In the presented case, we identified a previously unknown missense variant – c.587G > A – within exon 4 of the JAG1 gene, which results in an amino acid alteration (p.Cys196Tyr) in the DSL domain of JAG1 that is responsible for its binding to the Notch2 receptor . Exon 4 is the second most frequent location of pathogenic variants within the JAG1 gene .
Cysteine loss within the JAG1 protein frequently results in missense mutations in ALGS patients , likely due to the unique type of bonds (disulfide bonds) that cysteine forms. Disulfide bridges between cysteine molecules are crucial for the stability of the tertiary and quaternary structures of proteins and hence, for their function. The structure and function of a protein is very likely to be perturbed if a cysteine residue is substituted with tyrosine, as the latter is an aromatic amino acid with hydrophobic properties. A missense mutation c.686G > A causing the p.Cys229Tyr amino acid alteration, and thus disruption of the binding properties of the DSL domain, was reported in a patient with ALGS . In vitro testing showed that this variant causes perinuclear retention of the modified protein and the inability to activate Notch signaling . We hypothesize that the variant discovered in this patient case, which alters the DSL domain in a similar manner, is causative for the presenting phenotype.
In addition, two pathogenic variants were discovered in direct proximity to the one presented (at positions 588 and 582) in clinically diagnosed ALGS patients . They resulted in premature termination codons, whereas the variant discovered in our patient, who had incomplete ALGS, leads to substitution of cysteine for tyrosine. This corresponds to claims of some authors that subclinical presentations of ALGS are more frequently caused by missense mutations, rather than deletions and other truncating variants that are typical of complete ALGS .
The expressivity of JAG1 variants is variable and thus correlation between genotype and phenotype is dubious [2, 14]. In a study where 53 variant-positive relatives of ALGS patients were assessed, it was shown that 25 of them (47%) did not meet the classical clinical criteria, and moreover, two patients (4%) were completely symptom-free . In the absence of obvious liver disease, which is the hallmark of ALGS, it took years to correctly diagnose our patient as her main symptom was ToF. However, the liver disease could have been subclinical. Aspartate and alanine transferase oscillated within the high – normal range, but we noted elevated alkaline phosphatase levels, especially during episodes of peritonitis, and a slight increase of liver echogenicity in the ultrasound. Knowing there have been cases of bile duct paucity with no abnormalities revealed in laboratory and imaging examination , we cannot exclude that our patient had silent bile duct paucity. Previously, cholestasis was condition sine qua non for the diagnosis of ALGS and now it is known to be present in only 89% of cases , whereas cardiovascular anomalies are found in up to 94% of patients . Some researchers suggest that the developing heart might be more prone to decreased JAG1 dosages compared to liver . Clinicians should therefore be urged to treat cardiovascular abnormalities as equally important as liver involvement when diagnosing ALGS.
Although surgical repairs for treating ToF are considered “curative”, long-term follow-ups show that despite the operations, patients experience complications caused by hemodynamic changes [17, 18]. The prevalence of significant renal impairment (GFR < 60) is 35-fold higher in cyanotic patients with congenital heart defects, even if they are treated, than in a general population . This phenomena of co-occurrence of renal impairment secondary to chronic heart disease is known as cardiorenal syndrome, type 2 . Bearing in mind the mild proteinuria in our patient, cardiorenal syndrome could have been one of the possible mechanisms contributing to the deterioration of renal function.
Other mechanisms in which cyanotic heart defects damage kidneys include poor blood oxygenation followed by polycythemia and increased blood viscosity. In patients with congenital heart disease, cyanosis doubles the risk of significant renal impairment . Twenty years of follow-up of patients with ToF treated with a Blalock-Taussig shunt showed that hematocrit levels correlate with increasing filtration fraction , which in turn, leads to structural changes in the glomeruli, such as glomerulosclerosis [22, 23]. Our patient suffered from cyanosis throughout the major part of her life and at the age of 33, she had hematocrit as high as 73.5% – based on the aforementioned data, her filtration fraction could have exceeded 40% . Although none of the patients treated with a Blalock-Taussig shunt developed significant renal impairment in the aforementioned study , and another study denies increased prevalence of CKD in patients with polycythemia , the observation periods in both cases could have been too short. Therefore, it cannot be excluded that long lasting polycythemia may be associated with severe deterioration of renal function. We suspect polycythemia played an important role in the development of ESRD in our patient.
Polycythemia is also associated with increased prevalence of hypertension  and we suspect it could have been the reason for the early onset of hypertension in our patient. Although hypertension is a known cause of CKD, blood pressure in our patient was well-controlled until renal function started to deteriorate and even later, it never reached values which could lead to ESRD if not accompanied by other factors. We believe that pre-existing renal anomalies due to ALGS were more important in the development of ESRD in our patient.
A recent retrospective cohort study involving pediatric patients with pathogenic variants of JAG1 showed renal involvement in 39% of cases. The most common findings included renal dysplasia (23% of patients) and renal tubular acidosis (9.5% of patients) . Our patient suffered from horseshoe kidney with increased echogenicity (accounting for renal dysplasia) and acidosis refractory to dialysis – likely due to renal tubular acidosis. Knowing that isolated ToF very rarely leads to ESRD , we suspect that renal abnormalities must have been an important precondition for it. According to the pediatric study, deterioration of renal function starts early – in underage patients with any form of renal involvement, CKD is present in 5.4%, and ESRD in 4.1% of cases . Though common and typical in patients with pathogenic variants of JAG1, renal abnormalities are not included in the classic criteria for ALGS. What is more, variants of JAG1 are present in 43, 51, 47 and 86% of patients corresponding to 2, 3, 4 and 5 systems affected . Knowing these numbers, the need for full revision of the clinical diagnostic criteria for ALGS becomes clear.
Since only several case reports on ESRD requiring dialysis in ToF have been published, and none in ALGS, we faced a major challenge in choosing the appropriate therapeutic strategy. Renal or combined kidney-heart-lungs transplant were impossible due to significant hemodynamic and vascular stress. In addition, the intellectual abilities of the patient posed a contraindication as full cooperation on part of the patient was not guaranteed. Hemodialysis would have required the creation of a vascular access, which is a hemodynamically stressful event and could result in cardiac failure. Central line was not considered due to risk of infections and difficulties with maintaining patency in view of extreme polycythemia. Furthermore, polyglobulia may disable adequate heparinization which could lead to both clotting of the blood within the hemodialysis machine and embolism in the patient. What is more, the distance between the dialysis unit and the patient’s place of residence could be a major obstacle in achieving effective treatment. Given these factors, the final viable therapeutic tool was CAPD, which reduces cardiovascular stress because there is minimal variation in circulating blood volume. We were concerned about the intellectual abilities of the patient, but good cooperation with the mother allowed for successful peritoneal care at home for over 4 years and significantly enhanced the patient’s quality of life.
Although ToF rarely leads to ESRD requiring dialysis, if treated palliatively and combined with renal dysplasia (typical for ALGS), it can result in severe renal failure as in the presented case. Therefore, clinicians should aim at early recognition of JAG1 pathogenic variants – firstly, to offer genetic counseling to patients and secondly, to predict the course of the disease. We suggest that pathogenic variants of JAG1 should always be considered in ToF patients and that they should be offered early nephrological care. We also suggest considering use of CAPD in the treatment of ALGS patients who have ESRD and severe cardiac involvement such as ToF.
Finally, clinical criteria for the diagnosis of ALGS require revision in view of cardiovascular anomalies being more prevalent than liver involvement and common renal impairment in the condition.