytes in one subject, and decreased 18-carbon fatty acid production in the affected adipocytes of the second subject . The new fatty acid production in the first subject, as measured by incorporation of 14 C, was diminished in the 18:1 fatty acids, but the analysis did not distinguish between oleic and vaccenic acids. A more recent study  reported increased proportions of 18:1 fatty acid percent of total in 13 DD subjects in comparison to controls, but the control subjects were healthy and leaner with an average BMI of only 26 kg/m2 while the DD subjects averaged 33.5 kg/m2. It is unknown how the DI of our study subjects would compare to those of non-obese controls or to unaffected adipose tissue from the same subjects. However, in many individuals with DD, MSL and FML, all SAT tissue is affected, making an intra-person comparison of SAT very difficult. Nevertheless, our data supports decreased desaturation in DD SAT compared to SAT of other obesity phenotypes.
We were unable to identify changes in SCD1 gene expression in our samples. Possible explanations for low gene expression include steady-state conditions, or low enzyme turnover. Altered protein expression and/or enzyme activity are potential causes of changes in the desaturation indicies in the absence of changes in gene expression. Discordant SCD1 mRNA and protein expression has been demonstrated in adipose tissue from obese humans . These possibilities were not able to be explored within the scope of the present study and should be investigated in the future.
A lower desaturation index or a shift towards increased saturated fatty acids has been associated with inflammatory states. SCD1-deficient mice are protected against diet-induced obesity, however, they have increased plasma inflammatory markers and increased risk for atherosclerosis . In a mouse model of colitis, fatty acid constituents of lysophosphatidylcholines were shifted towards increased saturated species relative to monounsaturated species, consistent with deceased SCD1 expression levels in the liver . In patients with coronary artery disease, epicardial fat exhibits higher SFA, lower unsaturated fatty acids, and secretes more proinflammatory cytokines than SAT . Induction of the adipocyte immune response by saturated fatty acids has been proposed to be mediated through the toll-like receptor-4/NFkB pathway . In addition, adipose tissue can hypertrophy near areas of inflammation such as infected lymph nodes  or inflamed tissues . Although obesity is associated with a low-grade, chronic inflammation in the adipose tissue, DD adipose tissue exhibits increased expression of IL-6 mRNA and evidence of pro-inflammatory macrophages, even in comparison to BMI- and weight-matched obese controls . Decreased desaturation leads to decreased turnover of proinflammatory saturated fatty acids , and we see in our DD subjects an increased proportion of saturated myristic acid in comparison to Controls. A possible relationship between DD fatty acid metabolism and inflammation in our subjects should be further investigated as others found no evidence of increased inflammation in the DD SAT based on counting primarily perivascular immune cells in the DD SAT compared to controls . An alternative explanation, that the lower DI in DD SAT reflects lower caloric intake cannot be dismissed but is less likely due to the presence of increased SAT in DD.
Additional contributing factors to inducing SAT growth other than lifestyle and inflammation have been reported. In mice with Prox1 haploinsufficiency, leakage of lymphatic fluid induced growth of fat . Lymph can be a strong inducer of adipogenesis. It is interesting to note that manual lymphatic drainage decreased the amount of fat in a case of DD . In a study on lipomas, increased lipoprotein lipase activity was implicated in the development of the abnormal adipose tissue masses .
With respect to PUFA, a low percent of total PUFA in MSL and a lower trend in DD were observed in comparison to obese controls. n-6 and n-3 PUFAs have been considered as anti-inflammatory fatty acids, with n-3 PUFAs having more potent anti-inflammatory effects. PUFA-deficiency may lead to consideration of whether inflammation is present in MSL. However, the DIs were not decreased in MSL. While LA trended lower in MSL, its downstream product AA trended higher than in DD, suggesting decreased AA turnover. AA is further metabolized to eicosanoids which can be either inflammatory or proinflammatory, so the significance of increased AA in MSL compared to DD is uncertain. An assessment of affected fat in MSL after intervention with dietary PUFAs (such as those found in fish oil) may be performed to determine the clinical significance of PUFA deficiency in MSL. In contrast, no clinical signs or studies in FML have suggested an inflammatory component in the development of the abnormal SAT in this RAD to date, and the cause for the growth of the hundreds of fatty nodules in this disorder remains unknown.
The relationships between desaturase activity and clinical markers of metabolic risk have yet to be determined. Although there were no differences that could be discerned in the correlations between clinical data and the palmitoleic/palmitic and oleic/stearic DI between groups, both DI were found to be correlated with the BMI and percent body fat in the total cohort. This is consistent with data showing that the DI in adipose tissue of obese mice are associated with BMI and the adiposity index . The plasma DI has also been previously described as correlating to waist circumference in adults  and to the waist-hip ratio in adolescents . Larger numbers of subjects in each of our study groups would be necessary to draw conclusions about whether any group differs in strength of association from the others, and to assess the influence of age and gender.
Although we were limited by availability of data on adiponectin, we found negative correlations between adiponectin levels and the oleic/stearic acid, vaccenic/stearic, and the (palmitoleic + vaccenic)/palmitic acid DI. An association between desaturase activity and adiponectin would be consistent with previously published data that demonstrated an inverse association between the plasma oleic/stearic DI and adiponectin in adolescent girls . In Crohn's disease patients, mesenteric adipose tissue has been found to exhibit more saturated fatty acids [3, 39], and a separate study showed increased adiponectin expression .
Our pilot study was limited by small subject numbers in the groups, lack of adipose tissue samples from a non-obese control group, and limited clinical and biochemical data. Subject numbers in the groups are small due to the rarity of these adipose disorders, and therefore the ability to examine correlations within groups is limited. Complete anthropometric data to assess degree of adiposity, as well as insulin resistance measures, would also have been desirable. Adipose tissue samples were also of insufficient quantity to perform protein expression studies or enzyme activity assays. Further studies are needed to explore the potential relationships between adipose tissue desaturase activity, inflammation, and other lipid parameters in people with and without RADs.
Increased SCD1 activity has been implicated not only in obesity, but also in the associated disorders of insulin resistance  and fatty liver . Moreover, SCD1 activity may affect cell signaling processes through changes in the proportion of MUFA to SFA in cell membranes, and its role in cell proliferation and cancer is being investigated. Recent studies have implicated upregulation of SCD1 in lung cancer , prostate cancer , and breast cancer . Pharmacologic inhibition of SCD1 has demonstrated a decrease in the MUFA/SFA ratio and cell proliferation in lung cancer cells , and has blocked signaling processes involved in oncogenesis and cancer progression in prostate cancer cells . Therefore, SCD1 is emerging as an enzyme with diverse biological roles and requires further investigation.