In our result, 21% of patients had glomerular hyperfiltration (e-GFR > 125 ml/min/1.73 m2), while 31% of them had reduced e-GFR and the rest had normal ranges from 90 to 125 ml/min/1.73 m2. Similarly, (Ghobrial et al. 2016) reported glomerular hyperfiltration (177.44 ± 35.6 mL/min/1.73 m2) in patients with SCD, so It was recommended to monitor the renal function of children with sickle cell disease, especially in homozygotic (Hb SS) patients [18].

Furthermore, Nnaji et al. 2020 indicated that abnormally high e-GFR in children with asymptomatic sickle cell anemia was observed and assumed to progress to chronic kidney disease; therefore, regular monitoring of renal function in asymptomatic pediatric patients with sickle cell disease and implementing management protocol is crucial to avoid anemic or crises episodes and advanced kidney disease.

Glomerular damage in sickle cell anemia was suggested to be caused by the damage of the cytoskeleton of cells called podocytes that line the visceral surface of Bowman’s capsule. Such damage was assumed to be caused by chronic ischemia–reperfusion injury occurring during vaso-occlusion episodes in sickle cell disease, therefore subsequent activity of TGF-β1 could lead to further damage and apoptosis of podocytes and glomerular damage [19]. Agata et al. 2014 noted that in patients with sickle cell disease, due to the internal medulla microenvironment being hypoxic, acidic, and hyperosmolar, polymerization of deoxygenated hemoglobin S results in RBC sickling and microinfarction causing reduced medullary blood flow, but if the hypoxia deteriorates, prostaglandins are released causing marked vasodilation and glomerular hyperfiltration [20].

In the current study, microalbuminuria was diagnosed in fourteen patients (14%) and none had macroalbuminuria or edema. Such results were in agreement with Belisário et al.,2020 study that reported the earliest manifestation of renal disease in pediatric SCD is an increase in the glomerular filtration rate and the occurrence of microalbuminuria [21].

Persistent proteinuria in children with SCD usually follows the occurrence of glomerular hyperfiltration, then the GFR is reduced as the sickle nephropathy progresses [22].

Microalbuminuria was found to be a good preclinical marker of glomerular damage predicting progressive renal failure in pediatric patients with SCD [23]. Ocheke and his colleagues reported that anemia and high e-GFR are risk factors for microalbuminuria and that the glomerular filtration rate was higher in children with microalbuminuria than those who do not (p ≤ 0.01) and it was also higher in children with sickle cell disease than in control [24]. The vaso-occlusive crisis was diagnosed in 82% of patients and 96% of them (n = 75) needed hospitalization. The vaso-occlusive crisis in sickle cell disease is due to the young sticky erythrocytes containing hemoglobulin S attaching to the walls of capillary venules leading to narrowing of their lumens which leads to the decrease in blood velocity and an increase in erythrocytes transient time; therefore, HB S become deoxygenated and subsequent erythrocyte sickling occurs; moreover, necrosis of the affected vascular area and inflammatory response are initiated which produce pain [25].

A previous study indicated that reduced e-GFR occurred during the vaso-occlusive crisis [26]. Moreover, Sarray et al. observed reduced IL-10 and increased IL-6 and TNFα levels during the vaso-occlusive crisis in pediatric sickle cell disease patients [27].

Hydroxyurea was the key treatment in 85% of our patients. In a previous study, it was found to be an effective and proven medication to reduce the frequency of painful episodes by 50% in sickle cell disease, it also decreases the rate of blood transfusions by inducing the production of Hb F [28].

In our study, four patients developed gallbladder stones and needed cholecystectomy. Cholelithiasis results from the chronic accelerated rate of erythrocyte destruction in individuals with sickle cell disease leading to the formation of insoluble calcium bilirubin that precipitates to form gallstones [29]. The rs1800469 polymorphism changes codon 25 which encodes arginine into proline in the signal peptide of TGF-β1. The amino acid substitution affects signal peptide properties that may inhibit the transport of TGF-β1 into the endoplasmic reticulum and eventually decline cytokine production. The arginine substitution into proline decreased the polarity of the signal peptide for TGF-β1. The increased hydrophobicity with increased binding energy of the signal peptide for TGF-β1 to signal recognition particle and translocon of endoplasmic reticulum implies decreased protein complex stability in potentially blocking the transport of TGF-β1 into the endoplasmic reticulum. This transport retention possibly hampers the synthesis and maturation of TGF-β1 leading to decreased cytokine production [30]. Other authors have proved that higher TGF-β1 is associated with higher susceptibility to different infections, even for septicemia [31]. Both findings show that more susceptibility to infections may increase sickling attacks [32].

In the current study the rs 1800471 G/G genotype had a higher incidence of vaso-occlusive attacks and earlier onset of the disease-related symptoms that was not attributed to increased HbS concentration alone, could suggest a modifying effect on the HbS polymerization, which should be further studied, As TGF-β 1 was found to be associated with hemolysis, leukocytes, platelets, and lipid metabolism, this provides evidence that this immunomarker likely modulates the inflammatory response in SCD in previous studies [33]. Previous studies described the effect of the presence of the G allele of rs 1800471 as associated with increased TGF-β1 production, this may explain a higher incidence of vaso-occlusive attacks and earlier onset of the disease-related symptoms [34].

In our study, a comparison between variants of the TGF-β1 gene between cases and controls. indicated the presence of a statistically significant difference regarding the C allele of rs1800471, that the C allele was found more in the control group.

Previous studies noted that TGF- β1 polymorphism would regulate its expression and mediate the occurrence of several diseases such as rheumatoid arthritis, colorectal carcinoma, diabetes mellitus, osteoporosis, asthma, Crohn’s disease, and fibrotic diseases of the skin and kidney [35].

El-Sherbini et al. 2013 demonstrated that when TGF‐β binds to its receptors, it exerts signals via SMAD, activating MAP kinases. Such pathway controls cell proliferation, apoptosis, and response to tissue injury, infection, bone homeostasis, endothelial growth, diabetic nephropathy, pulmonary fibrosis, inflammation, immune regulation, and extracellular matrix synthesis [36]. Moreover, Santiago et al. 2021 revealed that the increased TGF-β1 levels play essential roles in vascular remodeling, vasculopathy, angiogenesis, and inflammation in pediatric patients with sickle cell disease [33].

Regarding the hypothesis of using the urinary TGF‐β as a marker of renal dysfunction in sickle cell disease, a previous study by Ghobrial et al. 2016 [37] revealed that urinary excretion of TGF-β1 was higher in sickle cell disease patients than in control children (p < 0.001).

Contrarily Sundaram, 2011 [38] indicated that urinary TGF‐β levels did not show any relationship with albuminuria in patients with sickle cell disease. Moreover, Mohtat, 2010 [39] reported elevated urine TGF- β1 levels in patients with sickle cell disease, but there was no correlation between urinary TGF-β1 and microalbuminuria or eGFR.

Our results indicated no significant effect of TGF‐β1 rs1800469 and rs1800471 genetic variants on the albumin-to-creatinine ratio (ACR), creatinine, and e-GFR in the case group. however prolonged period of microalbuminuria precedes persistent proteinuria, which is followed by renal failure in SCD patients. Therefore, early detection of microalbuminuria may allow earlier intervention to prevent renal complications [40].

In progressive kidney failure, previous studies indicated the correlation between TGF‐β 1 genetic variants and the progression of chronic kidney failure and that TGF‐β1 single nucleotide variants are helpful as a prognostic indicator of progressive kidney disease [41].

A recent study suggested that genetic variants in the TGF-β1 and IL-4 genes rs1800469, rs1800470, rs1800471, and rs8179190 may play a role as a genetic contributor to the susceptibility of chronic kidney disease [10]. In addition, TGF β1 was found to induce renal hypertrophy and fibrosis [10].

Saraf et al. 2015 study on the genetic markers of sickle nephropathy indicated the association of APOL1 G1/G2 with kidney disease in sickle cell disease through increased risk of hemoglobinuria and associations of HMOX1 variants with kidney disease through reduced protection of the kidney from hemoglobin-mediated toxicity [42].

This study had some limitations, as not all cases had histological evidence of SCN, also, the limited literature on the two studied genetic variants in the pathogenesis of SCN necessitates more studies on different ethnic populations, and different age groups with metanalysis studies.