Non-cirrhotic intrahepatic portal hypertension (NCIPH) is one of the common causes for portal hypertension in the Indian sub-continent [1, 2]. Due to overlapping features with cirrhosis, a diagnosis of NCIPH is often missed. However, liver biopsy, Tc-sulfur colloid nuclear scan and non-invasive tests such as transient elastography or magnetic resonance elastography can help differentiate the two entities [1,2,3]. Despite advances, the pathogenesis of NCIPH still remains unknown, with multiple etiologies being proposed varying from infection, prothrombotic state, immune disorder and genetic factors [4]. Of these etiological factors, genetic factors remain the least explored.

To identify the genetic factors associated with NCIPH, families with more than one patient affected with NCIPH may provide a chance to uncover the responsible genes. In a study of children with obliterative portal venopathy (OPV), the authors reported 14 families in whom two or three siblings were affected, with no disease in the parents. It may suggest to some an autosomal recessive mode of inheritance, while others may blame the common environmental factors. After all, genes are just loaded guns and the environment pulls the trigger. In contrast, afflicted members in nine other families belonged to different generations, indicating a different mode of transmission, though the environment could still be shared. The existence of OPV in children with a combined extrahepatic genetic condition, a vascular developmental defect or a neurologic developmental defect raises the possibility that OPV may indeed, even though occasionally, result from a developmental genetic abnormality [5].

We read with great interest the article by Aaron et al. reporting the association of genetic factors with NCIPH [6]. Next-generation sequencing (NGS) analysis was done in the present study on 54 consecutive NCIPH patients. Potential candidate variants were identified for further analysis using the sequencing data of these 54 patients. The focused genetic panel included 11 genes: (1) VWF-ADAMTS13 pathway: VWF, ADAMTS13, (2) Complement pathway: MBL2, CFB, CFH, C3, CD46, CFI, (3) Vitamin B12 metabolism: MTHFR and (4) Familial NCIPH: KCNN3, DGUOK. These variants were subsequently examined using Restriction Fragment Length Polymorphism (RFLP) or Sanger sequencing in all 84 NCIPH patients and 103 controls. Variants in the genes MBL-2, CD46 and VWF were found to be either related to or predisposed to NCIPH. The frequency of the c.7624A > G variant of VWF (OR 7.8, p = 0.03) was significantly higher in cases than in controls, while the frequency of VWF variant c.2365 A > G and c.2385 T > C was higher in controls. The authors hypothesized that the MBL2 polymorphism leads to a higher risk of infection and complement activation, which, along with VWF polymorphism, may predispose to an increased risk of microthrombi.

Alteration in the VWF-ADAMTS13 pathway has shown an association with both arterial as well as venous thrombosis [7]. In patients with compensated cirrhosis, ADAMTS13 activity has been shown to independently predict the development of portal vein thrombosis (PVT) [8]. A previous study analyzing the association of portal microangiopathy in NCIPH with ADAMTS13 deficiency reported significantly lower ADAMTS13 activity in all patients compared to controls [9]. The authors also reported that 36% of patients with NCIPH had abnormally large VWF multimers compared to 0% in controls. Thus, ADAMTS13 deficiency causes large, uncleaved VWF accumulation in the liver, where it interacts with platelet glycoprotein 1b to activate the development of thrombi [9]. Thus, a disorder of the VWF-ADAMTS13 pathway forms the primary pathogenic mechanism for developing NCIPH, in which an alteration in the complement pathway may synergize.

Multiple other genes have been shown to have an association with familial NCIPH. One study reported a mutation in the KCNN3 gene (encoding a small conductance calcium-activated potassium channel involved in the regulation of arterial and venous vascular tone) in a person affected by idiopathic non-cirrhotic portal hypertension (INCPH) along with three of his four children, suggesting autosomal dominant transmission [10]. Another case series reported a mutation in DGUOK (deoxyguanosine kinase required for mitochondrial DNA replication) in three patients of NCIPH from two consanguineous families [11]. Warasnhe et al. reported two siblings with NCIPH and hepatopulmonary syndrome to have a mutation in the mitochondrial tRNA methyltransferase 5 (TRMT5) gene, which is essential for the accuracy and fidelity of mitochondrial DNA translation [12]. This suggests a potential role of mitochondrial toxicity in the development of NCIPH.

In a recent report, the authors sequenced the genomes of two healthy persons and four patients from a large, multi-generational Lebanese family with portosinusoidal vascular disease (PSVD). In a heterozygous deleterious variant (c.547C > T, p.R183W) of FCH and double SH3 domains 1 (FCHSD1), an uncharacterized gene was identified, with pattern of inheritance being autosomal dominant. On the introduction of this FCHSD1 variant by CRISPR/Cas9 genome editing into 15 mice, nine (60%) developed portal hypertension features suggestive of PSVD. This FCHSD1R183W mutation was found to cause hyperactivation of the mechanistic target of rapamycin (mTOR) signaling pathway and the authors proposed that mTOR pathway antagonists may offer therapeutic benefits in PSVD [13].

Aaron et al. should be applauded for this first study from India on the genetic profile of patients with NCIPH, which provides insight into the pathogenesis of NCIPH in the Indian population. It is an excellent preliminary exploration into an uncommon entity, but results are not ripe enough yet to apply the Bradford Hill criteria and there are multiple limitations that must be kept in mind. First, except for the VWF variant c.7624A > G, the strength of association with other variants is small. Second, regarding coherence, the measurement of VWF level could have helped establish the relation between the genotype and phenotypic alteration. Third, further studies are required to analyze the reproducibility of the present study’s findings. Fourth, the study reports that none of the family members of the patients in the present study had biopsy-proven NCIPH, although five patients had a family history of significant liver disease. Further evaluation of these patients and their relatives might have provided further insight into the genetics of familial NCIPH in India. Lastly, a recent study from India in abstract form reported one or more thrombophilic conditions in one-third of patients, with anti-phospholipid antibodies being the most common (23.2%) [14]. Analysis of genetic thrombophilic conditions may help understand the association with NCIPH.

The present study also opens a few questions which need to be answered. In the study, VWF c.2365 and c.2385 were significantly higher in controls rather than cases. Is it possible that these variants confer protection against the development of the disease? Compared to previous reports of familial NCIPH, the present study did not find KCNN3 and DGUOK mutations in any of the patients. Thus, potential genes responsible for NCIPH in India may be different from those in the western population and needs comparative studies between the Asian and Caucasian population. Lastly, whether whole-genome/exon sequencing could have helped identify additional genes associated with NCIPH remains to be seen.

We all know that genes are not about inevitabilities but about potentials and vulnerabilities. The work by Aaron et al. may have identified some of the genetic factors that play a significant role in the pathogenesis of NCIPH. It is also possible that environmental factors such as infection or toxins may provide a second or even a third hit for developing NCIPH in genetically predisposed individuals in India. Future studies will crystallize the real importance of genetic factors in the pathogenesis of NCIPH.