Willemijn Witkam

All Publications

• Distinct bacterial facial cutaneous microbiota composition in adolescents with acne vulgaris: a population-based cross-sectional observational study BRITISH JOURNAL OF DERMATOLOGY Witkam, W. M., Mathieu, A., Dal Belo, S. E., Moreau, M., Aguilar, L., Verlouw, J. A. M., Nijsten, T. E. C., Pardo, L. M. 2026

  Abstract

  Acne vulgaris (acne) is a prevalent dermatological condition associated with distinct facial cutaneous microbiota compositions. However, existing research has often methodological limitations such as insufficient statistical power and the underrepresentation of diverse adolescents populations, which are highly relevant for understanding acne pathogenesis.We aimed to compare bacterial facial cutaneous microbiota compositions between acne cases and controls within a large, multiethnic adolescent population, while adjusting for potential confounders.We compared bacterial facial cutaneous microbiota compositions using 16S rRNA sequencing between physician-evaluated acne cases (N=399) and controls (N=527) in adolescents (median age 13) in a population-based study. We additionally evaluated the independent associations of biological sex, puberty stage, perceived skin color, ethnicity, and weight status with facial cutaneous microbiota compositions. Our analytical approaches included assessment of alpha and beta diversity (including Permutational Multivariate Analysis of Variance), relative abundances, univariate coordinate analysis (Analysis of Compositions of Microbiomes with Bias Correction 2), and phylogenetic analyses.The overall microbiota composition of acne cases was less rich and diverse than controls (Chao1 β -44.98 (SE 4.36) Shannon β -1.01 (SE 0.07), P<0.01). While acne status was a major contributor to variations in overall microbiota compositions (R2 4.32%, P<0.01), skin color, sex, puberty stage, and weight status also independently contributed to variations (R2 0.44-1.05%, P<0.01). Specifically, acne cases showed higher relative abundances of Cutibacterium and Staphylococcus, and lower abundance of Streptococcus. Following confounder adjustment, bias correction, and phylogenetic analysis, Cutibacterium granulosum (C. granulosum), Cutibacterium acnes (C. acnes), and Staphylococcus epidermidis (S. epidermidis) emerged as the most differentially abundant species in acne cases versus controls (log fold changes: 0.75-2.17, P<0.01).Our findings reveal distinct facial microbiota compositions in adolescents with acne, identifying C. granulosum, C. acnes, and S. epidermidis as key microbial signature. This study suggests a significant clinical relevance for C. granulosum in acne pathology and a potential protective role for Streptococcus in acne-free skin. These results underscore the importance of representative study populations in microbiome research and provide important methodological and technical insights for future investigations.

  View details for DOI 10.1093/bjd/ljaf508

  View details for Web of Science ID 001671789000001

  View details for PubMedID 41370181

• Management Strategies and Corticophobia among Healthcare Professionals Involved in the Care for Atopic Dermatitis: A Dutch Survey. Dermatology (Basel, Switzerland) Ragamin, A., van Halewijn, K. F., Schappin, R., Witkam, W. C., Arends, N. J., Hoogeterp, J., Bohnen, A. M., Elshout, G., Schuttelaar, M. L., Pasmans, S. G. 2025; 241 (2): 101-112

  Abstract

  Various healthcare professionals (HCPs) deliver care for patients with atopic dermatitis (AD). Although pivotal, management strategies and the relation with corticophobia among HCPs have not been investigated. This study aimed to investigate management strategies for AD and its relation with corticophobia among HCPs.Dutch general practitioners (GPs), youth healthcare physicians (YHPs), pediatricians, dermatologists, pharmacists, and pharmacy assistants participated in a survey on management strategies and corticophobia. The Topical Corticosteroid Phobia questionnaire for professionals (TOPICOP-P) was used to measure attitudes toward topical corticosteroids (TCSs). Higher scores reflect a more negative attitude.A total of 407 HCPs (124 GPs, 33 YHPs, 51 pediatricians, 56 dermatologists, 58 pharmacists, and 85 pharmacy assistants) participated. Compared to dermatologists, other HCPs showed greater reluctance to TCS. This difference was highlighted by the finding that half of GPs reported to prescribed only TCS of mild potency for infants with severe AD, while few dermatologists (9%) reported a similar approach. Dermatologists had lowest TOPICOP-P scores (median: 19, IQR: 12-28). GPs and pharmacy assistants had highest scores (GPs median: 36, IQR: 31-44, pharmacy assistants: median: 36, IQR: 31-42). More corticophobia was significantly associated with prescription of a lower TC potency class in prescribing HCPs (B -0.04, 95% CI: -0.07 to 0.01, p = 0.01), and a trend was found between more corticophobia and longer perceived durability of one TCS tube.This study shows the differences in management of AD and reluctance toward TCS in HCPs. Furthermore, corticophobia among HCPs and its influence on the selection of TCS potency class and recommendations were demonstrated. To reduce corticophobia and improve care for AD, more education is needed.Various healthcare professionals (HCPs) deliver care for patients with atopic dermatitis (AD). Although pivotal, management strategies and the relation with corticophobia among HCPs have not been investigated. This study aimed to investigate management strategies for AD and its relation with corticophobia among HCPs.Dutch general practitioners (GPs), youth healthcare physicians (YHPs), pediatricians, dermatologists, pharmacists, and pharmacy assistants participated in a survey on management strategies and corticophobia. The Topical Corticosteroid Phobia questionnaire for professionals (TOPICOP-P) was used to measure attitudes toward topical corticosteroids (TCSs). Higher scores reflect a more negative attitude.A total of 407 HCPs (124 GPs, 33 YHPs, 51 pediatricians, 56 dermatologists, 58 pharmacists, and 85 pharmacy assistants) participated. Compared to dermatologists, other HCPs showed greater reluctance to TCS. This difference was highlighted by the finding that half of GPs reported to prescribed only TCS of mild potency for infants with severe AD, while few dermatologists (9%) reported a similar approach. Dermatologists had lowest TOPICOP-P scores (median: 19, IQR: 12-28). GPs and pharmacy assistants had highest scores (GPs median: 36, IQR: 31-44, pharmacy assistants: median: 36, IQR: 31-42). More corticophobia was significantly associated with prescription of a lower TC potency class in prescribing HCPs (B -0.04, 95% CI: -0.07 to 0.01, p = 0.01), and a trend was found between more corticophobia and longer perceived durability of one TCS tube.This study shows the differences in management of AD and reluctance toward TCS in HCPs. Furthermore, corticophobia among HCPs and its influence on the selection of TCS potency class and recommendations were demonstrated. To reduce corticophobia and improve care for AD, more education is needed.

  View details for DOI 10.1159/000542421

  View details for PubMedID 39496255

  View details for PubMedCentralID PMC11965863

• Genome-wide association meta-regression identifies stem cell lineage orchestration as a key driver of acne risk. medRxiv : the preprint server for health sciences Maxwell, J., Mitchell, B. L., DuHarpur, X., Pardo, L. M., Witkam, W. C., Dand, N., Bartels, M., Betti, M. J., Boomsma, D. I., Dong, X., Gerring, Z., Finer, S., Hagenbeek, F. A., Hottenga, J. J., Hripcsak, G., Huilaja, L., Hveem, K., Jacobs, B. M., Kals, M., Kaufman-Cook, J., Kettunen, J., Khan, A., Kingo, K., Kiryluk, K., Løset, M., Lunter, G., Lupton, M. K., Min, J. L., Martin, N. G., Medland, S. E., Neijzen, D., Nijsten, T. E., Nikopensius, T., Olsen, C. M., Petukhova, L., Reigo, A., Rentería, M. E., Rispoli, R., Saklatvala, J., Sliz, E., Tasanen-Määttä, K., Thomas, L., Trembath, R. C., Vaht, M., van Heel, D. A., Weng, C., Whiteman, D. C., Barker, J. N., Smith, C., Simpson, M. A. 2025

  Abstract

  Over 85% of the population experience acne at some point in their lives, with its severity spanning a quantitative spectrum, from mild, transient outbreaks to more persistent, severe forms of the condition. Moderate to severe disease poses a substantial global burden arising from both the physical and psychological impacts of this highly visible condition. The analytical approach taken in this study aimed to address the impact of variation in the dichotomisation of acne case control status, driven by ascertainment and study design, on effect size estimates across independent genetic association studies of acne. Through a fixed intercept meta-regression framework, we combined evidence genome-wide for association with acne across studies in which case-control status had been ascertained in different settings, allowing for different severity threshold definitions. Across a combined sample of 73,997 cases and 1,103,940 controls of European, South Asian and African American ancestry we identify genetic variation at 165 genomic loci that influence acne risk. There is evidence for both shared and ancestry specific components to the genetic susceptibility to acne and for sex differences in the magnitude of effect of risk alleles at three loci. We observe that common genetic variation explains 13.4% of acne heritability on the liability scale. Consistent with the hypothesis that genetic risk primarily operates at the level of individual pilosebaceous units, a polygenic score derived from this case-control study of acne susceptibility is associated with both self-reported and clinically assessed acne severity in adolescence, further strengthening the link between genetic risk and disease severity. Prioritisation of causal genes at the identified acne risk loci, provides genetic validation of the targets of established and emerging acne therapies, including retinoid treatments. The identified acne risk loci are enriched for genes encoding downstream effectors of RXRA signalling, including SOX9 and components of the WNT and p53 pathways. Illustrating that the control of stem cell lineage plasticity and cellular fate are important mechanisms through which genetic variation influences acne susceptibility within the pilosebaceous unit.

  View details for DOI 10.1101/2025.06.27.25330406

  View details for PubMedID 40666320

  View details for PubMedCentralID PMC12262744

• Association between air pollutant exposure and acne severity in an adolescent cohort in the Netherlands JOURNAL OF THE EUROPEAN ACADEMY OF DERMATOLOGY AND VENEREOLOGY Witkam, W. M., Essers, E., Petricola, S., Dal Belo, S. E., Aguilar, L., Nijsten, T. E. C., Guxens, M., Pardo, L. M. 2025; 39 (10): e857-e859

  View details for DOI 10.1111/jdv.20571

  View details for Web of Science ID 001413079100001

  View details for PubMedID 39898779

• Undertreatment of acne vulgaris in Dutch adolescents: A complex interplay of socioeconomic, sex and ethnic-related differences JEADV CLINICAL PRACTICE Witkam, W. M., Buckers, P. P., Dal Belo, S. E., Pardo, L. M., Nijsten, T. 2024; 3 (4): 1290-1293

  View details for DOI 10.1002/jvc2.402

  View details for Web of Science ID 001303850500054

• The epidemiology of acne vulgaris in a multiethnic adolescent population from Rotterdam, the Netherlands: A cross-sectional study. Journal of the American Academy of Dermatology Witkam, W. C., Dal Belo, S. E., Pourhamidi, S., Raynaud, E., Moreau, M., Aguilar, L., Jansen, P. W., Nijsten, T. E., Pardo, L. M. 2024; 90 (3): 552-560

  Abstract

  Although acne is a prevalent multifactorial inflammatory skin condition, few studies were performed in multiethnic populations.To study the prevalence and determinants of acne in a multiethnic study at the start of puberty.This cross-sectional study is embedded in Generation R, a population-based prospective study from Rotterdam, the Netherlands. Three-dimensional facial photos at the center visit in 2016-2019 (of ∼13-year-olds) were used to grade acne severity using the Global Evaluation of the Acne Severity (GEA). Analyses were stratified by biological sex and explored through chi-square tests and multivariable ordinal logistic regression.A total of 4561 children (51% girls) with a median age of 13.5 (IQR 13.3-13.6) were included. The visible acne prevalence (GEA 2-5) for girls vs boys was 62% vs 45% and moderate-to-severe acne (GEA 3-5) 14% vs 9%. Higher puberty stages (adjusted odds ratios: 1.38 [1.20-1.59] and 2.16 [1.86-2.51] for girls and boys, respectively) and darker skin colors V and VI (adjusted odds ratios: 1.90 [1.17-3.08] and 2.43 [1.67-3.56]) were associated with more severe acne in both sexes, and being overweight in boys (adjusted odds ratio: 1.58 [1.15-2.17]).Cross-sectional design.Acne prevalence was high at the age of 13 years and was associated with advanced puberty, darker skin color, and weight status.

  View details for DOI 10.1016/j.jaad.2023.10.062

  View details for PubMedID 37967670

• Long term effects of digital education among healthcare professionals in paediatric dermatology: Opportunities for improving care. Skin health and disease Ragamin, A., Schappin, R., Witkam, W. C., Spiering, M., Mendels, E., Schuttelaar, M. L., Pasmans, S. G. 2022; 2 (3): e143

  Abstract

  Topical corticosteroids (TCS) are the cornerstone of treatment for patients with atopic dermatitis (AD). Unfortunately, anxiety and misplaced beliefs on TCS, known as corticophobia, is common among health care professionals (HCPs) and could influence their practices, resulting in suboptimal patient care.To investigate the effects of digital education (DE) on the knowledge of TCS, practices and corticophobia among HCPs in paediatric dermatology.HCPs registered for an interactive online masterclass on paediatric dermatology including the treatment of AD and TCS were invited to participate in a survey on knowledge of TCS, self-reported practices and corticophobia. Corticophobia was measured using the TOPICOP-P questionnaire (range: 0%-100%, with higher scores indicating more corticophobia). Participants received the survey before, directly after, and 6 months after DE.Of the 86 participants, 66 (77%) completed the survey before the masterclass, 76 (88%) directly after, and 34 (40%) 6 months after. Key components of knowledge on TCS and self-reported practices improved greatly after DE, such as correct prescription amount of TCS (45%, 91%, 88%) and application instructions (56%, 99%, 94%). Overall corticophobia decreased after DE with median scores dropping from 33% before DE to 25% after DE (p < 0.01) and remained 25% 6 months later.Interactive DE for HCPs is an efficient tool to attain prolonged improvements of knowledge on TCS, practices, and corticophobia. All these factors are important for optimal care for patients. This study shows great opportunities for improving care by investing in HCPs.

  View details for DOI 10.1002/ski2.143

  View details for PubMedID 36092257

  View details for PubMedCentralID PMC9435444