Evidence Against the Causal Relationship Between a Putative Cis-Regulatory Variant of MYH3 and Intramuscular Fat Content in Pigs.

Cong Huang,Liepeng Zhong,Yizhong Huang,Liping Cai,Junwu Ma,Xiaoxiao Zou

doi:10.3389/fvets.2021.672852

Cong Huang, Liepeng Zhong + Show 4 more

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https://doi.org/10.3389/fvets.2021.672852

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Abstract

Improving meat quality has become the main goal of modern pig breeding. Intramuscular fat content (IMF) is an important trait influencing meat quality of livestock, but the molecular mechanism behind this trait is still unclear. Recently, Cho et al. reported the discovery of the first causal mutation affecting IMF and red flesh color (a*) in pigs, namely XM_013981330.2:g.−1805_−1810del, a 6-bp deletion variant in the porcine MYH3 promoter region. The objective of this study was to reassess the causality of this mutation for its potential commercial application. By Sanger sequencing, we firstly identified several new variants (including a 4-bp deletion) at or near the 6-bp deletion site, which formed four haplotypes in multiple breeds. Unexpectedly, the 6-bp deletion allele, previously determined as the MYH3 Q allele because of its significantly positive effect on IMF and a*, was found not only in Chinese indigenous breeds, but also in four western commercial breeds with relatively lower IMF levels, including Duroc, Large White, Landrace and Pietrain. More surprisingly, we found that the MYH3 Q allele and the haplotypes harboring it had no significant effects on IMF, marbling and color score in three large-scale divergent pig populations: the heterogeneous F6 and F7 pigs and commercial crossbred Duroc × (Landrace × Yorkshire) pigs. Transient transfection analysis in porcine satellite cells showed that the 6-bp deletion variants had a negligible effect on transcription of reporter gene, but could attenuate the MRF (myogenesis regulatory factors)-induced increase in luciferase activity of the MYH3 promoter vector. The MYH3 protein level in muscle did not differ significantly among the haplotype groups. Therefore, our results cannot support the causal relationship between the 6-bp deletion in MYH3 and IMF trait, suggesting that the causal mutation for the IMF QTL on SSC12 needs to be further identified.

Highlights

Intramuscular fat (IMF) content plays an important role in determining the eating quality of pork, such as tenderness, juiciness and flavor
We found that the MYH3 Q allele was present in Chinese native breeds and in four western commercial breeds including Duroc, Large White, Landrace and Pietrain
We noted that the PCR-RFLP with HpyCH4IV endonuclease can only distinguish the mutate type H2 from the other three haplotypes, while the two novel haplotypes H3 and H4, albeit harboring the 6-bp deletion and the 4-bp deletion, respectively, were wrongly judged as corresponding to the q allele as the wild-type haplotype H1 (Table 1)

Summary

Introduction

Intramuscular fat (IMF) content plays an important role in determining the eating quality of pork, such as tenderness, juiciness and flavor. A study conducted by Cho et al presented evidences that a 6-bp deletion (XM_013981330.2:g.−1805_−1810del) in the promoter region of MYH3 is the first causal mutation that was identified for IMF and red flesh color (a∗) in domestic pigs [11] They firstly identified a 488.1-kb critical region on porcine chromosome 12 (SSC12) that affects both IMF and a∗ by a joint linkage-linkage disequilibrium analysis in two independent F2 crosses between Korean native pigs (KNPs) and Western commercial breeds (Landrace and Duroc). Only the MYH3 gene, encoding myosin heavy chain 3, was found to be preferentially overexpressed in the skeletal muscle of KNPs than in that of Western commercial breeds They further validated MYH3 as a quantitative trait gene (QTG) using transgenic mice, and discovered the XM_013981330.2:g.−1805_−1810del variant in the 5′-flanking region of MYH3, which deletion (Q) allele carriers exhibited significantly higher values of IMF and a∗ than wild-type (q) allele carriers. They found that the MYH3 Q allele occurred exclusively in Asian domestic breeds (such as Chinese Neijiang, Chinese Putian, Chinese Xiang, and KNP) and Asian wild boars, but was almost absent in European and African wild boars, as well as European commercial pig breeds (including Large White, Landrace and Duroc, etc.), which indicates that this allele is of Asian origin

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