Increased Skeletal Muscle Volume in Women With Familial Partial Lipodystrophy, Dunnigan Variety

Abstract

Introduction:

Familial partial lipodystrophy, Dunnigan variety (FPLD), an autosomal dominant disorder caused by LMNA mutations, is characterized by fat loss from the extremities. However, it is unclear whether these patients appear muscular because of a lack of subcutaneous fat or have an actual increase in muscle mass. Therefore, we compared muscle mass and volume of selected muscles in women with FPLD and control subjects using dual-emission x-ray absorptiometry (DXA) and magnetic resonance imaging (MRI).

Methods:

Whole-body axial MRI and DXA scans were obtained on 39 women, aged 18 to 65 years, with FPLD and 17 healthy women matched for body mass index and age (group 1). Volumes of muscles in both the thighs, calves, and psoas were calculated from MRI scans and muscle mass in extremities were calculated from DXA. In addition, abdominal MRI and DXA scans were analyzed from 129 healthy, frequency-matched women (group 2). Comparisons between women with FPLD and control subjects were made using ANOVA, adjusting for height, body mass index, and age.

Results:

Patients with FPLD, compared with control subjects had significantly greater volumes of the thigh muscles, (6358 ± 1491 vs 5198 ± 716 mL, P = .002), calf muscles (3133 ± 713 vs 2397 ± 335 mL; P < .001), and psoas muscles (210 ± 51 vs 175 ± 34 [group 1] and 165 ± 38 mL [group 2], P < .001). Patients with FPLD also had significantly increased arm and leg muscle masses when measured by DXA (P < .001). Insulin sensitivity, assessed by insulin tolerance tests, was negatively correlated to the calf muscle volume.

Conclusions:

Female patients with FPLD have increased skeletal muscle volume and mass compared with those of normal women.

Familial partial lipodystrophy, Dunnigan variety (FPLD) is an autosomal dominant disorder caused by lamin A/C mutations. All patients have normal body fat at birth and during early childhood. During late childhood and extending into puberty, patients begin to lose sc fat in the extremities and truncal region. Simultaneously, increased deposition of fat occurs in some areas such as the face, chin, anterior and posterior neck, and inside the abdomen and the labia majora in women. Patients with FPLD are predisposed to metabolic complications such as insulin resistance, diabetes mellitus, hypertriglyceridemia, and hepatic steatosis (1). Patients have increased prevalence of acanthosis nigricans and polycystic ovary syndrome (PCOS) and develop a muscular appearance in the extremities and truncal region (2). It is, however, not known whether this muscular appearance is the result of sc fat loss or an increase in the size of the muscles.

The aim of this investigation was to compare both total and regional skeletal muscle mass in female patients with FPLD and matched normal control subjects using dual-emission X-ray absorptiometry (DXA) and to compare volume of the selected muscles in the 2 groups using magnetic resonance imaging (MRI). We also studied the relationship of increased muscle volume with insulin sensitivity in a subset of women with FPLD.

Subjects and Methods

The study protocol was approved by the institutional review board of the University of Texas Southwestern Medical Center, and all patients and control subjects signed an informed consent form. We obtained axial, whole-body MRI scans of 39 adult female patients with FPLD belonging to 21 different families (aged 18–65 years; 36.7 ± 13.7 years [mean ± SD]). Twenty patients had p.R482W LMNA mutation, 9 had p.R482Q mutation, 4 had p.K515E mutation, 2 had p.R582H mutation, and 1 each had a p.R582S, p.R62G, p.D192V, and p.K486N mutation. Within the FPLD group, 27 had hypertriglyceridemia, 16 had diabetes, 10 had hypertension, 8 had PCOS, 6 showed acanthosis nigricans, 3 were postmenopausal, and 1 had coronary heart disease.

Men were excluded from the study because of the relatively low number of patients with FPLD in our database. Because full skeletal muscle development occurs after puberty, we excluded women younger than 18 years old. MRI scans were performed using a quadrature body coil with thickness of 10 mm, gap of 5 mm, repetition time of 580 to 581 ms, echo time of 8 ms, matrix of 256 × 256 pixels, field of view of 450 mm, rectangular field of view of 65%, and number of signals averaged of 1 (Philips Intera, Andover, Massachusetts). DXA scans were performed using a Discovery W (S/N 80502) model machine (Hologic Inc, Bedford, Massachusetts).

Seventeen healthy women (ages >18 years with a body mass index [BMI] of 18–31.9 kg/m²), frequency-matched to the FPLD group by age and BMI, were recruited for control group 1. This number was chosen to provide enough power to detect differences in muscle volume seen in other studies (3). Subjects were excluded if they had medical problems requiring prescription drugs or supplements, were using tobacco, consumed more than 14 alcoholic drinks per week, or exercised more than 1 hour/d on average. Each received a full-body, axial MRI scan using the same parameters as those for the patients with FPLD, a DXA scan, and a blood test to screen for metabolic abnormalities. Axial abdominal MRI scans from another 129 healthy, frequency-matched women from the Dallas Heart Study were analyzed for psoas muscle volume, and their DXA scan data were also used for comparison.

MRI software was used to calculate the volume of skeletal muscles. In the thighs and calves, areas of signal intensity too low to be muscle or too bright to be muscle were removed, and the remaining voxel volume was calculated (Figure 1). The psoas muscle was traced directly, and the voxel volume was calculated. The thigh, calf, and psoas muscles were defined as, bilaterally, between the ischial tuberosity and 6 cm above the patella, between the proximal tibiofibular articulation and 4.5 cm superior to the distal tibiofibular articulation, and the entire psoas muscle above the pelvic brim, respectively. These areas were chosen because previous studies have examined these muscles to establish muscular hypertrophy (3). Measurements were validated for reproducibility with 2% variability over 3 measurements. Muscle mass in the extremities and soft tissue mass in the trunk was calculated from DXA scans by subtracting the bone mass and fat mass from the total mass. In addition, genotyping for LMNA mutations was performed in patients with FPLD.

Figure 1.

Axial magnetic resonance images from the thigh, calf, and abdomen of a female patient with FPLD and a control female. MRI of the thigh of a patient with FPLD (A) with fat in green background and bone in blue and skeletal muscle in dark gray. Compared to the thigh of a control subject (B), there is almost complete sc fat loss seen throughout the thigh, with only small amounts of fat in between the muscles. MRIs of the calf of a patient with FPLD (C) and a control subject (D) and of the abdomen of a patient with FPLD (E) and a control subject (F). Psoas muscles are outlined in green in panels E and F. There is almost complete loss of sc fat seen in the FPLD subject as well as visibly larger cross-sectional areas of most muscles.

Some patients with FPLD also underwent an intravenous insulin tolerance test with 0.2 U of regular insulin per kg body mass. Insulin was injected, and plasma glucose was measured at −1, 5, 10, 15, 20, and 30 minutes. The K constant (K_ITT) was calculated (4).

Statistical analysis

The primary endpoint of this study was the muscle volumes of the psoas, thigh, and calf muscles. The data were tested for normality and analyzed for differences between the FPLD group and normal control subjects using linear models, unadjusted and adjusted for combinations of age, height, and BMI. Secondary endpoints were lean muscle measurements taken by DXA scan. Comparisons were also made within the FPLD group among patients with different mutations, using analysis of covariance and 1-way ANOVA. Associations between muscle volume and insulin resistance variables were assessed with Spearman ρ correlations. Statistical analysis was performed using SPSS 19 (IBM Corporation, Armonk, New York) and SAS 9.3 (SAS Institute, Cary, North Carolina).

Results

Patients with FPLD, compared with control group 1, had 22% (Δ mean difference [95% confidence interval]: 1159 [396–1923] mL) higher thigh muscle volume, 30% (Δ 736 [372–1100] mL) higher calf muscle volume, and 19% (Δ 35.0 [11.5–58.6] mL) higher psoas muscle volume. Psoas muscle volume was also significantly higher in patients with FPLD compared with control group 2 (Table 1). Regression with adjustment for age and height and age and BMI yielded similar and significant differences in all comparisons.

Table 1.

General characteristics and skeletal muscle volume and mass of affected women with FPLD and the 2 control groups

Variable	FPLD (n = 39)	Control Subjects		P Values ANOVA (ANCOVA)
		Group 1 (n = 17)	Group 2 (n = 129)
Age, y	36.7 ± 13.7	34.8 ± 11.5	38.4 ± 7.7	.29
Weight, kg	66.7 ± 9.8	62.6 ± 5.6	67.0 ± 11.0	.29
Height, cm	163.3 ± 5.6	164.3 ± 5.6	162.0 ± 8.0	.38
BMI, kg/m2	25.0 ± 3.3	23.2 ± 2.2	25.5 ± 3.6	.05
Body fat, %	20.1 ± 5.5	33.5 ± 5.9	34.7 ± 6.2	<.0001
Psoas muscle volume, mLa	210 ± 51	175 ± 34	165 ± 38	<.0001 (<.0001)‡
Thigh muscle volume, mLa	6358 ± 1491	5198 ± 716	NA	.004 (.002)c
Calf muscle volume, mLa	3133 ± 713	2397 ± 335	NA	.0002 (<.0001)c
Arm muscle mass, gb	5827 ± 1,207	3771 ± 599	4389 ± 770	<.0001 (<.0001)c
Leg muscle mass, gb	16 100 ± 2725	12 954 ± 1527	13 888 ± 2483	<.0001 (<.0001)c
Trunk soft tissue mass, gb	25 208 ± 3930	18 800 ± 1940	21 037 ± 2777	<.0001 (<.0001)c

Abbreviation: ANCOVA, analysis of covariance; NA, not available.

^aDetermined by axial MRI.

^bDetermined by whole-body DXA scans.

^cP value adjusted for age, BMI, and height.

With DXA, patients with FPLD, compared with control group 1 had 55% (Δ 2055 [1557–2554] g) increased arm muscle mass, 20% increased leg muscle mass (Δ 3146 [1726 to 4566] g), and 34% increased trunk soft tissue mass (Δ 6408 [4685–8130] g). Similarly increased muscle mass was observed in the FPLD group in comparison with control group 2 (Table 1). The p.R482W mutation (n = 20) was associated with less psoas volume than other mutations when adjusted for age, BMI, and height (191.3 ± 45.6 vs 230.3 ± 50.2 mL; P = .03). The p.K515E mutation (n = 4) was associated with more psoas muscle volume than other mutations (252.2 ± 37.0 vs 205.5 ± 50.8 mL; P = .02).

The K_ITT values ranged from 1.29 to 1.41 and showed a significant negative correlation to calf muscle volumes when adjusted for height, age, and BMI (r = −0.68, P = .02, n = 15), but there was no significant correlation with the psoas or thigh muscle volumes.

Discussion

Many patients with FPLD report muscularity, most often noticed in the thighs and calves (5). Previously, objective assessment of differences in muscle mass or volume in patients with FPLD compared with those in unaffected individuals have not been done (1, 6, 7). Our study documents, for the first time, a significantly higher muscle volume and mass in women with FPLD compared with those in similarly sized healthy women using state-of-the-art MRI and DXA techniques. Although we did not perform muscle biopsies, a previous study in a small number of patients with FPLD (n = 4) has shown increased diameter of both type I and II muscle fibers compared with those in control subjects (6). In contrast with patients with congenital generalized lipodystrophy, who have skeletal muscle hyperplasia, patients with FPLD may have muscle hypertrophy to account for their increased muscle volume and mass (8).

The precise mechanisms of increased muscle mass and fiber hypertrophy are not clear. Increased muscle volume could be the result of myopathy. Mutations in LMNA can cause Emery-Dreifuss muscular dystrophy, limb girdle muscular dystrophy, and multisystem overlap syndrome (9–11). Both autosomal dominant Emery-Dreifuss muscular dystrophy and limb girdle muscular dystrophy present with calf hypertrophy and muscle weakness, yet we observed increased muscle volume in both the thighs and calves (7, 10, 12).

Increased muscularity could also be due to insulin resistance and specificity spillover effects of hyperinsulinemia on IGF-I receptors (13). IGF-I is important for increasing muscle mass due partially to the expression of different isoforms of Akt (14, 15). This pathway has been shown to play a role in load-induced hypertrophy of skeletal muscles. Thus, nonspecific activation of IGF-I receptors on muscles could influence muscle growth regulation (13).

In addition, FPLD predisposes women to development of PCOS, associated with high muscle mass (16). Thus, hyperandrogenemia and hyperinsulinemia may both play a role in increased muscle volume in women with PCOS (17). Our data suggested that with increased insulin resistance, there is increased volume of calf muscle.

Lastly, the muscle hypertrophy could also be due to a build up of lipids within the myocytes because insulin resistance has also been shown to be related to intramyocellular triglyceride levels, and the inability to store sc fat in the extremities and trunk may lead to aberrant deposition in the muscle (18). Studies of muscle metabolism suggest defects in metabolism due to lamin A/C dysfunction as well as alteration in systemic metabolism caused by lack of body fat. This could lead to aberrant lipid deposition; however, a previous study showed normal levels of lipids in the skeletal muscle (19).

In conclusion, We found that female patients with FPLD have increased skeletal muscle volume and mass compared with those of normal women. The exact molecular mechanisms involved in increasing muscle mass in patients with FPLD remain to be determined; however, it may be attributed to a combination of high insulin and androgens, myopathy, and aberrant lipid deposition.