Christine M. Alvarez

Disruption of the endocytic protein HIP1 results in neurological deficits and decreased AMPA receptor trafficking

Huntingtin interacting protein 1 (HIP1) is a recently identified component of clathrin‐coated vesicles that plays a role in clathrin‐mediated endocytosis. To explore the normal function of HIP1 in vivo, we created mice with targeted mutation in the HIP1 gene (HIP1−/−). HIP1−/− mice develop a neurological phenotype by 3 months of age manifest with a failure to thrive, tremor and a gait ataxia secondary to a rigid thoracolumbar kyphosis accompanied by decreased assembly of endocytic protein complexes on liposomal membranes. In primary hippocampal neurons, HIP1 colocalizes with GluR1‐containing AMPA receptors and becomes concentrated in cell bodies following AMPA stimulation. Moreover, a profound dose‐dependent defect in clathrin‐mediated internalization of GluR1‐containing AMPA receptors was observed in neurons from HIP1−/− mice. Together, these data provide strong evidence that HIP1 regulates AMPA receptor trafficking in the central nervous system through its function in clathrin‐mediated endocytosis.

The genotype–phenotype correlation of hereditary multiple exostoses

Hereditary multiple exostoses (HME) is an autosomal dominant condition with a wide spectrum of clinical presentations. The purpose of this study was to determine the relationship between the genotype and the phenotype in HME. Thirty‐two affected individuals from 10 families participated in the study. An extensive description of HME phenotype in terms of the anatomical burden of disease involved clinical and radiographic examinations and evaluation of 76 parameters. Mutations were determined by sequencing the EXT 1 and EXT 2 genes. Mutations were found in eight families (26 individuals), with one mutation previously reported in the literature and seven novel mutations. There were seven subjects with an EXT 1 mutation and 16 with an EXT 2 mutation. Patients with EXT 1 mutation were found to have more exostoses, more limb malalignment with shorter limb segments and height, and more pelvic and flatbone involvement. A genotype–phenotype correlation exists in HME, with patients with EXT 1 mutations having a higher degree of anatomical burden.

Treatment of idiopathic clubfoot utilizing botulinum A toxin: a new method and its short-term outcomes.

A pivotal point in most clubfoot management protocols is Achilles tendon lengthening or tenotomy to address hindfoot deformity. The effectiveness of botulinum A toxin (BTX-A) in attenuating the function of the triceps surae muscle complex as an alternative to tenotomy was investigated. Fifty-one patients with 73 idiopathic clubfeet were recruited. Outcome measures included surgical rate, Pirani clubfoot score, ankle dorsiflexion with knee in flexion and extension, and recurrences. Patients were divided according to age: group 1 (<30 days old) and group 2 (>30 days and <8 months old). Ankle dorsiflexion in knee flexion and extension remained above 20 degrees and 15 degrees, respectively, and Pirani scores below 0.5 following BTX-A injection for both groups. One of the 51 patients required limited posterior release and 9 patients required repeat manipulation and casting plus or minus BTX-A injection. The use of BTX-A as an adjunctive therapy in the noninvasive approach of manipulation and casting in idiopathic clubfoot is a safe and effective treatment.

Normative data for the dynamic pedobarographic profiles of children

In order to establish the clinical utility of pedobarography in the treatment of childhood foot pathology, a reliable set of pedobarograph data describing non-pathologic feet is required. The purpose of this study was to describe the pedobarographic profiles of normal children across all ages, with specific focus on young children and explore age-related differences in foot pressure patterns. The Tekscan HR Mat pressure measurement system was used in a protocol involving a dynamic test at self-selected speed and walking pattern of 146 normal children (age range 1.6-14.9 years). Relative force and timing data were obtained across five foot segments (heel, lateral midfoot, medial midfoot, lateral forefoot, and medial forefoot). Analysis of variance (ANOVA) techniques were applied to determine if there were any age-related differences in foot pressure profiles in children across four a priori pedobarograph variables: % of stance at initiation at the heel, % of stance at initiation at the medial midfoot, maximum % force at the heel, maximum % force at the medial midfoot. Differences in foot pressure profiles were distinguished across three age groups: (1) Group 1: <2 years; (2) Group 2: 2-5 years; and (3) Group 3: >5 years. Age-related differences in initiation patterns, force transmission, and the amount of time spent on each foot segment provide evidence for maturation of childrens foot pressure profiles from a flatfoot pattern in the young child to a curvilinear pattern in the older child.

Evaluation of the anatomic burden of patients with hereditary multiple exostoses.

Hereditary multiple exostosis (HME) is an autosomal dominant condition resulting predominantly from mutations in the exostosin 1 (EXT1) and exostosin 2 (EXT2) genes. We asked two questions in our study: first, what is the anatomic burden of subjects with HME; second, is there a difference in anatomic burden in subjects with EXT 1 versus EXT 2. The anatomic burden experienced by HME patients was defined according to three domains: (1) lesion quality; (2) limb malalignment and deformity; and (3) limb segment lengths and percentile height. Seventy-nine subjects with HME were included in this study. Of these 79 phenotypes were completed. Forty-eight genotypes were confirmed leaving 48 complete genotype-phenotype profiles for analysis. Analysis of the coding and flanking intronic regions of EXT1 and EXT2 was performed in each patient by direct sequencing of PCR-amplified genomic DNA. All three domains of anatomic burden showed a wide range of presentation in the HME study sample. More lesions and greater tendency to flat bone occurrence was associated with EXT1. EXT1 patients were shorter. All limb segments tended to be shorter for EXT1 subjects. EXT1 subjects showed more anatomic burden with respect to lesion quality and height. Level of Evidence: Level II, prognostic study. See Guidelines for Authors for a complete description of levels of evidence.

Studies of TBX4 and chromosome 17q23.1q23.2: an uncommon cause of nonsyndromic clubfoot

Clubfoot is a common birth defect characterized by inward posturing and rigid downward displacement of one or both feet. The etiology of syndromic forms of clubfoot is varied and the causes of isolated clubfoot are not well understood. A microduplication of 2.2 Mb on chromosome 17q23.1q23.2 which includes T‐box 4 (TBX4), a hindlimb‐specific gene, and 16 other genes was recently identified in 3 of 66 families reported as nonsyndromic clubfoot, but additional non‐foot malformations place them in the syndromic clubfoot category. Our study assesses whether variation in or around TBX4 contributes to nonsyndromic clubfoot. To determine whether this microduplication was a common cause of nonsyndromic clubfoot, 605 probands (from 148 multiplex and 457 simplex families) with nonsyndromic clubfoot were evaluated by copy number and oligonucleotide array CGH testing modalities. Only one multiplex family (0.68%) that had 16 with clubfoot and 9 with other foot anomalies, had a 350 kb microduplication, which included the complete duplication of TBX4 and NACA2 and partial duplication of BRIP1. The microduplication was transmitted in an autosomal dominant pattern and all with the microduplication had a range of phenotypes from short wide feet and toes to bilateral clubfoot. Minimal evidence was found for an association between TBX4 and clubfoot and no pathogenic sequence variants were identified in the two known TBX4 hindlimb enhancer elements. Altogether, these results demonstrate that variation in and around the TBX4 gene and the 17q23.1q23.2 microduplication are not a frequent cause of this common orthopedic birth defect and narrows the 17q23.1q23.2 nonsyndromic clubfoot‐associated region.

Outcome of prenatally diagnosed isolated clubfoot

To analyze the aneuploidy risk and treatment outcome of prenatally diagnosed isolated clubfoot, to determine the false‐positive rate (FPR) of ultrasound diagnosis and to calculate the risk of diagnostic revision to complex clubfoot.

Variants in genes that encode muscle contractile proteins influence risk for isolated clubfoot.

Isolated clubfoot is a relatively common birth defect that affects approximately 4,000 newborns in the US each year. Calf muscles in the affected leg(s) are underdeveloped and remain small even after corrective treatment. This observation suggests that variants in genes that influence muscle development are priority candidate risk factors for clubfoot. This contention is further supported by the discovery that mutations in genes that encode components of the muscle contractile complex (MYH3, TPM2, TNNT3, TNNI2, and MYH8) cause congenital contractures, including clubfoot, in distal arthrogryposis (DA) syndromes. Interrogation of 15 genes encoding proteins that control myofiber contractility in a cohort of both non‐Hispanic White (NHW) and Hispanic families, identified positive associations (P < 0.05) with SNPs in 12 genes; only 1 was identified in a family‐based validation dataset. Six SNPs in TNNC2 deviated from Hardy–Weinberg equilibrium in mothers in our NHW discovery dataset. Relative risk and likelihood ratio tests showed evidence for a maternal genotypic effect with TNNC2/rs383112 and an inherited/child genotypic effect with two SNPs, TNNC2/rs4629 and rs383112. Associations with multiple SNPs in TPM1 were identified in the NHW discovery (rs4075583, P = 0.01), family‐based validation (rs1972041, P = 0.000074), and case–control validation (rs12148828, P = 0.04) datasets. Gene interactions were identified between multiple muscle contraction genes with many of the interactions involving at least one potential regulatory SNP. Collectively, our results suggest that variation in genes that encode contractile proteins of skeletal myofibers may play a role in the etiology of clubfoot.

A kinematic description of dynamic midfoot break in children using a multi-segment foot model

Midfoot break (MFB) is a foot deformity that occurs most commonly in children with cerebral palsy (CP), but may also affect children with other developmental disorders. Dynamic MFB develops because the muscles that cross the ankle joint are hypertonic, resulting in a breakdown and dysfunction of the bones within the foot. In turn, this creates excessive motion at the midfoot. With the resulting inefficient lever arm, the foot is then unable to push off the ground effectively, resulting in an inadequate and painful gait pattern. Currently, there is no standard quantitative method for detecting early stages of MFB, which would allow early intervention before further breakdown occurs. The first step in developing an objective tool for early MFB diagnosis is to examine the difference in dynamic function between a foot with MFB and a typical foot. Therefore, the main purpose of this study was to compare the differences in foot motion between children with MFB and children with typical feet (Controls) using a multi-segment kinematic foot model. We found that children with MFB had a significant decrease in peak ankle dorsiflexion compared to Controls (1.3 ± 6.4° versus 8.6 ± 3.4°) and a significant increase in peak midfoot dorsiflexion compared to Controls (15.2 ± 4.9° versus 6.4 ± 1.9°). This study may help clinicians track the progression of MFB and help standardize treatment recommendations for children with this type of foot deformity.

Health-related Quality of Life in People With Hereditary Multiple Exostoses

Background: Hereditary multiple exostoses (HME) is a rare genetic disorder, which can be associated with severe complications that may significantly affect the health-related quality of life (HRQL). Our primary objective was to describe the baseline HRQL in HME individuals at the British Columbia’s Children’s Hospital HME clinic and the Multiple Hereditary Exostoses Coalition compared with relevant Canadian and US population norms. This is the first study to explore the HRQL among adults and children with HME. Methods: Previously validated instruments Short Form-36 version 2, Short form-6D, and Child Health Questionnaire Parent Form 50 were used to assess the HRQL of individuals with HME. The scores from these instruments were compared with the relevant population norms. The British Columbia’s Children’s Hospital and Multiple Hereditary Exostoses coalition populations were also compared with each other. Results: The study sample consisted of 100 participants including 57 adults and 43 children. The mean age for Short Form 36 version 2 survey was 40.10±13.01 years and for Child Health Questionnaire Parent Form 50 was 9.93±3.48 years. Adult HME population had lower scores than both the US and Canadian general population in all domains except for emotional role limitations. Short Form -6D utility scores (0.65) indicates the quality of life for some individuals is near death and for others it is comparable or better than individuals with rheumatoid arthritis. Children with HME scored less than the US general population; particularly lower scores were seen in bodily pain (51.2 vs. 81.7) and emotional self-esteem (52.0 vs. 79.8). Conclusions: HME population has lower HRQL than the general population. These data provide a benchmark for individuals with HME. From such data, future research on HME disease progression and effectiveness of treatments/interventions can be tracked over time. Level of Evidence: Level II, This is a prognostic, prospective study with participants enrolled at different points in their disease.