1. Case presentation
The proband is a boy who was born after 36 weeks of pregnancy, with a birth weight of 2650 g. Various subtypes of epidermal nevus including nevus sebaceous, speckled lentiginous nevus and pigmented nevus were noted at birth. Motor development was delayed. He achieved head lifting at 8 months, independent sitting and standing at 18 months and 2 years respectively, and walking at 4 years old. He uttered his first word after 12 months old, however, could not be able to speak even a single sentence by the age of the last examination at 5 years and 9 months. He first developed seizures at 4 months of age and was subsequently admitted to our hospital. Brain MRI, blood gas analysis, and blood biochemistry did not reveal any significant abnormalities, except for slightly elevated levels of alanine aminotransferase and aspartate aminotransferase at that time. A diagnosis of West syndrome was made based on typical clinical and electroencephalographic manifestations [16]. After almost three years of antiepileptic medication and a ketogenic diet, no further seizures occurred. The patient also presented ocular anomalies, including exotropia, incomplete eyelid closure, conjunctivitis, and xanthelasmata on the left eyelid after birth. Vision could not be examined, due to the patient's young age and lack of cooperation during optical exams. In his early childhood, the boy frequently experienced thin stools, occurring 2–3 times a day, without any indication of infectious disease. Weight gain was slow.
At the age of 5 years and 2 months, the child refused to stand and was admitted for further investigations. His height parameter was below the third percentile (92 cm, -5.2SD) and The physical examination showed that the patient had a low nose bridge, binocular strabismus, mild pectus carinatum, and a slightly shorter right lower limb [Fig. 1]. Dermatological examination revealed pigmentation, sebaceous nevus, and epidermal verrucous nevus on the head, neck, chest, and upper limbs, as well as freckles. The scalp and face had mainly hairless, yellow-orange patches of varying sizes and shapes, with a few black papules. There were black, linear, rough patches on the neck following the Blaschko line, accompanied by pronounced verrucous hyperplasia in the shape of cauliflower, and dense needle-sized brown papules with soft pedicles. Additionally, the patient had large café au lait macules covered with dense tan spots on the abdomen and bilateral lower limbs. No obvious verrucous epidermal nevus was found, and the skin was smooth. The patient's parents and other family members did not report any similar symptoms.
The X-rays of both knees showed metaphyseal splaying and cupping [Fig. 2A-C]. Radiographs of long bones and chest revealed multiple abnormalities, including bone destruction accompanied by old pathological fractures in the right femoral diaphysis and the proximal tibia, as well as rickets. Further investigations showed low serum potassium levels ranging between 2.09 mmol/L and 3.5 mmol/L (normal range 3.7–5.2 mmol/L) and extremely low serum phosphorus levels between 0.28 mmol/L and 0.7 mmol/L (normal range for age: 1.37–1.99 mmol/L). Serum calcium ranged between 2.09 and 2.45 mmol/L (normal range 2.1–2.8 mmol/L). Serum alkaline phosphatase (649 U/L) and bone-specific alkaline phosphatase (223 ug/L) were increased. Normal results were found for serum osteocalcin, thyroid and parathyroid function, liver and kidney function, and calcitonin. Additionally, he was prescribed supplements for phosphate, potassium, and calcium.
2. Diagnostic assessment
A skin biopsy of his plantar lesion revealed shallow epithelial corners, pigmentation in the basal layer of the epidermis, hyperplasia of fibrous tissue in the dermis, and a small amount of lymphocyte infiltration [Fig. 1G]. Next-generation sequencing revealed a heterozygous missense mutation in HRAS gene, and then Sanger sequencing of DNA from affected skin tissue identified the missense variant c.37G > C, p.Gly13Arg in HRAS (NM_005343.4). The same variant has been reported in previous cases of CSHS [5] and is commonly found in sebaceous nevi [17]. Targeted high-depth sequencing showed that the HRAS p.Gly13Arg variant was present at a mosaic ratio of 20% in affected skin tissue, at about 1% in the proband’s blood and oral mucosa and at less than 1% in the proband’s hair follicle and normal skin [Supplementary Fig. S1 and Table S1]. The father’s blood contained a low number of reads of the variant, probably representing non-specific alignment. The variant was not found in DNA from the mother’s blood.
Quantification of serum FGF23 indicated an elevated level of 164.4 pg/ml (normal range 23.3–95.4 pg/ml).
3. Outcomes of therapeutic intervention
3.1 Mineral and bone metabolism assessments
Initially, the boy received oral phosphate (20–60 mg/kg/d, q6h), calcium (500 mg/d, qd) and calcitriol (0.25 µg/d, qd) supplementation for almost 5 months. The levels of alkaline phosphatase and bone specific alkaline phosphatase decreased. However, serum phosphorus remained low, between 0.55 mmol/L and 0.67 mmol/L. TmP/GFR was also at a low level of 0.51 mmol/L (age-appropriate normal range 1.2–2.6 mmol/L). After a washout period of conventional therapy for one week, the patient received burosumab injections at a dose of 0.8 mg/kg every 2–6 weeks because of the unexpected illness and constantly increased phosphorus level. The third dose of burosumab was delayed by intestinal obstruction for 4 weeks. Surprisingly, the serum phosphate level reached almost the normal range, even though the fourth injection was delayed for 1 week due to the boy having a fever. Because of a suspicious adverse event and a consistent increase in phosphorus levels, we decided to administer the injection at three-week intervals. As a result, serum Pi levels increased and remained within the normal range, rising from the baseline value of 0.67 mmol/L to 1.38 mmol/L after five doses over four months [Fig. 3A]. TmP/GFR increased from 0.51 mmol/L at baseline to 1.45 mmol/L [Fig. 3B]. Levels of calcium, 25-hydroxy vitamin D, alkaline phosphatase and bone specific alkaline phosphatase remained in normal range [Fig. 3C-F]. The β-isomerized C-terminal telopeptide of collagen type I increased from a normal value to 1.59 ng/ml at week 29 (normal range 0.3–0.6 ng/ml). Meanwhile, N-MID Osteocalcin showed a transient elevation, reaching a level of 83.59 ng/ml at week 21 (normal range 24–70 ng/ml). Calcitonin remained within the normal range, while parathyroid hormone increased from 2.7 pmol/L at baseline to 12.3 pmol/L at week 9 after 2 doses of burosumab. Parathyroid hormone levels then were within the normal range until week 29, where it increased to a higher value of 10 pmol/L again. As the blood parameters related to bone metabolism improved, the X-rays showed an improvement in the epiphyseal border of the long bones, with less splaying and cupping [Fig. 2D, E].
3.2 Skeletal and physical function evaluation
Burosumab treatment was associated with rapid catch-up growth. The patient experienced a significant improvement in their walking pattern after a 6-month course of FGF23 antibody treatment. Prior to receiving burosumab, the patient was unable to walk unassisted. After 6 months of treatment, the patient's height increased from 92 cm (-5.2SD) to 100 cm (-3.9SD). His PROMIS Pain Interference domain score improved from a baseline T-score of 65.9 ± 1.9 (mean ± SD) to 57.8 ± 2.0 after 6 months of burosumab therapy [Fig. 4A]. In the PROMIS Fatigue domain, the T-score at baseline was 73.4 ± 2.1, which decreased to 57.9 ± 2.1 after burosumab treatment [Fig. 4B]. Additionally, for PROMIS Physical Function Mobility, the T-score increased from 20.1 ± 2.5 to 28.1 ± 1.9 [Fig. 4C]. Furthermore, constant improvements in walking distances were observed. The mean distance covered during the 6-minute walk test (6MWT) increased gradually from 108 m (21.1% of the predicted distance) at week 0 to 180 m (37.2% of the predicted distance) at week 9, 220 m (42.4% of the predicted distance) at week 18, and finally to 270 m (52% of the predicted distance) at week 25 [Fig. 4D]. These changes in T-scores for both PROMIS and 6MWT results suggest meaningful improvements in the boy's physical function.
3.3 Safety
The patient experienced dynamic intestinal obstruction confirmed by abdominal X-ray examination after receiving two subcutaneous injections of burosumab [Fig. 2F]. As a result, conservative treatment was performed, and the injection interval was prolonged to 6 weeks. During the subsequent therapy course, burosumab was administered for 2–4 weeks based on the phosphate level, and intestinal obstruction did not recur. No other adverse events, such as hypercalciuria or nephrocalcinosis, were observed.