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    Biomechanics

    Sex and age specific thoracic aortic aneurysm outcomes in a mouse model of Marfan Syndrome

    (E-163) Sex and age specific thoracic aortic aneurysm outcomes in a mouse model of Marfan Syndrome

    Friday, October 13, 2023
    9:30 AM – 10:30 AM PDT
    Location: Exhibit Hall - Row E - Poster # 163
    Introduction::

    Thoracic aortic aneurysm (TAA) is characterized by progressive diameter dilation due to weakening of the aortic wall. TAA is associated with Marfan Syndrome (MFS), caused by mutations in fibrillin-1 (FBN1), an extracellular matrix (ECM) macromolecule [1]. Mutations in FBN1 cause alterations in other ECM constituents such as fragmented elastic fibers and improper deposition of collagen fibers, leading to changes in aortic geometry and compromised mechanical properties that contribute to TAA formation and eventual rupture. Male patients with MFS have a higher risk of TAA development, progression and rupture than age matched female patients with MFS, however the incidence of MFS is not sex dependent [2]. The mechanisms behind the sex specific outcomes of TAA are not clear. We hypothesize that time dependent changes in the mechanical properties of the aorta may differ in males and females and contribute to differences in TAA formation and progression. To address our hypothesis, we investigated geometry, wall structure, and biomechanics of the aorta in mgR/mgR (MU) (model of MFS) [3] and littermate wild type (WT) male and female mice at different ages (2, 3 and 4 months).The results of this study can provide biophysical biomarkers for predicting TAA outcomes and its management in males and females.



    Materials and Methods::

    We collected experimental data for MU and WT male and female mice at 2, 3, and 4 month of age. After genotyping, animals were separated into groups according to their genotype and sex where each group had 4-5 animals. Experimental steps included, cardiovascular ultrasound, biaxial mechanical tests, and structural characterization. For ultrasound, animals were maintained under 1.5-2% isoflurane anesthesia and a parasternal long axis view of the ascending aorta (AA) was obtained in B and M modes to measure systolic (SD) and diastolic (DD) diameters. Following the ultrasound, animals were euthanized by carbon dioxide inhalation in compliance with the Institutional Animal Care and Use Committee. Aortae were dissected, segmented into AA and descending aorta (DA) sections and then each segment was cannulated on a pressure myograph for biaxial mechanical tests. The inflation-extension tests on each vessel were performed according to the protocol described previously [4]. Experimental data such as pressure, diameter, and force corresponding to the in vivo stretch were used to determine the mechanical properties of the aorta. After mechanical testing, rings were cut to measure the unloaded vessel dimensions for stress-strain calculations. The incremental modulus was determined by the slope of the stress-strain curves at the mean blood pressure (105 mm Hg). For structural characterization, post-experimentation vessels were fixed in 4% paraformaldehyde, frozen in OTC compound, sectioned and imaged with multiphoton microscopy.



    Results, Conclusions, and Discussions::
    Survival analyses showed 50-60% mortality in male and 30-35% mortality in female MU mice up to 4 months of age with rupture of the ascending aorta in male MU mice. Compared to WT mice, MU mice had larger SD and DD at 3 and 4 months of age (Figure 1A), consistent with TAA formation. The percentage increase in SD and DD from 2 – 4 months of age was larger in MU male (81 and 130% increase in SD and DD, respectively) than MU female (38 and 98% increase in SD and DD, respectively) mice, showing faster growth of the TAA in males. Changes in the incremental modulus between 2 – 4 month old MU and WT mice was larger in male (370%) than female mice (170%), showing an increased rate of stiffening of TAA in males. The incremental modulus was higher overall in MU male compared to female mice (Figure 1B) and in AA compared to DA (supplementary Figure 1SA).  MU aorta showed a shift of the peak compliance towards lower pressure values, indicating the loss of elastic fibers [4]. These changes were more noticeable in male MU mice, AA, and at 4 months of age (Figure 1C, D; supplementary Figure 1SB, C). Preliminary imaging suggests increased elastic fiber fragmentation in MU aorta, with more fragmentation in males than females (Figure 1E). With low numbers in each group, statistical analyses were not performed at this time, but the study is ongoing with additional numbers expected to be included in next few months.


    Results show sex and age specific MFS variations in aortic geometry, mechanical properties, and elastic fiber fragmentation that can lead to sex specific TAA outcomes. The cause and effect relationships of these variables require further study. For example, it is not known if increased elastic fiber fragmentation in male MU mice allows increased diameter dilation or if increased diameter dilation in male MU mice puts stresses on improperly assembled elastic fibers causing fragmentation. Computational models of growth and development will be used in future work to better understand the biophysical mechanisms of TAA progression and identify biomarkers.


    Acknowledgements (Optional): : We are thankful to national institute of health for providing the research fund.

    References (Optional): :

    [1] Wagenseil, J. E., and Mecham, R. P., 2007, New Insights into Elastic Fiber Assembly, Birth Defects Res C Embryo Today.


    [2] Detaint, D., Faivre, L., Collod-Beroud, G., Child, A. H., Loeys, B. L., Binquet, C., et al., 2010,  Cardiovascular manifestations in men and women carrying a FBN1 mutation. Eur. Heart J. 31, 2223–2229.


    [3] Pereira L, Andrikopoulos K, Tian J, Lee SY, Keene DR, Ono R, Reinhardt DP, Sakai LY, Biery NJ, Bunton T, Dietz HC, Ramirez F. 1997, Targetting of the gene encoding fibrillin-1 recapitulates the vascular aspect of Marfan syndrome. Nat Genet. 17(2):218-22.


    [4] Le VP, Cheng JK, Kim J, Staiculescu MC, Ficker SW, Sheth SC, Bhayani SA, Mecham RP, Yanagisawa H, Wagenseil JE , 2015, Mechanical factors direct mouse aortic remodelling during early maturation. J R Soc Interface. 2015 6;12(104):20141350.