Curation of FBP1 pathogenic variants and allele frequency analysis
After pathogenicity assessment of FBP1 variants collected from the Chinese Children's Rare Disease Genetic Testing Clinical Collaboration System (CCGT), Human Gene Mutation Database (HGMD), ClinVar, China national knowledge infrastructure (CNKI), Web of Science, and PubMed, 97 P/LP variants were identified, including frameshift (35%, 34/97), missense (32%, 31/97), splicing (15%, 15/97), nonsense (10%, 10/97), CNV (5%, 5/97) and in-frame indel (2%, 2/97) variants (Fig. 1 and Additional file 1: Table S1). To compare allele frequency (AF) of these variants in Chinese population with other populations, we divided CCGT into two unrelated sub cohorts, the children cohort (CCGT_C) and the parents cohort (CCGT_P). Then, we excluded children diagnosed with FBP1D from CCGT_C and their parents from CCGT_P. Among the 97 P/LP variants, 38 have been documented in the CCGT, China Metabolic Analytics Project (ChinaMAP) and gnomAD (Additional file 1: Table S2). Eight of the 38 variants were observed in Chinese (Fig. 2). Of the eight variants, c.490G > A and c.355G > A had significantly higher AF in the Chinese population than in the non-East Asian populations, while c.841G > A had significantly lower AF value in the Chinese population than in the South Asian population (all P values < 0.05). In addition, c.490G > A is the most common variant in the Chinese population (AF in CCGT_C: 1/3216, in CCGT_P: 1/2232; and in ChinaMAP: 1/3025), while it has not been reported in African American, admixed American, Ashkenazi Jewish, Finnish in Finland and South Asian populations.
Estimation of FBP1D prevalence in the Chinese population
We estimated the FBP1D prevalence based on 20,904 pediatric patients in the CCGT_C cohort (12,771 males and 8,133 females) and 10,042 parental samples in the CCGT_P cohort (5,014 males and 5,028 females). The total number of individuals carrying P/LP FBP1 variants was 36 (25 males and 11 females) in the CCGT_C and 18 (11 males and 7 females) in the CCGT_P. Based on the carrier frequency, the estimated FBP1D prevalence were 1/1,348,695 in the CCGT_C and 1/1,244,960 in the CCGT_P. By the permutation and combination method, the estimated prevalence were 1/1,510,786 and 1/1,179,494 respectively. By the Bayesian framework, the estimated prevalence of FBP1D were 1/1,312,275 (95% confidence interval was 1/2,748,617 ~ 1/737,905) in the CCGT_C and 1/1,179,494 (95% confidence interval was 1/3,542,901 ~ 1/544,684) in the CCGT_P. In general, the estimated prevalence of FBP1D in the Chinese population is 1/1,310,034 by averaging all the above results (Table 1).
Table 1
FBP1D prevalence estimation in CCGT children cohort and parents cohort with estimated affected frequency by three methods
| Children Cohort | Parents Cohort |
Total number | 20904 | 10042 |
Gender (Female/Male) | 8133/12771 | 5028/5014 |
Carrier with P/LP variants (Female/Male) | 11/25 | 7/11 |
Carrier frequency | 1/580 | 1/558 |
Method 1: carrier frequency | 1/1348695 | 1/1244960 |
Method2: permutation and combination | 1/1510786 | 1/1309631 |
Method 3: Bayesian framework (95% CI) | 1/1312275 (1/2748617~ 1/737905) | 1/1179494 (1/3542901~ 1/544684) |
Average | 1/1310034 |
Genotype and phenotype of six Chinese FBP1D patients
Five patients were diagnosed as FBP1D in the CCGT_C. Another published FBP1D patient diagnosed in our center from other cohort was also collected in this study by literature review[16]. Here we add detailed clinical phenotypes and prognosis of this patient (Table 2 and Additional file 2: Table S3). Of these six patients, two presented phenotypes in the neonatal period, two in their early infancy (< 1 year) and two in childhood. Hypoglycemic episodes was the first symptom for most patients (5/6, Table 2), mostly associated with fever or infection (4/5). Two children presented with nervous system symptoms, and their prognosis was not very optimistic, and even one child died. Based on clear genetic test results and typical clinical manifestations, all patients were diagnosed with FBP1D. After early dietary guidance (avoid feeding the fructose-related foods and avoid prolonged fasting) and avoiding infection or other triggering factors, most patients (4/6) did not present poor prognoses and developed well.
Table 2
Genotype and clinical manifestations of six Chinese FBP1D patients
ID | Mutations | Gender | Age (years) | Age of onset | Phenotypes at onset | Prognosis |
1 | c.355G > A (paternal), c.960_961insG (maternal) | Male | / | 1 year and 8 months | Status epilepticus, vomiting, severe infection with fever | Death |
2 | c.490G > A (paternal), c.490G > A (maternal) | Female | 7 | 3 years | Convulsions, unconsciousness, hypoglycemia, fever | With brief epileptic seizure |
3 | c.841G > A (paternal), c.778G > A (maternal) | Male | 6 | 11 months | Rotavirus infection, hypoglycemia, metabolic acidosis, hepatomegaly | Without symptoms for 2 years |
4 | c.355G > A, exon1 deletion | Male | 9 | Neonatal | Episodic tachypnea, hypoglycaemic episodes, pneumonia | Occasional hypoglycemia |
5 | c.704del (paternal), c.959dup (maternal) | Male | 8 | Neonatal | Vomiting, hypoglycemia, hyperbilirubinemia | Occasional nighttime hypoglycemic attacks |
6* | c.333 + 2T > G (paternal), c.333 + 2T > G (maternal) | Male | 6 | 7 months | Hypoglycemia, lactic acidosis, metabolic acidosis, fever | Occasional vomiting and hypoglycemia |
* This patient has been reported in Wang’s study [16]. |
(Table 2 should appear here.)
Analysis of genotype-phenotype relationship in 122 FBP1D patients
We collected genotype and phenotype of 122 FBP1D patients, including the six patients mentioned above and other 116 patients from literature review (Additional file 2: Table S3). The most common clinical phenotype was hypoglycemia (111/113, 98.2%), and then was metabolic acidosis (102/121, 84.3%). Other common phenotypes included vomiting (64/107, 59.8%), ketosis (61/83, 73%), hepatomegaly (54/89, 61%), fever (49/94, 52%), respiratory distress (30/51, 59%) and seizures (27/72, 37%). When comparing combinations of variants, patients carrying homozygous exon1 deletion were more likely to present hepatic steatosis than without these variants (OR = 5.4, P value = 0.028), while carrying homozygous c.841G > A were more likely to present increased urinary glycerol than without these variants (P value = 0.010) (Additional file 2: Table S4). Mutation types of all variants in the 122 FBP1D patients were classified into missense, nonsense, splicing, frameshift, in-frame indel and CNV (Additional file 2: Table S5). Two missense variants were the most common mutation type combination in FBP1D patients (48/122, 39.3%), then were two frameshift variants (22/122, 18.0%) and two CNVs (15/122, 12.3%). For mutation type-phenotype analyses, we found that patients carrying two CNVs were more likely to present hepatic steatosis (OR = 17.9, P value = 0.009), and carrying two missense variants were not likely to present fever (OR = 0.16, P value = 0.001). For zygosity-phenotype analyses, patients carrying compound heterozygous pathogenic variants were more likely to present lethargy (P value = 0.040), and patients carrying homozygous pathogenic variants were more likely to present ketosis (P value = 0.007) and hepatic steatosis (P value = 0.015) (Additional file 2: Table S6).
Comparison of clinical manifestations between FBP1D and HFI
Hereditary fructose intolerance (HFI) is also a severe inborn errors of fructose metabolism caused by mutations in ALDOB gene. We extracted 33 common clinical phenotypes of FBP1D and 35 common clinical phenotypes of HFI from OMIM and HPO. Then we thoroughly calculated the frequency of these phenotypes in the 122 collected FBP1D patients and 68 HFI patients from our previous study[5]. Seventeen phenotypes are identical in these two diseases (Additional file 3: Table S7), while FBP1D has 16 relatively distinctive phenotypes and HFI has 18 relatively distinctive phenotypes (Additional file 3: Table S8). Though FBP1D and HFI are both metabolism disorders, FBP1D patients markedly have hypoglycemia and metabolic acidosis than HFI patients (OR = 88 and 7 respectively, all P value < 0.05). Besides, FBP1D patients more probably have seizures and coma than HFI patients (all P value < 0.05). Interestingly, we found that HFI patients are more likely to present diarrhea than FBP1D patients, though the difference was not significant after statistical adjustment. Comparing to HFI, fever and ketosis are considered as relatively distinctive phenotypes of FBP1D.