A Chinese study has for the first time identified a key protein, known as LIMA1, which was shown to regulate intestinal cholesterol absorption in both mouse models and in humans.
Inhibition of that protein will potentially provide a new direction for treating hypercholesterolemia, researchers led by Bao-Liang Song reported in the June 7, 2018, edition of Science.
A high concentration of low-density lipoprotein cholesterol (LDL-C) is an established risk factor for cardiovascular disease (CVD), the leading cause of mortality worldwide. Although cholesterol-lowering statins and inhibitors of cholesterol absorption, such as Zetia (ezetimibe, Merck & Co. Ltd.), are currently available, there remains a need for new, more effective treatments.
"Despite the availability of these therapeutics, the prevalence of CVD continues to rise, and many individuals are intolerant of statins and/or ezetimibe, or are unable to reach their target LDL-C levels using these strategies," said Song, a professor in the Department of Biochemistry of the College of Life Sciences at Wuhan University.
"Thus, there remains a need to identify targets and therapeutics that provide alternative ways to lower LDL-C and treat CVD, and inhibition of LIMA1 may provide a new direction for treating hypercholesterolemia."
The LDL-C concentration is complex and known to be influenced by both environmental and genetic factors. While around 40 to 50 percent of phenotypic differences in LDL-C are due to genetic factors, those have not been fully characterized.
For example, in a large-scale analysis, just 2.5 percent of subjects with severely high LDL-C had known genetic variants for familial hypercholesterolemia. Moreover, 10 to 20 percent of total variances in LDL-C could be attributed to single-nucleotide variants (SNVs) identified by genome-wide association studies.
In a previous cardiovascular risk survey of 14,122 persons, including 3,919 Kazakhs in western China, researchers had identified a Kazakh family with inherited low levels of LDL-C.
"To our knowledge, this was the first CVD survey done in Kazakhs," Song told BioWorld Asia.
A major ethnic group in western China, Kazakhs exhibit marked differences in SNVs across their genomes. They also live in remote regions and usually marry within their own ethnic group, making them particularly suitable to characterize ethnic-specific variants associated with LDL-C.
"Chinese Kazakhs eat more meat but have a lower incidence of CVD than other populations such as Han Chinese," said Song.
To identify causal SNVs, the researchers took samples from three Kazakh individuals who had low LDL-C and one with normal LDL-C. Those were then analyzed using whole-exome sequencing.
Song and his team identified a rare frameshift variant in the LIM domain and actin binding 1 (LIMA1) gene from the Kazakh family with inherited low LDL-C and reduced cholesterol absorption.
A frameshift variation is a genetic mutation caused by insertions or deletions of a number of nucleotides in a DNA sequence that is not divisible by three.
"We performed whole-exome sequencing to identify the mutation in the LIMA1 gene, which revealed an 8-nucleotide deletion in the coding region of LIMA1 causing the reading frameshift variation," said Song.
The researchers then studied the LIMA1 gene in a mouse model, in which it was found to be mainly expressed in the small intestine and localized on the brush border membrane.
The small intestine accounts for approximately half of daily cholesterol input, being the major tissue responsible for cholesterol absorption, which is mediated by the key transmembrane protein NPC1L1.
"The brush border membrane of the intestinal lumen is in direct contact with food, so the LIMA1 protein being localized in that structure is consistent with the protein's function in cholesterol absorption," explained Song.
The researchers then used techniques including immunostaining to demonstrate that LIMA1 bridged NPC1L1 to a transportation complex containing the protein myosin Vb and facilitated cholesterol uptake.
"NPC1L1 is known to be a key protein for cholesterol absorption and myosin Vb is essential for vesicle transport, so this finding reveals the mechanism whereby LIMA1 affects cholesterol uptake," said Song.
Similar to the human phenotype, LIMA1-deficient knockout mice displayed reduced cholesterol absorption and were resistant to diet-induced hypercholesterolemia.
"Inhibiting LIMA1 could therefore decrease cholesterol absorption and prevent hypercholesterolemia in humans, even if they eat too much fat. In addition, our human studies have shown that subjects with the LIMA1 mutation were healthy, so LIMA1 could be a safe new drug target to prevent CVD," said Song.
Although new targets and therapeutics for lowering LDL-C and treating CVD await identification, LIMA1 inhibition may hold the key to future such treatments.
"Our next step will be to find a compound to inhibit LIMA1, while we are also working to find new genes regulating cholesterol metabolism," said Song.