Clinical Significance of Benign Liver Function Abnormalities

Liver function tests, also known as liver biochemical index tests, are important methods for assessing liver damage or liver function decompensation. These tests typically include indicators such as TBil, DBil, IBil, ALT, AST, ALP, GGT, TBA, ChE, and Alb. Abnormal liver biochemical indicators usually indicate liver damage and are important references for diagnosing and assessing various acute and chronic liver injuries. Some genetic and immune factors can cause abnormal liver biochemical indicators without accompanying liver damage, and their long-term prognosis is generally good. The author refers to this condition as “benign liver function abnormalities,” and this article will provide a review of such diseases.

1Benign Hyperbilirubinemia

1.1 UGT1A1 Gene Disorders

The UGT1A1 gene is located on the long arm of chromosome 2 and consists of 13 promoters and 5 exons. Mutations in the UGT1A1 gene lead to impaired glucuronidation of IBil, resulting in hyperbilirubinemia. Clinically, it is classified into three types based on enzyme activity: Gilbert syndrome (GS), Crigler-Najjar syndrome type I (CNS-I), and Crigler-Najjar syndrome type II (CNS-II). CNS-I patients have severely reduced or absent enzyme activity, leading to severe jaundice and high mortality; GS and CNS-II are generally benign and do not progress to liver fibrosis or cirrhosis.

GS is the most common, with an incidence of 5% to 10%. It was first reported by Gilbert in 1901 and is considered an autosomal recessive hereditary disease, although some studies suggest it may be an autosomal dominant disorder with incomplete penetrance. GS patients have UGT1A1 activity at about 30% of normal levels, presenting with mild fluctuating jaundice. Jaundice may worsen under conditions such as fasting, exercise, emotional stress, pregnancy, and infection, but usually does not accompany any organic liver lesions, indicating a benign process. Microscopic examination shows normal liver tissue structure except for increased lipofuscin in liver cells. Current research indicates that GS has significant heterogeneity, resulting from the interaction of environmental and genetic factors. There are differences in UGT1A1 gene mutation polymorphisms among different ethnic groups. In populations of East Asian countries such as China and Japan, the p.G71R mutation is more common, while in Western populations, the A(TA)7TAA mutation is more prevalent. Additionally, mutations at c.686C>A in exon 1, c.1091C>T in exon 4, and c.1456T>G in exon 5 can also cause increased IBil. Therefore, comprehensive gene testing of the UGT1A1 gene is helpful for the early diagnosis of GS. Patients with GS generally have normal liver physiological functions and do not require special treatment. Some studies have suggested that bilirubin, rather than being merely a metabolic waste product, also acts as an antioxidant, possessing anti-inflammatory properties, protecting vascular endothelium, regulating metabolism, and modulating immunity. GS, through its mild hyperbilirubinemia, may provide protective effects against complications of diabetes, cardiovascular diseases, metabolic syndrome, non-alcoholic fatty liver disease, and colitis, but the specific mechanisms require further research.

In the 1960s, foreign scholars classified a type of benign jaundice responsive to phenobarbital as CNS-II. Patients with CNS-II have UGT1A1 activity of about 10% to 30% of normal levels and often present with persistent jaundice, which may worsen during infections or pregnancy. After phenobarbital treatment, bilirubin levels can decrease by more than 25%, and once bilirubin levels reach appropriate levels, medication can be gradually stopped, with a good prognosis. The physiological functions and intellectual development of CNS-II patients are generally unaffected. In CNS-II patients from China and Japan, p.G71R and p.Y486D homozygous or heterozygous mutations are common, while the single p.G71R mutation usually does not lead to significant reductions in UGT1A1 activity, but when combined with other mutations, it can cause CNS; the p.Y486D mutation affects the glucuronidation of all UGT1 family subtypes, leading to more severe manifestations. Although CNS-II patients rarely develop bilirubin encephalopathy, there remains a risk of neurological damage if timely diagnosis and treatment are not provided during fasting, infections, or pregnancy. Therefore, timely diagnosis, active treatment, and prevention of risk factors are crucial.

1.2 Dubin-Johnson Syndrome (DJS)

DJS is an autosomal recessive disorder caused by mutations in the organic anion transporter protein (ABCC2) gene, characterized primarily by elevated DBil. Liver biopsy specimens show a homogeneous black color, and microscopic examination reveals diffuse black pigment granules within liver cells. The ABCC2 gene is located on chromosome 10 and encodes multidrug resistance-associated protein 2 (MRP2), which is expressed on the canalicular membrane of liver cells and is responsible for the biliary excretion of various toxins. DJS patients experience elevated DBil due to the loss of MRP2 protein activity or synthesis impairment caused by ABCC2 mutations, leading to impaired secretion of DBil and other organic anions into the bile. The loss of MRP2 function can upregulate the transport function of MRP3, which can transport DBil back into the plasma from liver cells, further elevating DBil levels. DJS patients generally have good health and do not require special treatment, although symptomatic treatment using ursodeoxycholic acid may be applied in some cases of cholestasis. The mechanism of cholestasis in DJS remains unclear, with some studies suggesting that MRP2 may also mediate bile acid transport, requiring further investigation.

1.3 Rotor Syndrome (RS)

RS is an autosomal recessive disorder caused by homozygous or compound heterozygous mutations in the SLCO1B1 and SLCO1B3 genes, leading to functional defects in OATP1B1 and OATP1B3 proteins. The transfer of DBil from liver cells to the blood is mediated by ABCC3, and the reuptake of DBil back into liver cells is mediated by OATP1B1/OATP1B3, with any defects in this reuptake leading to elevated DBil levels in RS patients. A total of 26 mutations in the SLCO1B1 gene and 10 mutations in the SLCO1B3 gene have been identified. Recent studies have found that in some RS patients, the insertion of the retrotransposon LINE-1 into the SLCO1B1 gene can cause disease through transcriptional repression and promotion of exon inversion or skipping. Microscopic examination of the liver lobules in RS patients shows no specific changes, and generally, no special treatment is required. The loss or dysfunction of OATP1B1/OATP1B3 proteins may affect the hepatic uptake and clearance of certain drugs, such as ursodeoxycholic acid, fibrates, non-steroidal anti-inflammatory drugs, beta-blockers, rifampicin, anticancer drugs, antiviral drugs, and antifungal drugs. In RS patients, the use of these drugs may exacerbate jaundice. Therefore, genetic analysis should be performed to confirm the diagnosis of RS and guide therapeutic drug prescriptions and genetic counseling.

2Benign Elevation of Aminotransferases

Macroenzymes are enzyme complexes formed in plasma under pathological or physiological conditions through self-aggregation or binding with other components in plasma (mainly immunoglobulins). Their molecular weight is larger than that of ordinary enzymes, preventing them from passing through the glomerulus, leading to prolonged clearance time in the body, thus maintaining elevated activity for a long time. The exact cause of macroenzyme formation is unclear, but some studies suggest it may be related to immune factors. Macro-AST is a type of macroenzyme first reported by Konttinen et al. Elevated serum AST activity is considered a basic biochemical marker for liver, heart, muscle, endocrine, and metabolic disorders, but macro-AST is usually a benign condition. Triester et al. reported a case of a 63-year-old male who developed macro-AST after receiving allergen-specific immunotherapy for allergic rhinitis, suggesting that the formation of macro-AST may be a cross-reaction related to the allergen-specific immune response. Kulecka et al. screened 32 suspected familial macro-AST patients using whole-exome sequencing and verified variations in 92 related family members and 1,644 healthy controls, discovering a genetic variant (p.Gln208Glu, rs374966349) associated with macro-AST in the GOT1 gene. Analysis indicates that the negatively charged glutamic acid on the surface of GOT1 strongly adsorbs serum immunoglobulins, which may be one of the reasons for the formation of enzyme-immunoglobulin complexes. AST has two similar isoenzymes, located in the cytoplasm and mitochondria, respectively, encoded by GOT1 and GOT2 on chromosomes 10q24 and 16q12.

When macro-AST is suspected as the cause of abnormal AST activity, macro-AST detection can be performed using methods such as: (1) Cold storage method, which is the simplest clinical method, involves storing the plasma or serum of macro-AST patients at 2-8 °C for 48 hours. If AST activity decreases by more than 65%, it suggests the possibility of a macroenzyme, requiring further confirmation through precipitation methods. (2) Precipitation method, where polyethylene glycol or protein A/G is incubated with macro-AST serum to form precipitated immune complexes, with sensitivity and specificity of 82.4% and 88.9%, respectively. (3) Protein electrophoresis, where an abnormal band can be seen between soluble AST (cAST) and mitochondrial type (mAST), indicating the presence of macro-AST. Further application of specific serum antibodies can determine the subtype of immunoglobulin. (4) Gel filtration chromatography, which uses gels with certain pore sizes as the chromatography medium to separate substances of different molecular weights and shapes. The consistency between the precipitation method and electrophoresis method is good, but the precipitation method is simpler and can serve as a preliminary screening method. Currently, the most commonly used method for detecting macro-AST is the polyacrylamide gel S300 chromatography method, where large molecular weight macro-AST is eluted before normal AST, but this technique cannot determine which immunoglobulin AST is bound to.

Since the discovery of macro-AST, there has been no definitive evidence to establish whether macro-AST is related to any diseases. Shah-Khan et al. followed up a 66-year-old female with macro-AST for eight years, during which her AST peaked at 616 U/L, but the patient remained healthy with no abnormalities found in abdominal imaging. Testing with the polyethylene glycol precipitation method confirmed the presence of macro-AST. Fortunato et al. reported 10 children with persistently elevated AST without clinical symptoms, of which 4 were macro-AST. These children underwent liver function tests every six months to a year, and at the end of the follow-up period of 0.5 to 6.5 years, all were healthy. Available data suggest that macro-AST may represent a benign process that does not require excessive investigation or intervention, merely regular follow-up.

3Benign Elevation of ALP

Serum ALP originates from the bone, liver, kidney, intestine, and placenta. Routine biochemical tests cannot distinguish different ALP isoenzymes, but specialized electrophoresis techniques can identify them. ALP elevation is usually due to cholestasis; for example, simultaneous elevation of ALP and GGT suggests that the liver is the source of ALP elevation. Due to the presence of ALP subtypes in the placenta, serum ALP may also be elevated during pregnancy. Intestinal ALP can be detected in about 40% of healthy subjects. The presence of macro-ALP can also lead to elevated serum ALP. Cervinski et al. monitored a 74-year-old male with persistently elevated serum ALP for four years, with a peak of 1,034 U/L and stable levels at 600 U/L. During follow-up, the elevation of ALP was not accompanied by an increase in GGT, and the patient underwent CT and ultrasound examinations that ruled out tumors, bone diseases, and other causes of ALP elevation. Some studies have shown that patients with ulcerative colitis have a higher probability of macro-ALP than the general population, but no specific ALP isoenzymes have been found in macro-ALP. The exact mechanism of macro-ALP remains unclear and may be related to immune factors, requiring further research.

4Benign Elevation of GGT

Plasma GGT primarily originates from the liver and is associated with a variety of hepatobiliary diseases. De Grandi et al. studied two families with hypergammaglobulinemia where GGT fluctuated between 2,500 and 9,600 U/L. The metabolic substrates of GGT in the studied cases (including plasma amino acids, amino salts, cysteinyl leukotrienes, whole blood glutathione, urinary leukotriene E4, etc.) were all normal. Mutations in the GGT1 gene, such as c.44T>G (p.Leu15Arg) or c.28_54del (p.Leu10_Val18del), were detected in the cases, suggesting that GGT1 gene mutations can lead to autosomal dominant familial hypergammaglobulinemia. GGT consists of big GGT (b-GGT), medium GGT, small GGT, and free GGT (f-GGT). b-GGT is contained within vesicles and is released from the cell membrane into bile and plasma. GGT1 gene mutations disrupt the transmembrane domains of GGT1, causing hypergammaglobulinemia, but there are no related clinical symptoms. Analysis shows that the composition of GGT in patients has significantly changed, with b-GGT activity within normal ranges, but medium, small, and f-GGT activities significantly elevated, with f-GGT activity accounting for approximately 97% of total activity. f-GGT represents a form of the enzyme lacking the N-terminal anchoring peptide, produced by mutant alleles of GGT1. For patients with unexplained elevated GGT levels, GGT1 mutations and plasma GGT component analysis should be considered to reduce unnecessary repeat and invasive diagnostic tests.

5Benign Elevation of Bile Acids

Bile acids are steroid molecules synthesized in the liver and secreted into the bile ducts via the bile salt export pump on the canalicular membrane, subsequently entering the intestine, where they are reabsorbed at the terminal ileum and returned to the liver. The sodium-taurocholate co-transporting polypeptide (NTCP) on the liver sinusoidal side reabsorbs bile acids back into liver cells. Mutations in NTCP can lead to impaired NTCP function, preventing bile acids from being reabsorbed into liver cells and resulting in hypercholanemia. In 2015, Vaz et al. first reported congenital bile acid uptake disorders due to NTCP mutations. Erlinger reported a case of a child with significantly elevated serum bile acid levels without pruritus, cholestasis, liver function abnormalities, or liver diseases. Sequencing of the SLC10A1 gene encoding NTCP revealed a homozygous point mutation c.755G>a, which significantly reduced the uptake activity of taurocholic acid. Studies have indicated that insufficient bile acid uptake does not affect bile acid synthesis, and the FGF19-FGFR4 signaling pathway plays an important role in regulating bile acid synthesis. Chinese scholars established a cohort of children with NTCP deficiency and analyzed the mutation spectrum and clinical manifestations of SLC10A1, with 113 children diagnosed with NTCP deficiency from 109 unrelated families through SLC10A1 gene testing, discovering five SLC10A1 mutations: c.800C>T, c.263T>C, c.595A>C, c.374dupG, and c.682_683delCT, with c.800C>T accounting for 94.50% of all mutant alleles. Except for hypercholanemia, all clinical symptoms and laboratory abnormalities gradually disappeared or normalized in all children. Notably, vitamin D deficiency and elevated ALP gradually disappeared by six months of age, indicating that NTCP deficiency is a common benign condition. Clinicians should consider the possibility of NTCP mutations in cases of elevated bile acids without cholestatic clinical manifestations to reduce misdiagnosis and mistreatment.

6Congenital Pseudocholinesterase Deficiency

ChE is a class of hydrolases that exist in various isoenzyme forms, generally classified as acetylcholinesterase (AChE) and butyrylcholinesterase (BChE). AChE is primarily found in muscle and nerve tissues and has a strong and specific effect on acetylcholine; BChE is produced in the liver and is present in all tissues, hydrolyzing other cholinesterase types such as succinylcholine. The ChE in human plasma is mainly BChE, with a small portion being AChE. Clinically, the measurement of serum ChE is an important method for assessing liver reserve function and can assist in diagnosing organophosphate poisoning. Congenital BChE deficiency is inherited in an autosomal recessive manner. Rosenman et al. assessed serum ChE in 127 employees in the Midwest of the United States preparing to spray organophosphates, finding 5 cases (3.9%) with baseline measurements below normal laboratory reference values. Except for one case that did not undergo genetic testing, the other four (3.1%) were all defective heterozygotes, and none of these four showed clinical symptoms during exposure to organophosphates. Since AChE, which acts on nerve receptors, and BChE in the liver are controlled by different genes, individuals with congenital serum BChE deficiency do not have an increased risk of organophosphate toxicity. BChE can hydrolyze neuromuscular blocking agents used in general anesthesia (such as succinylcholine), and low BChE activity may significantly prolong the action of these drugs. Therefore, while congenital BChE deficiency does not affect health under normal circumstances, there is a risk of respiratory arrest after anesthesia, and liver function tests should be performed before general anesthesia to avoid risks associated with ChE deficiency during surgery.

7Conclusion

The liver is the largest biochemical factory in the human body, participating in material metabolism, bile production and excretion, detoxification, and many important biochemical reactions. Therefore, liver biochemical indicators are commonly used to reflect liver function. However, abnormal liver biochemical indicators do not necessarily equate to liver damage; some genetic and immune factors can also cause abnormal liver biochemical indicators. When routine examinations cannot clarify the diagnosis, clinicians should broaden their diagnostic and treatment approaches, considering congenital causes and immune factors to avoid misdiagnosis and mistreatment.

https://www.lcgdbzz.org/cn/article/doi/10.12449/JCH240231

Han Xu, Li Jia, Xiong Qingfang, et al. Clinical Significance of Benign Liver Function Abnormalities. Journal of Clinical Hepatobiliary Diseases, 2024, 40(2): 408-412