Biallelic variants in Plexin B2 (PLXNB2) cause amelogenesis imperfecta, hearing loss and intellectual disability.

BACKGROUND: Plexins are large transmembrane receptors for the semaphorin family of signalling proteins. Semaphorin-plexin signalling controls cellular interactions that are critical during development as well as in adult life stages. Nine plexin genes have been identified in humans, but despite the apparent importance of plexins in development, only biallelic PLXND1 and PLXNA1 variants have so far been associated with Mendelian genetic disease. METHODS: Eight individuals from six families presented with a recessively inherited variable clinical condition, with core features of amelogenesis imperfecta (AI) and sensorineural hearing loss (SNHL), with variable intellectual disability. Probands were investigated by exome or genome sequencing. Common variants and those unlikely to affect function were excluded. Variants consistent with autosomal recessive inheritance were prioritised. Variant segregation analysis was performed by Sanger sequencing. RNA expression analysis was conducted in C57Bl6 mice. RESULTS: Rare biallelic pathogenic variants in plexin B2 (PLXNB2), a large transmembrane semaphorin receptor protein, were found to segregate with disease in all six families. The variants identified include missense, nonsense, splicing changes and a multiexon deletion. Plxnb2 expression was detected in differentiating ameloblasts. CONCLUSION: We identify rare biallelic pathogenic variants in PLXNB2 as a cause of a new autosomal recessive, phenotypically diverse syndrome with AI and SNHL as core features. Intellectual disability, ocular disease, ear developmental abnormalities and lymphoedema were also present in multiple cases. The variable syndromic human phenotype overlaps with that seen in Plxnb2 knockout mice, and, together with the rarity of human PLXNB2 variants, may explain why pathogenic variants in PLXNB2 have not been reported previously.

Heterozygous COL17A1 variants are a frequent cause of amelogenesis imperfecta.

BACKGROUND: Collagen XVII is most typically associated with human disease when biallelic COL17A1 variants (>230) cause junctional epidermolysis bullosa (JEB), a rare, genetically heterogeneous, mucocutaneous blistering disease with amelogenesis imperfecta (AI), a developmental enamel defect. Despite recognition that heterozygous carriers in JEB families can have AI, and that heterozygous COL17A1 variants also cause dominant corneal epithelial recurrent erosion dystrophy (ERED), the importance of heterozygous COL17A1 variants causing dominant non-syndromic AI is not widely recognised. METHODS: Probands from an AI cohort were screened by single molecule molecular inversion probes or targeted hybridisation capture (both a custom panel and whole exome sequencing) for COL17A1 variants. Patient phenotypes were assessed by clinical examination and analyses of affected teeth. RESULTS: Nineteen unrelated probands with isolated AI (no co-segregating features) had 17 heterozygous, potentially pathogenic COL17A1 variants, including missense, premature termination codons, frameshift and splice site variants in both the endo-domains and the ecto-domains of the protein. The AI phenotype was consistent with enamel of near normal thickness and variable focal hypoplasia with surface irregularities including pitting. CONCLUSION: These results indicate that COL17A1 variants are a frequent cause of dominantly inherited non-syndromic AI. Comparison of variants implicated in AI and JEB identifies similarities in type and distribution, with five identified in both conditions, one of which may also cause ERED. Increased availability of genetic testing means that more individuals will receive reports of heterozygous COL17A1 variants. We propose that patients with isolated AI or ERED, due to COL17A1 variants, should be considered as potential carriers for JEB and counselled accordingly, reflecting the importance of multidisciplinary care.

Review of 1.75 million referrals over 34 months identifies the disruptive impact of the SARS-CoV-2 pandemic on oral surgery care in England: a service evaluation.

Aims This study aimed to use electronic referral management system (eRMS) oral surgery data across multiple sites in England to evaluate the service over a 34-month period in relation to: 1) pre- and post-pandemic referral rates in oral surgery; 2) examining the data for signs of inequality in obtaining a referral for oral surgery; and 3) considering the impact on service provision for oral surgery in England.Methods Oral surgery referral data were available from an eRMS for areas of England covered by this service for the 34-month period of March 2019 to December 2021 (inclusive), which included 12 months of pre-pandemic data and the first 22 months of the pandemic. The data were from the following regions in England: Central Midlands; Cheshire and Merseyside; East Anglia and Essex; Greater Manchester; Lancashire; Thames Valley; and Yorkshire and the Humber.Results The total number of referrals received was 1,766,895 during this 34-month period, with pre-pandemic referral levels averaging at 25,498 per month, with a reduction to 698 per month in April 2020. Referrals have risen to a peak of 217,646 for the month of November 2021. An average of 1.5% of referrals were rejected pre-pandemic, compared with 2.7% per month post-pandemic.Discussion Pre-pandemic referral numbers were predictably stable within a narrow range which have then increased dramatically post-pandemic. The variations in oral surgery referral patterns place significant strain on oral surgery services across England. This not only has consequences on the patient experience, but also on workforce and workforce development, to ensure that there is not a long-term destabilising impact.Conclusion Analysis of 1.75 million referrals to oral surgery services in England has highlighted the ongoing impact of the pandemic and the need to actively minimise adverse impacts on patients, NHS services and the workforce.

The changing face of a dental school: a Leeds perspective.

Time spent as a student at a dental school leaves a legacy that shapes how each of us develops professionally and more generally as our lives progress. Personal reminiscences relate to our own time as a student. Comments years later that 'things are not the same' are true. The reality is that dental schools are constantly reinventing themselves and can never afford to stand still.The aim of this opinion piece is to highlight the priorities and direction of the School of Dentistry at the University of Leeds and how these are informed by the past. Innovation in education and research are at the centre of the changes that are being driven forwards within a framework where equality, diversity and inclusion are increasingly incorporated as business as usual. COVID-19 disruption has driven change and brought a new confidence in our ability to deliver this. It is an exciting time to be part of the School.

An evaluation of referrer factors for 98,671 referrals made to the West Yorkshire oral surgery managed clinical network over a three-year period.

Introduction Patients referred from primary dental care to hospital-based specialists in high volumes can contribute to significant NHS service pressures. Surprisingly, little is understood about what contributes to referral factors.Aims To gain new insight into the referral factors from primary dental care by interrogating the tri-speciality West Yorkshire managed clinical network (MCN) referral pathway data for a 36-month period (2016-2019).Methods Anonymised referrals from the electronic referral management system were collated for analyses.Results There were 98,671 referrals within the 36-month period, 12.3% of which were rejected. Of those accepted for triage, 76% were directed at oral surgery, with >60% accounted for by exodontia. In total, 10% of referrers accounted for 60% of all referrals. Peak referral occurred five years after General Dental Council registration.Discussion This is the first report of referral data from a tri-speciality MCN with exodontia referrals predominating. The data set demonstrates variation in referrer behaviours despite referral guidance. Referrals should be based upon patient need but patterns observed in this study suggested possible associations with high and low referral patterns which warrant further research.Conclusions Interrogation of the referral database suggests that there are interesting patterns of referral which may be associated with characteristics of the referrer as well as their patients' needs. Further investigation could inform improved processes and service design, as well as education delivery and workforce development.

Spectrum of pathogenic variants and founder effects in amelogenesis imperfecta associated with MMP20.

Amelogenesis imperfecta (AI) describes a heterogeneous group of developmental enamel defects that typically have Mendelian inheritance. Exome sequencing of 10 families with recessive hypomaturation AI revealed four novel and one known variants in the matrix metallopeptidase 20 (MMP20) gene that were predicted to be pathogenic. MMP20 encodes a protease that cleaves the developing extracellular enamel matrix and is necessary for normal enamel crystal growth during amelogenesis. New homozygous missense changes were shared between four families of Pakistani heritage (c.625G>C; p.(Glu209Gln)) and two of Omani origin (c.710C>A; p.(Ser237Tyr)). In two families of UK origin and one from Costa Rica, affected individuals were homozygous for the previously reported c.954-2A>T; p.(Ile319Phefs*19) variant. For each of these variants, microsatellite haplotypes appeared to exclude a recent founder effect, but elements of haplotype were conserved, suggesting more distant founding ancestors. New compound heterozygous changes were identified in one family of the European heritage: c.809_811+12delinsCCAG; p.(?) and c.1122A>C; p.(Gln374His). This report further elucidates the mutation spectrum of MMP20 and the probable impact on protein function, confirms a consistent hypomaturation phenotype and shows that mutations in MMP20 are a common cause of autosomal recessive AI in some communities.

A missense mutation of Leu74Pro of OGR1 found in familial amelogenesis imperfecta actually causes the loss of the pH-sensing mechanism.

Ovarian cancer G protein-coupled receptor 1 (OGR1), also known as GPR68, is a proton-sensing G protein-coupled receptor (GPCR) coupling to Gq/11/phospholipase C/Ca2+ signaling pathways. The specific histidine residues at the extracellular surface of OGR1 are suggested to be involved in the proton sensing. Later, some metal ions, including nickel ion (Ni2+), are also indicated to be OGR1 ligands. OGR1 polymorphic variants have recently been found in three families with amelogenesis imperfecta, which suggested that OGR1 is required for the process of dental enamel formation. One of these families possesses a missense mutation from leucine to proline at 74 (L74P) of OGR1. In the present study, we characterized HEK293 cells with L74P OGR1 (L74P-OGR1) and hemagglutinin (HA)-tag, as compared with cells with wild-type OGR1 (WT-OGR1) and HA-tag. We found that either acidic pH or NiCl2 induced intracellular Ca2+ mobilization and morphological change in WT-OGR1-transfected cells; however, the extracellular stimulus-induced actions were severely damaged in L74P-OGR1-transfected cells. We further confirmed that either WT-OGR1 or L74P-OGR1 is localized mainly in the surface of the cells, but only WT-OGR1 is internalized in response to acidification or NiCl2. Thus, the L74P-OGR1 protein may be distributed in the plasma membranes but severely damaged in the receptor functions. We speculate that L74P in the second transmembrane domain in OGR1 may result in conformational changes in the receptor, thereby disturbing the sensing extracellular signals, i.e., protons or metal ions, and/or transducing them to the intracellular signaling machinery through G proteins.

A missense variant in specificity protein 6 (SP6) is associated with amelogenesis imperfecta.

Amelogenesis is the process of enamel formation. For amelogenesis to proceed, the cells of the inner enamel epithelium (IEE) must first proliferate and then differentiate into the enamel-producing ameloblasts. Amelogenesis imperfecta (AI) is a heterogeneous group of genetic conditions that result in defective or absent tooth enamel. We identified a 2 bp variant c.817_818GC>AA in SP6, the gene encoding the SP6 transcription factor, in a Caucasian family with autosomal dominant hypoplastic AI. The resulting missense protein change, p.(Ala273Lys), is predicted to alter a DNA-binding residue in the first of three zinc fingers. SP6 has been shown to be crucial to both proliferation of the IEE and to its differentiation into ameloblasts. SP6 has also been implicated as an AI candidate gene through its study in rodent models. We investigated the effect of the missense variant in SP6 (p.(Ala273Lys)) using surface plasmon resonance protein-DNA binding studies. We identified a potential SP6 binding motif in the AMBN proximal promoter sequence and showed that wild-type (WT) SP6 binds more strongly to it than the mutant protein. We hypothesize that SP6 variants may be a very rare cause of AI due to the critical roles of SP6 in development and that the relatively mild effect of the missense variant identified in this study is sufficient to affect amelogenesis causing AI, but not so severe as to be incompatible with life. We suggest that current AI cohorts, both with autosomal recessive and dominant disease, be screened for SP6 variants.

New missense variants in RELT causing hypomineralised amelogenesis imperfecta.

Amelogenesis imperfecta (AI) is a heterogeneous group of genetic diseases characterised by dental enamel malformation. Pathogenic variants in at least 33 genes cause syndromic or non-syndromic AI. Recently variants in RELT, encoding an orphan receptor in the tumour necrosis factor (TNF) superfamily, were found to cause recessive AI, as part of a syndrome encompassing small stature and severe childhood infections. Here we describe four additional families with autosomal recessive hypomineralised AI due to previously unreported homozygous mutations in RELT. Three families carried a homozygous missense variant in the fourth exon (c.164C>T, p.(T55I)) and a fourth family carried a homozygous missense variant in the 11th exon (c.1264C>T, p.(R422W)). We found no evidence of additional syndromic symptoms in affected individuals. Analyses of tooth microstructure with computerised tomography and scanning electron microscopy suggest a role for RELT in ameloblasts' coordination and interaction with the enamel matrix. Microsatellite genotyping in families segregating the T55I variant reveals a shared founder haplotype. These findings extend the RELT pathogenic variant spectrum, reveal a founder mutation in the UK Pakistani population and provide detailed analysis of human teeth affected by this hypomineralised phenotype, but do not support a possible syndromic presentation in all those with RELT-variant associated AI.

Novel DLX3 variants in amelogenesis imperfecta with attenuated tricho-dento-osseous syndrome.

OBJECTIVES: Variants in DLX3 cause tricho-dento-osseous syndrome (TDO, MIM #190320), a systemic condition with hair, nail and bony changes, taurodontism and amelogenesis imperfecta (AI), inherited in an autosomal dominant fashion. Different variants found within this gene are associated with different phenotypic presentations. To date, six different DLX3 variants have been reported in TDO. The aim of this paper was to explore and discuss three recently uncovered new variants in DLX3. SUBJECTS AND METHODS: Whole-exome sequencing identified a new DLX3 variant in one family, recruited as part of an ongoing study of genetic variants associated with AI. Targeted clinical exome sequencing of two further families revealed another new variant of DLX3 and complete heterozygous deletion of DLX3. For all three families, the phenotypes were shown to consist of AI and taurodontism, together with other attenuated features of TDO. RESULTS: c.574delG p.(E192Rfs*66), c.476G>T (p.R159L) and a heterozygous deletion of the entire DLX3 coding region were identified in our families. CONCLUSION: These previously unreported variants add to the growing literature surrounding AI, allowing for more accurate genetic testing and better understanding of the associated clinical consequences.

Dentin dysplasia: diagnostic challenges.

Dentin dysplasia(DD) is a rare autosomal dominant disorder associated with disturbance of the dentin. While the crowns appear clinically normal, on radiography, the pulp spaces appear partially or completely obliterated, with short blunted roots, and multiple periapical radiolucencies affecting the apparently sound teeth. Clinical signs include spontaneous abscess formation or increased tooth mobility which can lead to exfoliation. DD can therefore have a significant impact on the patient's dentition, and treatment is often challenging. Shields' classification of dentin disorders has been recently criticised for failing to consider differential variations and expressions of these disorders. This paper describes a case of a 23-year-old woman with previously undiagnosed DD, who presented with clinical and histological features belonging to several of these diseases, thus highlighting the potential diagnostic challenges faced with Shields' classification.

Amelogenesis Imperfecta; Genes, Proteins, and Pathways.

Amelogenesis imperfecta (AI) is the name given to a heterogeneous group of conditions characterized by inherited developmental enamel defects. AI enamel is abnormally thin, soft, fragile, pitted and/or badly discolored, with poor function and aesthetics, causing patients problems such as early tooth loss, severe embarrassment, eating difficulties, and pain. It was first described separately from diseases of dentine nearly 80 years ago, but the underlying genetic and mechanistic basis of the condition is only now coming to light. Mutations in the gene AMELX, encoding an extracellular matrix protein secreted by ameloblasts during enamel formation, were first identified as a cause of AI in 1991. Since then, mutations in at least eighteen genes have been shown to cause AI presenting in isolation of other health problems, with many more implicated in syndromic AI. Some of the encoded proteins have well documented roles in amelogenesis, acting as enamel matrix proteins or the proteases that degrade them, cell adhesion molecules or regulators of calcium homeostasis. However, for others, function is less clear and further research is needed to understand the pathways and processes essential for the development of healthy enamel. Here, we review the genes and mutations underlying AI presenting in isolation of other health problems, the proteins they encode and knowledge of their roles in amelogenesis, combining evidence from human phenotypes, inheritance patterns, mouse models, and in vitro studies. An LOVD resource (http://dna2.leeds.ac.uk/LOVD/) containing all published gene mutations for AI presenting in isolation of other health problems is described. We use this resource to identify trends in the genes and mutations reported to cause AI in the 270 families for which molecular diagnoses have been reported by 23rd May 2017. Finally we discuss the potential value of the translation of AI genetics to clinical care with improved patient pathways and speculate on the possibility of novel treatments and prevention strategies for AI.

A Fourth KLK4 Mutation Is Associated with Enamel Hypomineralisation and Structural Abnormalities.

"Amelogenesis imperfecta" (AI) describes a group of genetic conditions that result in defects in tooth enamel formation. Mutations in many genes are known to cause AI, including the gene encoding the serine protease, kallikrein related peptidase 4 (KLK4), expressed during the maturation stage of amelogenesis. In this study we report the fourth KLK4 mutation to be identified in autosomal recessively-inherited hypomaturation type AI, c.632delT, p.(L211Rfs*37) (NM_004917.4, NP_004908.4). This homozygous variant was identified in five Pakistani AI families and is predicted to result in a transcript with a premature stop codon that escapes nonsense mediated decay. However, the protein may misfold, as three of six disulphide bonds would be disrupted, and may be degraded or non-functional as a result. Primary teeth were obtained from one affected individual. The enamel phenotype was characterized using high-resolution computerized X-ray tomography (CT), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and microhardness testing (MH). Enamel from the affected individual (referred to as KLK4 enamel) was hypomineralised in comparison with matched control enamel. Furthermore, KLK4 inner enamel was hypomineralised compared with KLK4 outer enamel. SEM showed a clear structural demarcation between KLK4 inner and outer enamel, although enamel structure was similar to control tissue overall. EDX showed that KLK4 inner enamel contained less calcium and phosphorus and more nitrogen than control inner enamel and KLK4 outer enamel. MH testing showed that KLK4 inner enamel was significantly softer than KLK4 outer enamel (p < 0.001). However, the hardness of control inner enamel was not significantly different to that of control outer enamel. Overall, these findings suggest that the KLK4 c.632delT mutation may be a common cause of autosomal recessive AI in the Pakistani population. The phenotype data obtained mirror findings in the Klk4-/- mouse and suggest that KLK4 is required for the hardening and mineralization of the inner enamel layer but is less essential for hardening and mineralization of the outer enamel layer.

Defects in the acid phosphatase ACPT cause recessive hypoplastic amelogenesis imperfecta.

We identified two homozygous missense variants (c.428C>T, p.(T143M) and c.746C>T, p.(P249L)) in ACPT, the gene encoding acid phosphatase, testicular, which segregates with hypoplastic amelogenesis imperfecta in two unrelated families. ACPT is reported to play a role in odontoblast differentiation and mineralisation by supplying phosphate during dentine formation. Analysis by computerised tomography and scanning electron microscopy of a primary molar tooth from an individual homozygous for the c.746C>T variant revealed an enamel layer that was hypoplastic, but mineralised with prismatic architecture. These findings implicate variants in ACPT as a cause of early failure of amelogenesis during the secretory phase.

Amelogenesis imperfecta caused by N-terminal enamelin point mutations in mice and men is driven by endoplasmic reticulum stress.

'Amelogenesis imperfecta' (AI) describes a group of inherited diseases of dental enamel that have major clinical impact. Here, we identify the aetiology driving AI in mice carrying a p.S55I mutation in enamelin; one of the most commonly mutated proteins underlying AI in humans. Our data indicate that the mutation inhibits the ameloblast secretory pathway leading to ER stress and an activated unfolded protein response (UPR). Initially, with the support of the UPR acting in pro-survival mode, Enamp.S55I heterozygous mice secreted structurally normal enamel. However, enamel secreted thereafter was structurally abnormal; presumably due to the UPR modulating ameloblast behaviour and function in an attempt to relieve ER stress. Homozygous mutant mice failed to produce enamel. We also identified a novel heterozygous ENAMp.L31R mutation causing AI in humans. We hypothesize that ER stress is the aetiological factor in this case of human AI as it shared the characteristic phenotype described above for the Enamp.S55I mouse. We previously demonstrated that AI in mice carrying the Amelxp.Y64H mutation is a proteinopathy. The current data indicate that AI in Enamp.S55I mice is also a proteinopathy, and based on comparative phenotypic analysis, we suggest that human AI resulting from the ENAMp.L31R mutation is another proteinopathic disease. Identifying a common aetiology for AI resulting from mutations in two different genes opens the way for developing pharmaceutical interventions designed to relieve ER stress or modulate the UPR during enamel development to ameliorate the clinical phenotype.

Mutations in the pH-Sensing G-protein-Coupled Receptor GPR68 Cause Amelogenesis Imperfecta.

Amelogenesis is the process of dental enamel formation, leading to the deposition of the hardest tissue in the human body. This process requires the intricate regulation of ion transport and controlled changes to the pH of the developing enamel matrix. The means by which the enamel organ regulates pH during amelogenesis is largely unknown. We identified rare homozygous variants in GPR68 in three families with amelogenesis imperfecta, a genetically and phenotypically heterogeneous group of inherited conditions associated with abnormal enamel formation. Each of these homozygous variants (a large in-frame deletion, a frameshift deletion, and a missense variant) were predicted to result in loss of function. GPR68 encodes a proton-sensing G-protein-coupled receptor with sensitivity in the pH range that occurs in the developing enamel matrix during amelogenesis. Immunohistochemistry of rat mandibles confirmed localization of GPR68 in the enamel organ at all stages of amelogenesis. Our data identify a role for GPR68 as a proton sensor that is required for proper enamel formation.

Deletion of amelotin exons 3-6 is associated with amelogenesis imperfecta.

Amelogenesis imperfecta (AI) is a heterogeneous group of genetic conditions that result in defective dental enamel formation. Amelotin (AMTN) is a secreted protein thought to act as a promoter of matrix mineralization in the final stage of enamel development, and is strongly expressed, almost exclusively, in maturation stage ameloblasts. Amtn overexpression and Amtn knockout mouse models have defective enamel with no other associated phenotypes, highlighting AMTN as an excellent candidate gene for human AI. However, no AMTN mutations have yet been associated with human AI. Using whole exome sequencing, we identified an 8,678 bp heterozygous genomic deletion encompassing exons 3-6 of AMTN in a Costa Rican family segregating dominant hypomineralised AI. The deletion corresponds to an in-frame deletion of 92 amino acids, shortening the protein from 209 to 117 residues. Exfoliated primary teeth from an affected family member had enamel that was of a lower mineral density compared to control enamel and exhibited structural defects at least some of which appeared to be associated with organic material as evidenced using elemental analysis. This study demonstrates for the first time that AMTN mutations cause non-syndromic human AI and explores the human phenotype, comparing it with that of mice with disrupted Amtn function.

Spectrum of PEX1 and PEX6 variants in Heimler syndrome.

Heimler syndrome (HS) consists of recessively inherited sensorineural hearing loss, amelogenesis imperfecta (AI) and nail abnormalities, with or without visual defects. Recently HS was shown to result from hypomorphic mutations in PEX1 or PEX6, both previously implicated in Zellweger Syndrome Spectrum Disorders (ZSSD). ZSSD are a group of conditions consisting of craniofacial and neurological abnormalities, sensory defects and multi-organ dysfunction. The finding of HS-causing mutations in PEX1 and PEX6 shows that HS represents the mild end of the ZSSD spectrum, though these conditions were previously thought to be distinct nosological entities. Here, we present six further HS families, five with PEX6 variants and one with PEX1 variants, and show the patterns of Pex1, Pex14 and Pex6 immunoreactivity in the mouse retina. While Ratbi et al. found more HS-causing mutations in PEX1 than in PEX6, as is the case for ZSSD, in this cohort PEX6 variants predominate, suggesting both genes play a significant role in HS. The PEX6 variant c.1802G>A, p.(R601Q), reported previously in compound heterozygous state in one HS and three ZSSD cases, was found in compound heterozygous state in three HS families. Haplotype analysis suggests a common founder variant. All families segregated at least one missense variant, consistent with the hypothesis that HS results from genotypes including milder hypomorphic alleles. The clinical overlap of HS with the more common Usher syndrome and lack of peroxisomal abnormalities on plasma screening suggest that HS may be under-diagnosed. Recognition of AI is key to the accurate diagnosis of HS.

Heimler Syndrome Is Caused by Hypomorphic Mutations in the Peroxisome-Biogenesis Genes PEX1 and PEX6.

Heimler syndrome (HS) is a rare recessive disorder characterized by sensorineural hearing loss (SNHL), amelogenesis imperfecta, nail abnormalities, and occasional or late-onset retinal pigmentation. We ascertained eight families affected by HS and, by using a whole-exome sequencing approach, identified biallelic mutations in PEX1 or PEX6 in six of them. Loss-of-function mutations in both genes are known causes of a spectrum of autosomal-recessive peroxisome-biogenesis disorders (PBDs), including Zellweger syndrome. PBDs are characterized by leukodystrophy, hypotonia, SNHL, retinopathy, and skeletal, craniofacial, and liver abnormalities. We demonstrate that each HS-affected family has at least one hypomorphic allele that results in extremely mild peroxisomal dysfunction. Although individuals with HS share some subtle clinical features found in PBDs, the diagnosis was not suggested by routine blood and skin fibroblast analyses used to detect PBDs. In conclusion, our findings define HS as a mild PBD, expanding the pleiotropy of mutations in PEX1 and PEX6.