Primary Hyperoxaluria Type 1: Clinical, Paraclinical, and Evolutionary Aspects in Adults from One Nephrology Center

Introduction Primary hyperoxaluria type 1 (PH1) is a rare and inherited condition of urolithiasis. The aim of our study was to analyze clinical, paraclinical, and evolutionary aspects of PH1 in adult patients in our Nephrology department. Methods We conducted a retrospective single-center study between 1990 and 2021. We collected patients followed for PH1 confirmed by genetic study and/or histopathological features of renal biopsy and morphoconstitutional analysis of the calculi. Results There were 25 patients with a gender ratio of 1.78. The median age at onset of symptoms was 18 years. A delay in diagnosis more than 10 years was noted in 13 cases. The genetic study found the I244T mutation in 17 cases and 33-34 InsC in 4 cases. A kidney biopsy was performed in 5 cases, on a native kidney in 4 cases and on a graft biopsy in one case. The analysis of calculi was done in 10 cases showing type Ic in 2 cases. After a median follow-up of 13 years (1 year–42 years), 14 patients progressed to end-stage chronic renal failure (ESRD). The univariate study demonstrated a remarkable association with progression to ESRD in our population (44% vs. 56%) RR = 13.32 (adjusted ORs (95% CI): 2.82–62.79) (p < 0.01). Conclusion Progression to ESRD was frequent in our series. Early diagnosis and adequate management can delay such an evolution.


Introduction
Primary hyperoxaluria type 1 (PH1) is a rare and severe inherited disease afecting glyoxylate and oxalate metabolism [1,2]. It is an autosomal recessive (AR) disorder, classifed into 3 types depending on the mutated protein. Te clinical presentation is dominated by oxalocalcic urinary lithiasis and nephrocalcinosis.
In Tunisia, PH1 is responsible for 13% of end-stage renal disease (ESRD) in children compared to 0.3% in Europe and 0.7% in North America [3,4]. Te aim of our study was to assess the diferent clinical, paraclinical, and evolutionary aspects of this disease.

Study Design.
Tis was a retrospective and descriptive study conducted in our department over a 31-year period . From a series of 73 patients followed up for hereditary urinary lithiasis (HUL) of all causes, we collected 25 patients diagnosed with PH1. We documented those who were followed up in consultations or hospitalized for PH1.

Inclusion Criteria.
Inclusion criteria included patients who have been diagnosed with PH1, that is, confrmed by genetic study, renal histology, or the presence of a urinary stone type Ic on morphoconstitutional analysis.

Participants.
Te patients were referred to our department for 3 reasons: frstly, for an etiological assessment of a urinary lithiasis in 12 cases; secondly, for a follow-up in the adult age from the pediatrics department in 10 cases; and fnally, for an exploratory investigation of a renal failure in 3 cases. Oxaluria was measured in 11 of our patients. Concentration hyperoxaluria was defned as a concentration >0.3 mmol/l and fow hyperoxaluria as a fow rate ≥0.45 mmol/24 h or a urine oxalate to creatinine ratio (Ox/ Creat) >0.03. For the genetic study, only AGXT mutations were investigated.

Defnitions.
Glomerular fltration rate (GFR) was estimated by the MDRD formula for adults and the Shwartz formula for children. Chronic kidney disease (CKD) stages were determined according to the KDIGO recommendations [5].
Te urinary stones analyzed were classifed according to the morphoconstitutional classifcation of Michel Daudon [6].

Statistical
Analysis. Data were entered and analyzed using Excel 365 and SPSS 20.0 software. We calculated the mean annual decline in glomerular fltration rate by dividing the diference in the sum of glomerular fltration rates at the start of follow-up and at the last visit by the total follow-up time in years. We compared survival curves (using the Kaplan-Meier method) based on the log rank test and performed logistic regression using the Cox proportional hazards model. Te parameters studied were age, gender, family history of CKD, time to etiological diagnosis, recurrence, aetiologies of HUL, occurrence of urinary tract infections or acute renal failure, stage of CKD at diagnosis, bilaterality of stone, presence of coralliform stone or nephrocalcinosis, and urological treatment.
We calculated incidence rates of CKD by relating the number of patients who started renal replacement therapy to the duration of follow-up. Confdence intervals were calculated at 95% using standard formulae. In all cases, the test was considered signifcant when the p level of signifcance was below 0.05.

Results
Tere were 16 males and 9 females with a gender ratio of 1.78. Te age of onset of lithiasis ranged from 2 to 55 years with a median of 18 years. Te median time to etiological diagnosis was 10 years (range: 0-39 years). A delay in diagnosis of more than 10 years was noted in 13 cases (52%). Te presence of parental consanguinity was specifed in 22 cases. It was found in 20 cases (91%). It was of frst, third, and second degree in 13, 5, and 1 cases, respectively. One case was distantly related. A family history of urinary lithiasis was found in 17 cases (68%).
We identifed 4 clinical forms: the child and adolescent form with progressive alteration of renal function was noted in 10 of our patients, the adult form was observed in 13 of our patients, and recurrence after renal transplantation without a precise diagnosis before transplantation was noted in one case. One patient was found to have a family survey following in the presence of an index case. Te circumstances of disease discovery were specifed in 24 cases (Table 1). Te most common presenting symptom was renal colic.
Oxaluria was measured in 11 cases, with a mean oxaluria of 0.484 mmol/l (range: 0.238-0.953 mmol/l). We noted fow hyperoxaluria in all cases and concentration hyperoxaluria in 8 cases (73%). Twelve patients had normal renal function (48%). Tirteen had CKD (52%). One patient was at stage 3A, one was at stage 3B, and three were at CKD stage 4. Seven patients had end-stage CKD from the beginning. One patient had unclassifed CKD due to lack of height data at the age of 8 years. Ten patients had a stone analysis which was type Ia in 8 cases (80%) and Ic in 2 cases (20%). An additional infection lithiasis was noted in one case (10%). Crystalluria was positive in 12 cases. Whewellites was noted in 11 cases (92%) and weddellites in one case (8%).
On a kidney scan, urinary lithiasis was bilateral in 18 cases. Nephrocalcinosis was found in 7 cases, associated with urinary stones in 6 cases and isolated in one case. A coralliform stone was noted in 2 cases. Genetic study revealed the I244T mutation in 17 cases and the 33-34 InsC mutation in 4 cases. Renal biopsy was performed in 5 cases (6%). Four cases were oxalosis in native kidneys, one of which was postmortem, and one case was an unrecognized oxalosis diagnosed on graft biopsy. Figure 1 shows the histological appearance of renal oxalosis in the native kidney. Overall, PH1 diagnosis was confrmed by genetic study in 21 cases, by renal histology alone in 2 cases, and by morphological analysis of the stone which was type Ic in 2 cases (8%).
Medical treatment included a diuresis of 3 l/m 2 /24 h and a low oxalate diet for all our patients and vitamin B6 for 13 of our patients. After a median follow-up of 13 years (extremes: 1 year-42 years), 14 patients progressed to end-stage CKD with recourse to renal replacement therapy at a mean age of 36 years (extremes 18-54 years). Two patients had an isolated kidney transplant (RT). Te frst patient was transplanted at the age of 42 years from a brain-dead donor. Te course was marked by the occurrence of acute renal failure at D2 post-RT, nonreversible in relation to unrecognized oxalosis which was diagnosed on graft biopsy. Te second patient had an isolated RT after 7 years of extrarenal purifcation. She had PH1 related to a homozygous mutation of the I244T allele. Clinically, she had no organomegaly. Blood oxalinity was normal at 29, 13, and 31 μmol/l. She had no osteocondensing lesions on imaging and no oxalic deposits on fundus and osteoarticular biopsy. She was transplanted from a related living donor (her mother) carrying the same mutation in the heterozygous state. Te evolution was favourable after hyperhydration and vitamin B6 supplementation. She retained normal renal function after 7 years of transplantation.
A family screening was performed in 6 families, fve by genetic study and one by urine oxaluria determination. It led to a positive diagnosis of unrecognized urinary lithiasis in 4 families, with the number of patients screened varying between 1 and 3 per family.
Te incidence of CKD was high among our PH1 patients (n � 8/18; 3.42%) (adjusted ORs (95% CI): 1.05-5.79) compared to other patients with other hereditary urolithiasis in our department, but the diference was not statistically signifcant. Long-term renal survival in PH1 was observed to be 33% at 30 years.

Discussion
PH1 is predominantly a disease of young children, but the age range is from birth to over 60 years [2]. In our series, the onset of symptomatology was in adulthood in 13 cases (52%). Tis is explained by a selection bias as it is a recruitment of patients in an adult nephrology department. Te gender ratio varies according to the series, with a male predominance noted in the series by Soliman et al. [7] and Gargah et al. [8] with a gender ratio of 1.36 and 1.2, respectively, while a female predominance was noted in the series by Nagara et al. with a gender ratio of 0.63 [9]. In our study, we noted a male predominance with a gender ratio of 1.78. PH1 presents in 5 clinical forms [10]. Te frst form, known as infantile, is severe, with rapid progression to CKD as a result of high hepatic oxalate production. Tese patients are managed in paediatrics. Te second form is that of children and adolescents with progressive alteration of renal function. Tis form was noted in 10 of our patients. In the series by Gargah et al. [8], the most frequent initial symptoms of childhood were hyperuremia in 44% of cases, urinary tract infections in 26% of cases, and abdominal or back pain in 20% of cases. Te third form, that of the adult, was observed in 13 of our patients. Te fourth form is the recurrence after renal transplantation without precise diagnosis before the transplantation which was noted in one of our cases. Te ffth form is that of subjects identifed following family screening in the presence of an index case. Tis form was observed in one of our cases. In the series by Gargah et al. [8], a discovery in the context of a family investigation was noted in 9% of cases. PH1 is classifed into 3 types depending on the mutated protein. Type 1 is the most common and is due to a mutation in the AGXT gene coding for alanine glyoxylate aminotransferase (AGT).
In Tunisia, only the search for specifc mutations in PH1 is done [11]. Te AGXTmutations identifed difer according to geographical origin [3,9,12,13]. Te so-called "Maghrebian" I244T mutation is predominant in all Tunisian regions. It has been described in Morocco, Algeria, Libya, Turkey, Pakistan, and the Canary Islands [14]. It was also the most frequent mutation in our series.
Tere is prevalence of consanguineous marriages and geographic endogamy in Tunisia, mainly in the rural areas. Consanguinity rate ranges from 20.1% to 39.33% [15,16].
Due to the high occurrence of consanguineous unions, a previous study indicated a six-fold rise in the risk of AR diseases [17]. A retrospective analysis involving 425 Tunisian patients with AR disorders discovered consanguinity in 69.4% of the cases, with frst cousin marriages being prevalent at 48.94% [18]. In specifc southern regions, the rate of consanguinity rose to 65.26%. Te majority of mutations were observed in the homozygous state. Furthermore, the study noted that geographic endogamy was present in 93.92% of the cases investigated. Te authors estimated a seven-fold increase in the likelihood of AR diseases due to consanguinity, with certain cases having a risk as high as 24fold.
Te InsC 33-34 mutation was identifed in Kasserine, Mahdia, and Sfax, which are towns located in the central region of Tunisia, and was found in 4 of our cases. It has been described previously in Italy [3,9,12,13]. Te only known aspect of the genotype-phenotype relationship in PH1 is the response to pyridoxine in patients with Gly170Arg and Phe152Ile mutations, which may improve renal prognosis [10].
CKD at the time of the etiological diagnosis of urinary lithiasis was noted in 13 cases (52%), 7 of which were at stage 5 of CKD. Tis is explained by the severity of the disease and the delay in diagnosis in our patients. Te frequency of type Ia in our study is probably explained by a delay in diagnosis which causes a change in the morphology of type Ic.
In our series, a renal biopsy was performed in 5 cases. Four of these cases were oxalosis on native kidneys, one of which was postmortem, and one on graft biopsy. Te positive diagnosis was made on renal histology in 5 cases in the series by Soliman et al., 4 on native kidneys and one on renal graft [7], and in 9 cases in the series by Gargah et al. [8]. Renal biopsy found crystals of monohydrated calcium oxalate, arranged in a rosette or needle-like radial pattern. Tese crystals are mainly located in the tubular lumen. Tey are birefringent in polarised light (Figure 1). Te massive nature of the calcium oxalate deposits points to the primary origin of POH but does not allow its classifcation (Figure 1).
Te aim of treatment is to reduce calcium oxalate supersaturation and to delay progression to advanced renal failure and systemic oxalosis [19,20]. It involves a diuresis treatment of 3 l/m 2 /24 h regularly distributed throughout the day, especially at bedtime, to avoid nocturnal supersaturation, a diet low in oxalate and a crystallisation inhibitor (potassium citrate and magnesium salts). Pyridoxine (vitamin B6), a cofactor of AGT, is benefcial in patients with PH1 with residual enzyme activity by diverting oxalate metabolism to the more soluble glycocoll. It can reduce oxaluria by up to 30% in 30% of patients [20][21][22]. Vitamin B6 was prescribed for 13 of our patients. Several promising treatments are under investigation such as Oxalobacter formigenes, Lumasiran, Stirpental, gene therapy, and hepatocyte transplantation [1,2]. Combined liver-renal transplantation is the best strategy for CKD. Isolated preemptive liver transplantation can be proposed before stage 4 CKD but poses ethical problems, whereas isolated renal transplantation has a high risk of graft recurrence [2,10,11,23]. In our series, one patient presented with an early recurrence of PH1 without a defnite diagnosis before transplantation. Te second patient had an isolated kidney transplant from a related living donor. Te evolution was favourable with preventive measures combining hyperhydration and vitamin B6. She retains normal renal function after 7 years of transplantation.
CKD was common in our series at diagnosis and during follow-up. In an American study of primary hyperoxaluria, the annual decline in eGFR difered according to the stage of CKD. It was 2.3, 5.3, 14.7, and 16.6 ml/min for stages 2, 3A, 3B, and 4, respectively [24]. It has been reported that type 1 primary hyperoxaluria has the worst renal prognosis with a renal survival of 27% at 30 years for type 1 primary hyperoxaluria, compared with 92% and 95% for types 2 and 3, respectively [25]. It is explained by the massive deposition of calcium oxalate crystals in the renal interstitium where they induce an infammatory response and progressive interstitial fbrosis [11,24,26].
Our series is one of the few Tunisian and international series dealing with adult-onset PH1 with a good follow-up. However, the main limitations of the study were the retrospective character which confronted us with a number of missing data and patients lost to follow-up and the genetic study analyzing only the mutations of the alanine AGXT gene.

Conclusion
Our study sheds light on PH1 as a rare and an underdiagnosed condition. Te most frequent genetic mutation in our series was I244T. Our fndings reveal a poor renal prognosis for PH1, with more than half of our patient's experiencing progression to CKD. Timely diagnosis and efective management strategies are crucial in delaying the progression of PH1.

Data Availability
Te data used to support the fndings of this study are available from the corresponding author upon request.

Conflicts of Interest
Te authors declare that they have no conficts of interest.