All patients presented here had common initial findings such as waddling gait, limb pain and/or skeletal deformities. In addition, the presence of congenital torticollis and tracheolaryngomalacia which are neurologic type-specific findings, is notable in one CDSMA and two MD patients; these findings have been reported in some patients with MD [4]. The age of onset in our cohort varied from infancy to early and late childhood, as CDSMA + classical MD < SEDK + SEDM < BCMY3 similar to reported patients [3–5, 9, 10, 12].
We have described seven missense and one deletion mutation in TRPV4 in 13 patients. TRPV4 mutations, mostly of the missense type, are not well understood in their exact pathogenic mechanism causing chondrodysplasia. A gain-of-function mechanism has been proposed [14, 15], but the observation of a heterozygous in-frame deletion mutation rules out the possibility that they are all explained by gain-of-function [12]. Krakow et al. [9] detected heterozygous missense mutations in TRPV4 in six SMDK and two MD cases; two mutations in TRPV4 showed a good genotype-phenotype correlation: Arg594His in exon 11 and Pro799Leu in exon 15 for SMDK and MD, respectively. Five of the SMDK patients presented here had this recurrent p.Arg594His variant [16]. However, this variant was also observed in two MD cases and in cases with a phenotype between SMDK and MD [5, 12]. Two of our patients with the MD phenotype had the p.Pro799Arg and p.Pro799Leu variants in exon 15, which were mostly associated with the classic MD phenotype [12]. This mutation was found in patients with the SEDM phenotype [6, 11, 17]. The p.Glu797Gly variant that occurred in our third patient has not yet been identified, but another substitution at the same residue (p.Glu797Lys) has been reported in several cases with mild MD, SMDK, and SEDM [8, 11, 12].
It has been reported that the clinical and radiologic features of patients with classic MD and SMDK overlap considerably [3, 5]. When comparing patients in early childhood, it was easy to distinguish clinically between patients with SMDK and MD; the trunk was short in patients with SMDK, whereas it was narrow and long in MD, and the extremities were short with a prominent knee. However, it can be difficult to distinguish between MD and SMDK as the trunk becomes shorter and the dumbbell-like appearance of the limbs disappears in the MD phenotype in late childhood, as described in previous reports [5, 8–10]. In one of the MD patients presented here, whom we followed until the age of 10 years, the ratio of sitting height to height had decreased from 0.54 to 0.52. While the SD of the height of our three MD patients at the age of 5 years was − 2.2, it decreased to -2.8 SD in one patient at the age of 10 years. As this patient had been closely monitored since the age of one and had received preventive physiotherapy, he had not developed kyphoscoliosis, so his height was not very short. In one cohort, the final height varied between 135 and 107 cm [13], but a woman with a final height of 145.5 cm was also reported [18]. During the follow-up of our SMDK patients, the sitting height to height ratio, which was 0.48 at 3 years of age, gradually decreased to below 0.45 at 10 years of age and 0.40 in adulthood (Table 1). Short stature occurred in SMDK patients after the age of 5 years; one female patient reached a final height of -4.4 SDS, and two male patients reached approximately − 6.5 SDS. In 1973, Kozlowski et al. [19] reported that patients with SMDK reached a final height of 130–150 cm. There are no recent cases of TRPV4 mutations in SMDK patients that have been followed up in recent years and reported final height. Only one 33-year-old woman with SMDK was reported who was 145 cm (-3 SDS) tall and complained of intermittent back pain but had no limitations in her activities of daily living [6].
The SMDK patients presented here gradually developed a barrel-shaped chest and pain when walking, running and climbing stairs over longer distances from late childhood to adulthood. In addition, they exhibited a swan neck deformity that became apparent during puberty and had previously been observed in patients with an intermediate phenotype [20]. The main complications reported in a cohort of patients with classic MD were short thorax and progressive kyphoscoliosis, as well as pain and gait disturbances due to hip and knee contractures [13]. Follow-up of our MD patients revealed marked kyphosis at an early age, which is more severe than SMDK with forward bending gait, and pain when walking long distances and climbing stairs increases with age.
The patients presented here, who were associated with SMDK and MD, had radiologic findings in early childhood that are well described in the literature, such as wafer-thin platyspondyly with elongated vertebral bodies, over-faced pedicles, flat acetabula, and severely delayed ossification of the carpal bones [3, 5, 8–10]. However, metaphyseal enlargement of the long bones, resulting in a dumbbell-shaped appearance with short limbs, is a unique feature of MD patients [3, 5, 10]. In addition, we underline that pelvic radiographs showed differences between SMDK and MD phenotypes; widening, cupping, and irregularity of the growth plate of the proximal femurs were associated with the SMDK phenotype, whereas the halberd-shaped appearance of the pelvis and proximal femurs and wide femoral necks were associated with MD patients. Interestingly, in our second family with the SMDK phenotype (Table 1) in which the father and two children were affected, one of the children (P3) had an intermediate phenotype between SMDK and MD with a halberd-shaped pelvis and proximal femora. Andreucci et al. [3] found that one affected mother and her two sons had within-family variability, with one having radiographic features more consistent with MD and the other more consistent with SMDK.
In the follow-up radiographs of patients with SMDK and MD, the most significant change was the widening of the metaphyses of the long bones in infancy (dumbbell-shaped deformity), which gradually regressed with increasing age, as has already been reported in patients with MD [5, 9, 10, 13]. In two adult patients with SMDK presented here, vertebral heights increased only slightly with age, but platyspondyly was still pronounced and the thorax was severely shortened due to severe kyphosis. In the study of Krakow et al. [9], three out of six SMDK patients and two MD patients had congenital scoliosis. However, both in our study and in the two studies in which MD patients were observed [13, 18], scoliosis occurred less frequently; the main spinal complication is severe kyphosis. In both SMDK and MD patients, ossification of the carpal bones was severely delayed during growth, but the epiphyses of the carpal bones are not irregular.
The main pelvic change in our patients with classic MD phenotype was the marked widening of the femoral neck, which increased with age, while the irregularities of the acetabulum and proximal metaphyses of the femur gradually increased in the SMDK phenotypes, and there was destruction of the femoral head and loss of the femur in the adult patients. It has been reported that children with SMDK have mild coxa valga in early childhood, but with the progression of metaphyseal dysplasia in the proximal femora, coxa vara begins with shortening of the femoral neck in late childhood [5, 9, 12]. In our study, the patients with SMDK, the obvious metaphyseal irregularity observed in the proximal femora is not obvious in other metaphyses of the long bones. Nishimura et al. [5] reported that metaphyseal dysplasia in long bones other than the pelvis does not appear until the age of 4 to 5 years in children with SMDK and is considered mild after this age. In our SMDK patients, the metaphyses of other long bones were not affected, whereas in the MD patients, the widening of the metaphyses was visible but decreased with increasing age. Only the patient with an intermediate phenotype between SMDK and MD had irregular metaphyses of the distal radius, and femur and proximal tibia.
In this study, a 3 bp deletion (c.1412_1414del; p.Phe471del) in TRPV4 was found in one patient. Patients carrying this mutation have been associated with classic or fatal MD phenotypes [8, 12]. Our patient had a waddling gait, limb pain, a narrow, long rib cage, and brachydactyly at the age of 5 years. Radiographs showed mild platyspondyly with over-faced pedicles, epiphyseal involvement of the hand, hip, and knee, mild metaphyseal irregularity, and short metacarpals and phalanges. Based on these findings, we consider that the clinical phenotype is compatible with the SEDM phenotype. The main features of SEDM are a short trunk with short hands and involvement of the epiphyses [22–24]. The age-dependent development of the phenotype is not well understood as it has been observed mainly in older children and adults. The history of one patient reported a narrow trunk in childhood [11]. The patient presented here was followed until the age of 15; trunk shortening gradually appeared after the age of 10 years and on his radiographs, the epiphyseal irregularities in the hands and knees were still present, while the epiphyses of the femoral head had improved and rectangular vertebral bodies, mild S-shaped scoliosis, short femoral necks and coxa vara were obvious at this age. The final height is reported to be 111–130 cm (between − 7.5 and − 8.5 SDS) in a small number of patients with SEDM [11]. Our patient had a height SDS of -1.7 at the age of 15 years.
Nishimura et al. [5] reported that the radiologic features of the SEDM phenotype include epiphyseal dysplasia, especially at the hip and knee, leading to early degenerative joint disease, moderate to severe platyspondyly, hypoplasia of the ilium, and horizontal acetabula. In our patient, the epiphyseal irregularities of the hip and knee were not progressive; iliac hypoplasia and horizontal acetabula were not present, only brachydactyly and rectangular vertebral bodies were present. His height was also within the normal range at the age of 15 years. These findings resembled a milder form of SEDM previously described in two families [11, 21].
The fact that TRPV4 mutations cause skeletal dysplasias was first demonstrated by Rock et al. in 2008 [14], when the heterozygous mutation was discovered in a patient with BCYM3. A recurrent mutation (p.Arg616Gln) observed in patients with BCYM3 phenotype was also identified in one of our patients and her mother (P10, P11) with typical BCYM3 findings [14, 25]. The mutation (p.Thr593Ile) found in our third BCYM3 patient was also described in a previous study [26]. BCYM3, a mild phenotype of TRPV4-associated dysplasia, is clinically characterized by scoliosis and a short trunk that occurs in prepuberty. Radiologically, the patients show over-faced pedicles, rectangular platyspondyly, short femoral necks, and coxa vara [14, 25, 27]. Two of the children with BCYM3 presented here had limb pain and scoliosis that developed at the age of 12–14 years; radiographs showed over-faced pedicles, rectangular platyspondyly, and scoliosis.
In adulthood, two children had leg pain during long walks, but all were able to move normally. They had progressive scoliosis and one of the children had undergone surgery. On the initial pelvic radiograph, it was noted that the vertical lines of the proximal femoral metaphyses, which were more prominent on the right side, disappeared at the age of 18 years and a short femoral neck and coxa vara developed. Interestingly, this unique radiologic finding was reported in a 13-year-old boy with BCYM3, which disappeared on follow-up radiographs [26]. The final height of two patients was normal, while the 38-year-old mother of one of the patients had short stature (-3.9 SDS) due to severe scoliosis. Short stature is generally not present but may be mild in some patients [25]. In a large family study, patients had chronic pain in the spine and hips that became evident at school age; growth was not affected in early childhood, but some patients reported worsening with age due to increasing spinal involvement [25].
There are increasing reports of patients with pathogenic TRPV4 variants with mixed phenotypes, including skeletal involvement and peripheral neuropathy [6]. Jedrzejowska et al. [28] described two families carrying the same variant (c.806G > A) as our patient with an overlap of CDSMA/SPSMA phenotypes, and skeletal abnormalities such as short stature, scoliosis, hip and knee contractures with hip dislocation, PEV, torticollis, and laryngomalacia. Our patient, who was consulted in infancy, had similar clinical findings. Skeletal radiographs include scoliosis, mild platyspondyly, abnormally shaped vertebral bodies, and over-faced pedicles. At around 9 years of age, he was almost normal height and had no other problems apart from mild laryngomalacia, scoliosis, and mild knee limitation.
There are some limitations to this study. Since the MD and CDSMA patients were followed until the ages of 10 and 9 years, respectively, the radiographs of puberty and adulthood could not be analyzed.
In conclusion, we would like to emphasize that patients with SMDK and classic MD develop a short trunk, severe short stature, a barrel-shaped chest with kyphosis that begins at around 5 years of age and increases with age, as well as bone pain when running, walking and climbing stairs. All BCYM3 and CDSMA patients had severe scoliosis. However, at the follow-up, the BCYM3, SEDM, and CDSMA patients were able to walk normally and were of normal height. In the SMDK patients, a swan-neck deformity of the fingers were present. Radiologic differentiators between SMDK and MD phenotypes were primarily radiographs of the pelvis: SMDK patients had an irregular proximal femoral metaphysis, whereas MD patients typically had marked femoral neck widening. Interestingly, the metaphyseal dysplasia of the long bones other than the proximal femur in SMDK patients was so subtle that it could be ignored. In one patient, the clinical phenotype was consistent with the mild SEDM form. Metaphyseal vertical lines in the proximal femur, previously reported in only one case, were present in one of our BCYM3 patients and disappeared in adulthood. Intrafamilial variability was observed in the SMDK and BCYM3 families. We believe that the results of this study will be invaluable for the diagnosis and treatment of various forms of TRPV4-associated skeletal dysplasias.