Alpha-1 antitrypsin deficiency liver disease

　　One, Etiology

　　Alpha-1 antitrypsin (α1-AT) deficiency liver disease is a congenital metabolic disease caused by the deficiency of a protease antagonist component, alpha-1 antitrypsin, in serum.

　　Two, Pathogenesis

　　1. Serum proteases produced in all body cells can trigger an inflammatory cascade and complement activation in target organs. There is also a group of substances in the normal human body that inhibit protease activity, known as proteinase inhibitors (proteinase inhibitor), which are widely distributed in body fluids such as plasma, lymph, urine, saliva, tears, bronchial secretions, cerebrospinal fluid, cervical mucus, seminal fluid, colostrum, and cytoplasm of some tissue cells. Proteinase inhibitors are involved in various physiological and pathological processes. Alpha-1 antitrypsin (α1-AT) is the main protease inhibitor in serum, a glycoprotein synthesized by liver cells with a molecular weight of 50,000 to 60,000, which is located at the α1 position during electrophoresis separation. After being released into the plasma, it constitutes the main component of α1-globulin, accounting for about 90% of α1-globulin and 4% of all serum proteins. α1-AT is an acute phase reaction protease inhibitor that can inhibit various endogenous and exogenous proteases, such as trypsin, chymotrypsin, elastase, thrombin, plasmin, kallikrein, and products with protease properties from certain bacteria and viruses. It also has multiple functions such as neutralizing toxins, clearing toxins, controlling infection and inflammation, and preventing self-digestion. It can also stimulate the release of α1-AT during inflammation, stress, trauma, pregnancy, or tumor, and the level of this enzyme in serum can increase.

　　2、α1-AT与弹性蛋白酶形成1∶1的牢固的复合体，以后在血液循环中被分解。这种复合体可与肝细胞的血清蛋白酶受体结合，从而刺激肝细胞产生α1-AT。在正常情况下，α1-AT可控制90％以上的血清弹性蛋白酶的活性。α1-AT分子上的唾液酸(N-乙酰神经氨酸)对其生物活性非常重要，祛除唾液酸的残基α1-AT即失去活性，并且很快从血液循环中清除。不含唾液酸残基的α1-AT在肝细胞合成后不能被释放入血，蓄积在肝细胞，可引起肝细胞损伤。

　　3、迄今有关α1-AT缺乏的家系研究报告表明，α1-AT等位基因(alleles)呈等显性表达蛋白酶抑制物基因(Pi基因)。Fagerhol等认为，控制α1-AT合成的所谓Pi基因是位于常染色体上的等位基因。采用薄层凝胶聚焦技术分析人类α1-AT电泳迁移率，发现其在人群中存在多态现象。现已鉴定出75种以上的α1-AT变异体，但大多数无临床意义或很罕见，分别命名为B、C、D、E、F、G、L、M、N、P、S、V、W、X、Z等。各等位基因分别用PiM、PiS、PiZ等表示。纯合子的基因型用PiMM、PiSS等表示，杂合子用PiMZ、PiSZ等表示。以上统称为Pi基因系统。编码α1-AT的基因定位于14号染色体长臂(14q24.3-32.1)。Pi基因系统的各种表现型，其血清蛋白酶抑制活性与α1-AT的浓度是不同的。PiM是具有正常功能的基因，绝大多数正常人是PiM的纯合子(PiMM)，其血清中α1-AT含量正常，功能也正常。具有PiZ基因的纯合子(PiZZ)个体血清中α1-AT含量严重缺乏，仅为正常人的15％左右，这种人常发生阻塞性肺病和幼年型肝硬化。具有纯合子PiSS血清中α1-AT含量中度缺乏，约为正常人的60％，这种人亦有患肺气肿和肝硬化的倾向。杂合子PiMZ、PiSZ等个体也有发生肺气肿和肝硬化的倾向。Jeppson等分析肽图(peptidmapping)发现α1-AT缺乏症PiZZ变异型蛋白肽链中一个谷氨酸被赖氨酸所取代，一个谷氨酸被谷氨酰胺所取代，PiSS变异型系谷氨酸被缬氨酸所取代。

　　4. α1-AT is produced in the rough endoplasmic reticulum of liver cells and transported to the Golgi apparatus for secretion. There is a hypothesis that protein misfolding (misfolding) conformation related to allele mutations may cause α1-AT to remain in the endoplasmic reticulum and not be released to the Golgi apparatus. Due to this misfolding change, the normal hidden region may be exposed, thus contacting different ligand receptors and not acting as an effective molecule for release. Abnormal α1-AT retention in the endoplasmic reticulum leads to accumulation and reduced secretion. The degradation rate of α1-AT within the cell depends on gene regulation. The pathophysiology of liver cell damage caused by α1-AT deficiency is still controversial. Currently, it is believed that liver damage is secondary to the accumulation of α1-AT in the rough endoplasmic reticulum of liver cells and may alter the degradation of abnormal α1-AT within the liver cells. Homozygous and heterozygous liver cells of α1-AT deficiency patients show periodic acid Schiff (PAS) positive amylopectin granules, supporting this hypothesis.

　　5. Individuals with α1-AT deficiency have three outcomes: some may remain healthy throughout their lives; most develop severe emphysema in their middle and young adulthood; and some have liver diseases since infancy. However, it is rare for people to have both emphysema and liver cirrhosis. It is still unclear why some individuals develop severe liver diseases while others remain asymptomatic. It is generally believed that liver damage is caused by multiple factors. For example, elastase can break down elastic fibers, leading to emphysema lesions. However, under normal circumstances, elastase inhibitors can suppress the activity of this enzyme, preventing emphysema. Studies have found that PiZ is more susceptible to chronic obstructive pulmonary disease (COPD). Congenital α1-AT deficiency has a genetic susceptibility, which needs to be combined with external factors to produce pathogenic effects. Smoking poses a greater risk, as smokers have an increased number of pulmonary macrophages, larger lysosomes, and the NO2 produced by tobacco combustion can stimulate the release of elastase by pulmonary macrophages and neutrophils. Due to the weakened ability to inhibit proteases, α1-AT deficient individuals are more prone to lung tissue damage, leading to chronic obstructive pulmonary disease. There is no relation between liver cirrhosis caused by α1-AT deficiency and pulmonary diseases. α1-AT deficiency is the main factor in α1-AT deficiency liver disease, and other factors are also involved. An increased activity of proteases in the body increases the susceptibility of the liver to other pathogenic factors and toxic substances, leading to liver damage. Gam suggests that it may also be due to the destruction or defect of the intestinal barrier, where toxins are absorbed into the liver, and the lysosomal enzymes are taken up and released by the liver Kupffer cells. When the body lacks α1-AT, this enzyme becomes destructive; or due to the accumulation of α1-AT in liver cells, when enterotoxins enter the liver, the protective proteinases in liver cells are excessively inhibited by α1-AT, leading to liver cell damage; or due to an excessive amount of α1-AT in liver cells, it inhibits the production of endogenous proteases in the liver, resulting in the inability to counteract enterogenous toxic substances, thus causing liver damage.

　　Splenomegaly, upper gastrointestinal bleeding caused by esophageal varices, and if there is no improvement in neonatal liver cholestasis, it may gradually develop progressive liver damage, progressing to liver cirrhosis and even death. Pathological histological changes vary with patient age. Liver biopsy of affected infants shows a lack of bile ducts, bile stasis within liver cells with or without cell swelling, mild inflammatory changes or fatty changes, and liver cells may show characteristic PAS-positive amylase-like bodies, which can be strongly stained with fluorescent-labeled α1-AT antisera and have α1-AT antigenicity. These granular inclusions are located in the endoplasmic reticulum of liver cells. The lack of α1-AT in patients is due to the inability of synthesized α1-AT to be released into the blood, accumulating in liver cells with the pure heterozygous PiZZ phenotype. Patients with α1-AT deficiency in infancy may develop progressive liver damage if there is no improvement.

　　1. Liver disease due to α1-AT deficiency can be discovered for the first time during infancy, or there may be no liver disease manifestation during this period. Chronic liver disease symptoms may appear in adulthood. 8% to 12% of newborns with PiZZ type α1-antitrypsin deficiency develop cholestatic jaundice within one month after birth, with serum bilirubin levels as high as 340μmol/L, and serum alkaline phosphatase (ALP) activity reaching 150 to 1300U/L. Children may show slow weight gain, drowsiness, irritability, and the appearance of acholic stools. Half of children at 3 months of age show elevated transaminase levels, with serum aspartate aminotransferase (AST) activity reaching 80 to 600U/L. 12% to 15% of α1-antitrypsin deficiency patients develop liver cirrhosis, manifested by abdominal distension, splenomegaly, upper gastrointestinal bleeding caused by esophageal varices, and other bleeding tendencies such as purpura. Most patients show persistent cholestasis in the neonatal liver for about 7 months to 1 year, which may regress. If there is no improvement, it may gradually develop progressive liver damage, progressing to liver cirrhosis and even death. Liver cirrhosis caused by α1-antitrypsin deficiency can also occur in adulthood, but it is rare for those who develop liver cirrhosis in middle and old age. Adult onset is more common in patients with heterozygous α1-AT deficiency liver disease, with a slow progression and varied clinical manifestations. There are reports that the risk of liver failure in adult patients with heterozygous α1-AT deficiency liver disease is significantly increased. Sreger reported that among 120 PiZZ-type newborns, 14 had long-term obstructive jaundice, neonatal hepatitis, and childhood liver cirrhosis, adult emphysema. Adult α1-antitrypsin deficiency liver cirrhosis may be asymptomatic, but can present with the clinical manifestations of liver cirrhosis, and may also be associated with liver cancer, more common in patients with homozygous α1-AT deficiency over 50 years old.

　　2. For any patient with non-infectious chronic hepatitis, unexplained splenomegaly, liver cirrhosis, and portal hypertension, the possibility of alpha-1-AT deficiency liver disease should be considered, and it should also be cautious about the possibility that a few cases of liver cancer are caused by alpha-1-AT deficiency.

　　Serum protein electrophoresis often shows a lack of alpha-1 globulin, which commonly suggests the disease. Direct measurement of alpha-1-antitrypsin can confirm the diagnosis, but attention should be paid to genetic phenotypic analysis, as the production of alpha-1-AT is affected by these factors. Therefore, diagnosis should be based on phenotypic analysis, not solely on the detection of alpha-1-AT levels.

　　1. Abstain from smoking

　　Patients with alpha-1-antitrypsin deficiency should abstain from smoking absolutely, as smoking can worsen alpha-1-antitrypsin deficiency pulmonary emphysema.

　　2. Prevention and treatment of complications

　　For patients with only mild liver damage, long-term supportive treatment is required; for those with portal hypertension, portal caval or splenorenal shunt surgery can be performed.

　　3. Liver transplantation

　　Liver transplantation should be performed for patients with severe liver damage, as the liver is the only place for the synthesis of alpha-1-antitrypsin. Therefore, liver transplantation not only cures liver disease but also corrects the deficiency of alpha-1-antitrypsin. It is now considered an effective method for treating the end-stage liver cirrhosis of Pizz. Liver transplantation using donor livers with PiMM phenotypes is expected to improve survival rates and improve the condition.

　　4. Liver gene therapy

　　Prospects are promising, but it is still difficult to achieve. Correcting the abnormal expression of alpha-1-antitrypsin is the key to preventing the occurrence of liver damage and controlling its progression. Zern successfully suppressed the abnormal expression of alpha-1-antitrypsin in a liver tumor cell line using specific ribozymes, achieving an inhibition rate of 70%. This lays the foundation for preventing the occurrence of liver damage due to alpha-1-antitrypsin deficiency.

　　1. Measurement of serum alpha-1-antitrypsin concentration: a reduction of 10% to 15% compared to normal may be helpful for diagnosis but cannot confirm it, as the concentration of serum alpha-1-antitrypsin may increase during acute inflammation.

　　2. Phenotypic analysis of alpha-1-antitrypsin using isoelectric focusing or agarose gel electrophoresis under acidic conditions can establish a diagnosis. Currently, PCR technology is used to detect variants of alpha-1-antitrypsin, which is not only rapid and highly sensitive but also requires only a small amount of cellular material. This technology is useful for confirming the diagnosis, population screening, and prenatal diagnosis.

　　3. Liver biopsy: shows liver cirrhosis, PAS staining shows characteristic inclusions in liver cells, and fluorescence staining shows blue granules accumulated in liver cells, i.e., α1-antitrypsin antibody fluorescence band.

　　Patients with α1-antitrypsin deficiency liver disease should consume foods rich in fat-soluble vitamins in conjunction with medication. Those with α1-antitrypsin deficiency should absolutely refrain from smoking, as smoking can worsen α1-antitrypsin deficiency emphysema..

　　1. α1-antitrypsin supplementation therapy

　　It aims to increase the endogenous α1-antitrypsin release in the liver, thereby increasing the activity of elastase inhibitors, and like reducing lung damage, it aims to reduce liver damage. However, this method also increases the binding of α1-antitrypsin to the serum protease complex receptor, which stimulates the increase of abnormal α1-antitrypsin products, leading to their accumulation in liver cells, thereby aggravating the damage to the liver. Therefore, this method is not suitable for the treatment of α1-antitrypsin deficiency liver disease.

　　2. Liver Transplantation

　　It has been used to treat patients with advanced α1-antitrypsin deficiency liver disease, which is one of the most common metabolic liver diseases suitable for liver transplantation treatment. Liver transplantation, in addition to replacing the damaged liver, can also correct metabolic defects to prevent progression to systemic lesions.

　　α1-antitrypsin deficiency liver disease is one of the many diseases that can be considered for gene therapy to reconstruct the normal genotypes. Its potential benefits are to reduce the need for liver transplantation. The gene therapy for α1-antitrypsin deficiency liver disease is to add the normal α1-AT gene within the genome of α1-AT-deficient liver cells, so that the cells can synthesize normal α1-AT. In addition, other treatment methods have also been studied. Such as developing serum protease complex receptor blockers (serping-enzymecomplexreceptorblocker) for reducing the production of abnormal α1-AT, or blocking the endoplasmic reticulum α1-AT binding site to avoid effectively secreting abnormal α1-AT. This is the forefront of treatment research.