LMX1B gene is located on chromosome 9q34, including eight exons, producing an mRNA transcript approximately 7 kb in length.[9] It has two isoforms, containing 395 and 402 amino acids respectively[18]. The protein contains two zinc-binding LIM domains at the NH2-terminus and a DNA-binding homeodomain (HD). The LIM domains mediate protein-protein interactions[19]. The homeodomain is a highly conserved domain initially characterized in Drosophila[20], constituting a sequence-specific DNA-binding motif and involved in modulating gene transcriptional programs. Additionally, homeodomain proteins play regulatory roles in homeotic transformations that result in the morphological conversion of one body segment to another. The first LIM domain is generated from exon 2, while exon 3 encodes the second LIM domain. The homeodomain is encoded by exons 4 through 6.[6] The majority of pathogenic LMX1B mutations in nail-patella syndrome are located in exons 2 to 6, affecting the LIM domains and homeodomain[18, 21, 22]. Mutations in LIM domains can affect its function of binding the zinc, whereas mutations in the homeodomain perturb sequence-specific DNA binding.
Haploinsufficiency is the main cause of NPS[21, 22]. In patients with various LMX1B mutations, including missense, nonsense, frameshift or splice-site mutations, even in complete deletions, the range and severity of symptoms showed no obvious difference. What’s more, no pathogenic LMX1B genotype was found by sequence analysis[22]. The minimum LMX1B dose required for development differs between species. LMX1B heterozygous (+/-) mice are completely normal, while the LMX1B(-/-) mice will show symptoms [7]. While in another study, heterozygous LMX1B knockout mice (LMX1B+/-) showed reduced compensatory kidney growth compared to wildtype LMX1B mice (LMX1B+/+) after inducing kidney damage through unilateral nephrectomy[23]. This further supports the idea that a minimum level of LMX1B is critical for normal kidney development, which also indicates that the phenotype of NPS is related to the dosage of LMX1B.
A distinct genotype-phenotype correlation could not be established in NPS patients. Even within the same family, substantial phenotypic heterogeneity is observed. In our case, the proband had more severe elbow contractures than his mother. In radiography, the proband showed bilateral radial head dislocation, but on his mother, dislocation only presented on the left side. The proband displayed foot involvement, while his mother had acetabular dysplasia. The proband also showed more severe nail and patella hypoplasia relative to his mother and sister. Gene sequencing revealed the mutation was located in exon 5, encoding the homeodomain region of the protein. Previous reports associated mutations in this region with increased risk of nephropathy[24]. However, our patients mainly present with skeletal manifestations, with normal kidney function.
LMX1B is essential for dorsal-ventral patterning during limb development[25], which is possibly the mechanism of musculoskeletal symptoms. LMX1B is responsible for establishing dorsal identity in the limb mesoderm, where its expression is induced by the dorsal ectodermal signal Wnt7a[26]. Loss of LMX1B in mice leads to defective patterning of dorsal autopod (hand/foot) and zeugopod (forearm/leg). The autopods display symmetrical ventral characteristics, including dorsal-type footpads, absent dorsal hair follicles and nails, as well as matching ventral-type muscles, tendons and ligaments. And the ectopic expression of LMX1B at the ventral part of the limb will make the muscles and tendons more dorsally[26]. Mutations in LMX1B impair dorsalization during limb development, leading to hypoplasia of dorsal structures, such as nails and patella.
Musculoskeletal deformities can affect multiple anatomical regions throughout the body, including nail, patella, elbow, hip and feet. Nail involvement is one of the cardinal features of the NPS, which can be seen in 95% of patients, including triangular or absent of the lunula, anonychia, hypoplasia, longitudinal ridging, splitting, and koilonychia[27]. The severity decreases from the ulnar side to the radial side. The thumbnails are the most often infected. The toenails are rarely involved. The dorsal skin of the fingers can also get involved. The loss of creases overlying the dorsal surface of finger joints can lead to restricted flexion of distal interphalangeal (DIP) joints. The severity gradient of dorsal skin is the same as that in nail changes, with index fingers most severely impacted. Hyperextension of proximal interphalangeal (PIP) joints along with DIP flexion ("swan-necking") and fifth finger clinodactyly also be found in some cases[1].
Patella deformity is another main manifestation of NPs. The patella can show asymmetric hypoplasia, irregular shape or even absence, with lateral and superior displacement. Most patella deformities are asymptomatic[28]. When symptomatic, manifestations containing pain, instability, locking, clicking, patella dislocation and inability to fully extend the knee may be displayed, which may relate to underdevelopment of the vastus medialis muscle. Knee flexion contractures can result from tight hamstrings. Other possible findings are osteochondritis dissecans, synovial plicae, anterior cruciate ligament absence. Early onset osteoarthritis is common.
Dislocation or subluxation of the radial head is the common manifestation of elbows in NPS. The antecubital pterygium, which refers to webbing of skin across the anterior elbow, can also be discovered. Dysplasia of the elbow is usually asymptomatic, but some patients may also get involved in restricted range of motion in extension, pronation and supination[29]. For those with limited range of motion, soft tissue releases or radial head excision may be necessary[4].
Another pathognomonic manifestation of NPS is the iliac horns, which refers to pathognomonic bony projections arising from the central part of the iliac bones, pointing posteriorly and laterally[30]. They are typically asymptomatic and are usually detectable only by radiography, although large ones may be palpable. Iliac horns do not need any treatment.
Foot abnormalities in NPS conclude talipes equinovarus, calcanovarus, pes planus, calcaneovalgus, and hyperdorsiflexion[1]. Tight Achilles tendons is also a common manifestation. Unlike nail or patella disorders, the incidence of foot involvement is relatively lower. But foot deformities are the primary indication for surgery treatment[4].
LMX1B mutations also disrupt expression of collagen IV chains and podocyte differentiation, resulting in the glomerular basement membrane abnormalities and renal disease[30]. There is substantial individual variability in manifestations, even within a single family, from proteinuria to renal failure. In one pair of identical twins with nail-patella syndrome, one exhibited rapid progression to renal failure while the other displayed persistent proteinuria only[31]. In one extreme case, renal function rapidly declined and the child died from renal failure at 8 years old[32]. The initial renal manifestation in NPS is typically proteinuria[33]. After proteinuria present, it may lead to various outcomes. It may spontaneously resolve, persist as asymptomatic manifestation, advance to nephrotic syndrome or nephritis, or end up as renal failure[30]. Although mutations in the HD domain of the LMX1B gene may tend to display renal dysfunction[2], factors precipitating renal function decline remain unclear.
LMX1B is high-expressed at the anterior segment during ocular development[34]. The low dosage protein will cause ocular dysplasia In NPS patients. The predominant ocular manifestations are primary open angle glaucoma and Lester's sign which refers to a cloverleaf or flower-shaped zone of pigmentation around the central iris[28]. Other ocular symptoms include microcornea, sclerocornea, congenital cataract and iris processes.
Identifying potential genotype-phenotype correlations represents a pivotal research direction in nail-patella syndrome. Further investigations are warranted to illuminate the upstream regulators and downstream effectors influencing LMX1B functionality. For instance, Haro et al. identified two Lmx1b-associated regulatory modules, LARM1 and LARM2, which are bound by Lmx1b to potentiate its expression during limb dorsalization in discrete spatial patterns. Ablation of these elements exclusively disrupted dorsalization, leaving other LMX1B-mediated processes intact[31, 35]. Other groups have delineated that E47 upregulates the transcriptional activity of LMX1B[16]. LDB1 downregulates the activity of LMX1B through E47. As a transcription factor implicated in diverse physiological pathways, substantial research is necessitated to disambiguate the complex roles of LMX1B and the pathogenesis of nail-patella syndrome.