Marfan syndrome is a hereditary connective tissue disease with high variability. Amongst over thirty different symptoms, the most prominent ones are lesions on the skeletal, cardiovascular, and ocular tissues . In clinical cases, ectasia of aorta and aortic dissection are the two most lethal and important phenomena of MFS . MFS caused by the FNB1 variant accounts for approximately 70–80% of all cases , while other cases may be caused by variants in potentially relative genes such as ACTA2, MYH11, MYLK, SMAD3, TGFBR1, TGFBR2, etc.
The FBN1 gene, located on chromosome 15, q21.1, encodes 2871 amino acids. Currently, over 2200 different variations have been discovered, and most of them relating to MFS. There are two major types of influences that FBN1 genetic variants can have on the structure of fibrillin-1: (1) nonsense variants, creating truncated proteins; (2) missense variants, leading to abnormal fibrillin-1 with mistakenly displaced amino acids . In some nonsense variants, mRNA containing premature translational-termination codon (PTC) rarely generates truncated proteins because they are disintegrated through a process known as nonsense-mediated mRNA decay (NMD) . In cases of missense variants, many displacements of amino acids influence components of crucial residues, such as the cysteine residue on cb-EGF structural domains. These displacements lead to possible mis-folding of structural domains and cause retention of the molecule in particular cases [14, 15]. Some other variants may affect the binding process between calcium and the cb-EGF domain, lowering the calcium affinity levels of the binding domain, altering the interaction patterns between cb-EGF domains in series with each other, and subsequently changing the rigidity of the molecule [16, 17].
In this research, we discovered that while the patient showed symptoms of classical MFS, a c.3217G > T heterozygous variant occurred on the26th exon of the FBN1 gene. This nonsense variant turned the 1073th amino acid, a Glutamic acid, into a stop codon. In the meantime, only a wild type base was found in the cDNA sequence of the patients and the target mRNA expression level was only 21% compared to the controls. Thus, we speculated that the cell degraded mRNA through NMD, which led to a significant decrease in the expression level of the fibrillin-1 protein, eventually causing MFS [18, 19]. Tjeldhorn  et al. pointed out back in 1995 that nonsense or missense variants could cause alterations of mRNA expression, which agreed with our results. The mRNA relative expression level of another MFS patient (FBN1 c.4414 T > C: p.Cys1472Arg) in our center tested by RT-PCR was 145.6% compared to the control samples, once again confirming the possible PTC mechanism.
On the other hand, a very similar variant of c.3217G > A, p.Glu1073Lys reported by Nijbroek , caused a severe symptom in newborn MFS. This variant affects the cb-EGD12 domain of the fibrillin-1 protein and each EGF-like domain has six highly conserved cysteine residues, which form disulfide bonds to establish the tertiary structure of the protein. The c.3217G > A: p.Glu1073Lys variant causes the replacement of glutamic acid by lysine, which changes the electric charge of the amino acid from negative to positive in the anterior of first cysteine in the domain. And it is highly correlated with a decrease in the binding affinity between the cb-EGF12 domain and calcium . “Dominant negative mutation” is a term for a pathogenic mechanism currently proposed for many autosomal dominant diseases, including MFS [23, 24]. According to this model, a mutant gene product has the capacity to impair the function of the wide-type protein produced by the normal allele. When considering a multimeric protein, interaction between mutant monomer and the wild-type product can result in the formation of abnormal multimer and hence the disease phenotype . This is why the variant of c. 3217 G > A caused a severe MFS symptom. The variant of c.3217G > T found in this study was a nonsense variant, which produced a premature translational-termination codon that reduces the stability of the variant transcript and consequently reduces protein production from the variant copy of the gene . The production of the protein was decreased, but the protein produced by wild type gene was still there, giving rise to a mild MFS phenotype by haploinsufficiency. The mechanisms and pathway affected by different variants at the same DNA position would probably lead to entirely different phenotypes. This hypothesis requires further studies to be verified.
In conclusion, we discovered a nonsense variant of the FBN1 gene, c.3217G > T, p.Glu1037Ter, which is very like to directly cause Marfan syndrome. This finding extended the variant spectrum of FBN1 gene and will provide a solution for patients to bear healthy offspring by preimplantation genetic testing or prenatal diagnosis. This study is based on data at the molecular level, and more experiments are needed to clarify the mechanism in the future.