Primary cutaneous lymphoma: recommendations for clinical trial design and staging update from the ISCL, USCLC, and EORTC.

The number of patients with primary cutaneous lymphoma (PCL) relative to other non-Hodgkin lymphomas (NHLs) is small and the number of subtypes large. Although clinical trial guidelines have been published for mycosis fungoides/Sézary syndrome, the most common type of PCL, none exist for the other PCLs. In addition, staging of the PCLs has been evolving based on new data on potential prognostic factors, diagnosis, and assessment methods of both skin and extracutaneous disease and a desire to align the latter with the Lugano guidelines for all NHLs. The International Society for Cutaneous Lymphomas (ISCL), the United States Cutaneous LymphomaConsortium (USCLC), and the Cutaneous Lymphoma Task Force of the European Organization for the Research and Treatment of Cancer (EORTC) now propose updated staging and guidelines for the study design, assessment, endpoints, and response criteria in clinical trials for all the PCLs in alignment with that of the Lugano guidelines. These recommendations provide standardized methodology that should facilitate planning and regulatory approval of new treatments for these lymphomas worldwide, encourage cooperative investigator-initiated trials, and help to assess the comparative efficacy of therapeutic agents tested across sites and studies.

Evaluating the Accuracy of Self-Collected Swabs for the Diagnosis of Mpox.

We evaluated the accuracy of patient-collected skin lesions, oropharyngeal, and rectal swabs among 50 individuals enrolled in a study of mpox viral dynamics. We found that the performance of self-collected samples was similar to that of physician-collected samples, suggesting that self-sampling is a reliable strategy for diagnosing mpox.

Clinical presentation and virological assessment of confirmed human monkeypox virus cases in Spain: a prospective observational cohort study.

BACKGROUND: In May, 2022, several European countries reported autochthonous cases of monkeypox, which rapidly spread globally. Early reports suggest atypical presentations. We aimed to investigate clinical and virological characteristics of cases of human monkeypox in Spain. METHODS: This multicentre, prospective, observational cohort study was done in three sexual health clinics in Madrid and Barcelona, Spain. We enrolled all consecutive patients with laboratory-confirmed monkeypox from May 11 to June 29, 2022. Participants were offered lesion, anal, and oropharynx swabs for PCR testing. Participant data were collected by means of interviews conducted by dermatologists or specialists in sexually transmitted infections and were recorded using a standard case report form. Outcomes assessed in all participants with a confirmed diagnosis were demographics, smallpox vaccination, HIV status, exposure to someone with monkeypox, travel, mass gathering attendance, risk factors for sexually transmitted infections, sexual behaviour, signs and symptoms on first presentation, virological results at multiple body sites, co-infection with other sexually transmitted pathogens, and clinical outcomes 14 days after the initial presentation. Clinical outcomes were followed up until July 13, 2022. FINDINGS: 181 patients had a confirmed monkeypox diagnosis and were enrolled in the study. 166 (92%) identified as gay men, bisexual men, or other men who have sex with men (MSM) and 15 (8%) identified as heterosexual men or heterosexual women. Median age was 37·0 years (IQR 31·0-42·0). 32 (18%) patients reported previous smallpox vaccination, 72 (40%) were HIV-positive, eight (11%) had a CD4 cell count less than 500 cells per µL, and 31 (17%) were diagnosed with a concurrent sexually transmitted infection. Median incubation was 7·0 days (IQR 5·0-10·0). All participants presented with skin lesions; 141 (78%) participants had lesions in the anogenital region, and 78 (43%) in the oral and perioral region. 70 (39%) participants had complications requiring treatment: 45 (25%) had a proctitis, 19 (10%) had tonsillitis, 15 (8%) had penile oedema, six (3%) an abscess, and eight (4%) had an exanthem. Three (2%) patients required hospital admission. 178 (99%) of 180 swabs from skin lesions collected tested positive, as did 82 (70%) of 117 throat swabs. Viral load was higher in lesion swabs than in pharyngeal specimens (mean cycle threshold value 23 [SD 4] vs 32 [6], absolute difference 9 [95% CI 8-10]; p<0·0001). 108 (65%) of 166 MSM reported anal-receptive sex. MSM who engaged in anal-receptive sex presented with proctitis (41 [38%] of 108 vs four [7%] of 58, absolute difference 31% [95% CI 19-44]; p<0·0001) and systemic symptoms before the rash (67 [62%] vs 16 [28%], absolute difference 34% [28-62]; p<0·0001) more frequently than MSM who did not engage in anal-receptive sex. 18 (95%) of 19 participants with tonsillitis reported practising oral-receptive sex. The median time from onset of lesions to formation of a dry crust was 10 days (IQR 7-13). INTERPRETATION: In our cohort, monkeypox caused genital, perianal, and oral lesions and complications including proctitis and tonsillitis. Because of the variability of presentations, clinicians should have a low threshold for suspicion of monkeypox. Lesion swabs showed the highest viral loads, which, combined with the history of sexual exposure and the distribution of lesions, suggests close contact is probably the dominant transmission route in the current outbreak. FUNDING: None.

Whole-body imaging of lymphovascular niches identifies pre-metastatic roles of midkine.

Cutaneous melanoma is a type of cancer with an inherent potential for lymph node colonization, which is generally preceded by neolymphangiogenesis. However, sentinel lymph node removal does not necessarily extend the overall survival of patients with melanoma. Moreover, lymphatic vessels collapse and become dysfunctional as melanomas progress. Therefore, it is unclear whether (and how) lymphangiogenesis contributes to visceral metastasis. Soluble and vesicle-associated proteins secreted by tumours and/or their stroma have been proposed to condition pre-metastatic sites in patients with melanoma. Still, the identities and prognostic value of lymphangiogenic mediators remain unclear. Moreover, our understanding of lymphangiogenesis (in melanomas and other tumour types) is limited by the paucity of mouse models for live imaging of distal pre-metastatic niches. Injectable lymphatic tracers have been developed, but their limited diffusion precludes whole-body imaging at visceral sites. Vascular endothelial growth factor receptor 3 (VEGFR3) is an attractive 'lymphoreporter' because its expression is strongly downregulated in normal adult lymphatic endothelial cells, but is activated in pathological situations such as inflammation and cancer. Here, we exploit this inducibility of VEGFR3 to engineer mouse melanoma models for whole-body imaging of metastasis generated by human cells, clinical biopsies or endogenously deregulated oncogenic pathways. This strategy revealed early induction of distal pre-metastatic niches uncoupled from lymphangiogenesis at primary lesions. Analyses of the melanoma secretome and validation in clinical specimens showed that the heparin-binding factor midkine is a systemic inducer of neo-lymphangiogenesis that defines patient prognosis. This role of midkine was linked to a paracrine activation of the mTOR pathway in lymphatic endothelial cells. These data support the use of VEGFR3 reporter mice as a 'MetAlert' discovery platform for drivers and inhibitors of metastasis.

Identifying unmet needs and challenges in the definition of a plaque in mycosis fungoides: An EORTC-CLTG/ISCL survey.

BACKGROUND: Consensus about the definition and classification of 'plaque' in mycosis fungoides is lacking. OBJECTIVES: To delineate a comprehensive view on how the 'plaque' entity is defined and managed in clinical practice; to evaluate whether the current positioning of plaques in the TNMB classification is adequate. METHODS: A 12-item survey was circulated within a selected panel of 22 experts (pathologists, dermatologists, haematologists and oncologists), members of the EORTC and International Society for Cutaneous Lymphoma. The questionnaire discussed clinical and histopathological definitions of plaques and its relationship with staging and treatment. RESULTS: Total consensus and very high agreement rates were reached in 33.3% of questions, as all panellists regularly check for the presence of plaques, agree to evaluate the presence of plaques as a potential separate T class, and concur on the important distinction between plaque and patch for the management of early-stage MF. High agreement was reached in 41.7% of questions, since more than 50% of the responders use Olsen's definition of plaque, recommend the distinction between thin/thick plaques, and agree on performing a biopsy on the most infiltrated/indurated lesion. High divergence rates (25%) were reported regarding the possibility of a clinically based distinction between thin and thick plaques and the role of histopathology to plaque definition. CONCLUSIONS: The definition of 'plaque' is commonly perceived as a clinical entity and its integration with histopathological features is generally reserved to specific cases. To date, no consensus is achieved as for the exact definition of thin and thick plaques and current positioning of plaques within the TNMB system is considered clinically inadequate. Prospective studies evaluating the role of histopathological parameters and other biomarkers, as well as promising diagnostic tools, such as US/RM imaging and high-throughput blood sequencing, are much needed to fully integrate current clinical definitions with more objective parameters.

Brentuximab vedotin in the treatment of cutaneous T-cell lymphomas: Data from the Spanish Primary Cutaneous Lymphoma Registry.

BACKGROUND: Brentuximab vedotin (BV) has been approved for CD30-expressing cutaneous T-cell lymphoma (CTCL) after at least one previous systemic treatment. However, real clinical practice is still limited. OBJECTIVES: To evaluate the response and tolerance of BV in a cohort of patients with CTCL. METHODS: We analysed CTCL patients treated with BV from the Spanish Primary Cutaneous Lymphoma Registry (RELCP). RESULTS: Sixty-seven patients were included. There were 26 females and the mean age at diagnosis was 59 years. Forty-eight were mycosis fungoides (MF), 7 Sézary syndrome (SS) and 12 CD30+ lymphoproliferative disorders (CD30 LPD). Mean follow-up was 18 months. Thirty patients (45%) showed at least 10% of CD30+ cells among the total lymphocytic infiltrate. The median number of BV infusions received was 7. The overall response rate (ORR) was 67% (63% in MF, 71% in SS and 84% in CD30 LPD). Ten of 14 patients with folliculotropic MF (FMF) achieved complete or partial response (ORR 71%). The median time to response was 2.8 months. During follow-up, 36 cases (54%) experienced cutaneous relapse or progression. The median progression free survival (PFS) was 10.3 months. The most frequent adverse event was peripheral neuropathy (PN) (57%), in most patients (85%), grades 1 or 2. CONCLUSIONS: These results confirm the efficacy and safety of BV in patients with advanced-stage MF, and CD30 LPD. In addition, patients with FMF and SS also showed a favourable response. Our data suggest that BV retreatment is effective in a proportion of cases.

Post hoc Analysis of a Randomized, Controlled, Phase 2 Study to Assess Response Rates with Chlormethine/Mechlorethamine Gel in Patients with Stage IA-IIA Mycosis Fungoides.

BACKGROUND: Mycosis fungoides (MF) is the most common form of cutaneous T-cell lymphoma. Patients can be treated using chlormethine gel, a skin-directed therapy developed and approved for MF. In the randomized, controlled 201 trial, chlormethine gel was found to be noninferior to equal-strength chlormethine ointment. However, there remains a need to gain more insight into outcome measures after treatment. OBJECTIVE: The aim of this study was to further investigate the potential of chlormethine gel treatment through a novel post hoc analysis of the 201 trial data (NCT00168064). METHODS: Patients were randomized to chlormethine gel or ointment; response assessments included Composite Assessment of Index Lesion Severity (CAILS) and total body surface area (BSA). In this post hoc analysis, additional subgroup response analyses were performed for stage IA/IB-IIA MF. Very good partial response (75 to <100% improvement) was included as an additional response category. Time to response and overall response trends were determined. Finally, multivariate time-to-event analyses were performed to determine whether associations were observed between treatment frequency, response, and adverse events. RESULTS: Response rates were significantly higher for patients with stage IA MF for CAILS (intent-to-treat [p = 0.0014] and efficacy-evaluable [EE; p = 0.0036] populations) and BSA (EE population [p = 0.0488]) treated with gel versus ointment. Time to first CAILS response and response trends were better for all-stage gel-treated patients overall. No association was seen between treatment frequency and response or occurrence of adverse events at the following visit. An association was observed between the occurrence of contact dermatitis and improved clinical response at the next visit (p = 0.0001). CONCLUSION: This post hoc analysis shows that treatment with chlormethine gel may result in higher and faster response rates compared with chlormethine ointment, which confirms and expands results reported in the original analysis. The incidence of contact dermatitis may potentially be a prognostic indicator for clinical response; this needs to be confirmed in a larger population.

Real-life experience of using pegylated liposomal doxorubicin in primary cutaneous T-cell lymphomas.

Mycosis fungoides (MF) is the most prevalent subtype of primary cutaneous T-cell lymphomas (CTCLs). Sézary syndrome (SS) is another entity defined by leukaemic involvement, lymphadenopathy and erythroderma. Pegylated liposomal doxorubicin (PEG-DOXO) is an anthracycline used in the management of advanced primary CTCL, particularly in induction strategies. However, there are limited data on its effectiveness and tolerability in real-life patients. We report 36 patients who received PEG-DOXO for MF or SS in our centre, describing the patients' characteristics, response rates and tolerance to the treatment. The best overall responses were observed for the skin, with lower response rates for nodal involvement and moderate responses for blood disease. The treatment was mainly well tolerated, without severe adverse events, and no cardiotoxicity was observed on cardiac function monitoring.

The changing therapeutic landscape, burden of disease, and unmet needs in patients with cutaneous T-cell lymphoma.

Cutaneous T-cell lymphomas (CTCLs) have a chronic, relapsing course, and the most common subtypes are mycosis fungoides and Sézary syndrome. The disease causes visible skin alterations and can also cause alopecia, pruritus and pain, all of which can impact patients' health-related quality of life (HRQoL). The goal of treatment is to reduce symptoms and prevent disease progression. However, treatment recommendations are often based on low levels of evidence due to the lack of well-designed randomised clinical trials and treatment guidelines, and approved drugs vary considerably across different countries and regions. Currently, available treatments rarely lead to durable remissions and eventually become less effective, meaning patients often require multiple therapy changes. Skin-directed therapies (SDTs) are first-line treatments for early-stage CTCL, whereas systemic therapies may be needed for early-stage disease that does not respond to SDT or for advanced-stage disease. However, patients can experience significant side-effects with these treatments or may be unable to tolerate them. Hence, there is an unmet need for effective therapies with good safety profiles for the treatment of early- and late-stage CTCL. Here, we review current treatment guidelines, investigational and approved treatments, the impact of CTCL on patients' HRQoL, and the treatment of pruritus.

The Course of Mycosis Fungoides under Cytokine Pathway Blockers: A Multicentre Analysis of Real-life Clinical Data.

Literature regarding the effect of biologics on the course of mycosis fungoides (MF) is scarce. This multicentre study analysed retrospective data on 19 patients with MF, who were treated with biologics; 12 for inflammatory conditions coexisting with MF, and 7 for MF misdiagnosed as an inflammatory skin disease. Eight patients were treated with anti-tumour necrosis factor-α-monotherapy; 6 had early-stage MF, in 3 patients MF preceded and in 3 MF was diagnosed after initiation of biologics, with no stage-progression or with stable disease, respectively (median treatment time concurrent with MF 57 months). Two patients had advanced stage MF: IIB, treated for 15 months with no stage-progression, and IVA1, treated for 8 months, died of disease 10 months later. The other 11/19 patients received anti-interleukin-17A and/or anti-interleukin-12/23 or anti-interleukin-23 (with/without anti-tumour necrosis factor-α/anti-interleukin-4/13), with stage-progression in 8 patients after a median of 8 months' treatment. Although, in general, biologics should be avoided in patients with MF, these results indicate that anti-tumour necrosis factor-α-monotherapy might not aggravate the disease course in early-stage patients. Interleukin-17A, interleukin-12/23 and interleukin-23 pathway-blockers may prompt progression of MF.

Omalizumab as an alternative therapeutic tool in the treatment of bullous pemphigoid: A case report.

Bullous pemphigoid (BP) is an acquired autoimmune bullous disease characterized by autoantibodies against the hemidesmosomal proteins found in the basal keratinocytes of the basement membrane zone (BMZ): a 180 kDa protein (type XVII collagen) mainly and the 230 kDa antigen. There is such evidence that the antibodies against the BMZ components are not only of IgG type, but also this bullous disease may have IgE antibodies directed to the BMZ that contribute to the pathogenesis of the disorder. IgE is not only thought to contribute to the pathogenesis of BP, it has also been suggested that eosinophils play a role in the development of the first signs associated with BP. A humanized monoclonal antibody directed to IgE, omalizumab, is approved for the treatment of severe asthma and chronic spontaneous urticaria, and it may be useful in the treatment of BP in the first stages of the disease.

Mogamulizumab versus vorinostat in previously treated cutaneous T-cell lymphoma (MAVORIC): an international, open-label, randomised, controlled phase 3 trial.

BACKGROUND: Cutaneous T-cell lymphomas are rare non-Hodgkin lymphomas with substantial morbidity and mortality in advanced disease stages. We compared the efficacy of mogamulizumab, a novel monoclonal antibody directed against C-C chemokine receptor 4, with vorinostat in patients with previously treated cutaneous T-cell lymphoma. METHODS: In this open-label, international, phase 3, randomised controlled trial, we recruited patients with relapsed or refractory mycosis fungoides or Sézary syndrome at 61 medical centres in the USA, Denmark, France, Italy, Germany, the Netherlands, Spain, Switzerland, the UK, Japan, and Australia. Eligible patients were aged at least 18 years (in Japan, ≥20 years), had failed (for progression or toxicity as assessed by the principal investigator) at least one previous systemic therapy, and had an Eastern Cooperative Oncology Group performance score of 1 or less and adequate haematological, hepatic, and renal function. Patients were randomly assigned (1:1) using an interactive voice web response system to mogamulizumab (1·0 mg/kg intravenously on a weekly basis for the first 28-day cycle, then on days 1 and 15 of subsequent cycles) or vorinostat (400 mg daily). Stratification was by cutaneous T-cell lymphoma subtype (mycosis fungoides vs Sézary syndrome) and disease stage (IB-II vs III-IV). Since this study was open label, patients and investigators were not masked to treatment assignment. The primary endpoint was progression-free survival by investigator assessment in the intention-to-treat population. Patients who received one or more doses of study drug were included in the safety analyses. This study is ongoing, and enrolment is complete. This trial was registered with ClinicalTrials.gov, number NCT01728805. FINDINGS: Between Dec 12, 2012, and Jan 29, 2016, 372 eligible patients were randomly assigned to receive mogamulizumab (n=186) or vorinostat (n=186), comprising the intention-to-treat population. Two patients randomly assigned to mogamulizumab withdrew consent before receiving study treatment; thus, 370 patients were included in the safety population. Mogamulizumab therapy resulted in superior investigator-assessed progression-free survival compared with vorinostat therapy (median 7·7 months [95% CI 5·7-10·3] in the mogamulizumab group vs 3·1 months [2·9-4·1] in the vorinostat group; hazard ratio 0·53, 95% CI 0·41-0·69; stratified log-rank p<0·0001). Grade 3-4 adverse events of any cause were reported in 75 (41%) of 184 patients in the mogamulizumab group and 76 (41%) of 186 patients in the vorinostat group. The most common serious adverse events of any cause were pyrexia in eight (4%) patients and cellulitis in five (3%) patients in the mogamulizumab group; and cellulitis in six (3%) patients, pulmonary embolism in six (3%) patients, and sepsis in five (3%) patients in the vorinostat group. Two (67%) of three on-treatment deaths with mogamulizumab (due to sepsis and polymyositis) and three (33%) of nine on-treatment deaths with vorinostat (two due to pulmonary embolism and one due to bronchopneumonia) were considered treatment-related. INTERPRETATION: Mogamulizumab significantly prolonged progression-free survival compared with vorinostat, and could provide a new, effective treatment for patients with mycosis fungoides and, importantly, for Sézary syndrome, a subtype that represents a major therapeutic challenge in cutaneous T-cell lymphoma. FUNDING: Kyowa Kirin.

Brentuximab vedotin or physician's choice in CD30-positive cutaneous T-cell lymphoma (ALCANZA): an international, open-label, randomised, phase 3, multicentre trial.

BACKGROUND: Cutaneous T-cell lymphomas are rare, generally incurable, and associated with reduced quality of life. Present systemic therapies rarely provide reliable and durable responses. We aimed to assess efficacy and safety of brentuximab vedotin versus conventional therapy for previously treated patients with CD30-positive cutaneous T-cell lymphomas. METHODS: In this international, open-label, randomised, phase 3, multicentre trial, we enrolled adult patients with CD30-positive mycosis fungoides or primary cutaneous anaplastic large-cell lymphoma who had been previously treated. Patients were enrolled across 52 centres in 13 countries. Patients were randomly assigned (1:1) centrally by an interactive voice and web response system to receive intravenous brentuximab vedotin 1·8 mg/kg once every 3 weeks, for up to 16 3-week cycles, or physician's choice (oral methotrexate 5-50 mg once per week or oral bexarotene 300 mg/m2 once per day) for up to 48 weeks. The primary endpoint was the proportion of patients in the intention-to-treat population achieving an objective global response lasting at least 4 months per independent review facility. Safety analyses were done in all patients who received at least one dose of study drug. This trial was registered with ClinicalTrials.gov, number NCT01578499. FINDINGS: Between Aug 13, 2012, and July 31, 2015, 131 patients were enrolled and randomly assigned to a group (66 to brentuximab vedotin and 65 to physician's choice), with 128 analysed in the intention-to-treat population (64 in each group). At a median follow-up of 22·9 months (95% CI 18·4-26·1), the proportion of patients achieving an objective global response lasting at least 4 months was 56·3% (36 of 64 patients) with brentuximab vedotin versus 12·5% (eight of 64) with physician's choice, resulting in a between-group difference of 43·8% (95% CI 29·1-58·4; p<0·0001). Grade 3-4 adverse events were reported in 27 (41%) of 66 patients in the brentuximab vedotin group and 29 (47%) of 62 patients in the physician's choice group. Peripheral neuropathy was seen in 44 (67%) of 66 patients in the brentuximab vedotin group (n=21 grade 2, n=6 grade 3) and four (6%) of 62 patients in the physician's choice group. One of the four on-treatment deaths was deemed by the investigator to be treatment-related in the brentuximab vedotin group; no on-treatment deaths were reported in the physician's choice group. INTERPRETATION: Significant improvement in objective response lasting at least 4 months was seen with brentuximab vedotin versus physician's choice of methotrexate or bexarotene. FUNDING: Millennium Pharmaceuticals Inc (a wholly owned subsidiary of Takeda Pharmaceutical Company Ltd), Seattle Genetics Inc.

Subcutaneous panniculitis-like T-cell lymphoma: Clinical features, therapeutic approach, and outcome in a case series of 16 patients.

BACKGROUND: Subcutaneous panniculitis-like T-cell lymphoma (SPTCL) is a rare primary cutaneous lymphoma of mature cytotoxic T cells. Initially, patients with SPTCL were treated with doxorubicin-based polychemotherapy. OBJECTIVE: To analyze clinical, biologic, immunophenotypical, molecular, imaging, treatment, and outcome data reflecting the current state of knowledge. METHODS: A retrospective multicenter study of 16 patients with SPTCL that was diagnosed between 1996 and 2016. RESULTS: The female-to-male ratio was 1.7. The median age at diagnosis was 46.5 years. Patients presented with multiple nodular or plaque-like lesions preferentially affecting the legs and/or trunk. Histopathology typically showed a lobular panniculitis with individual adipocytes surrounded by atypical lymphocytes, usually with a CD3+, CD4-, CD8+, CD56-, TIA1 cytotoxic granule associated RNA binding protein 1-positive phenotype and high proliferation rate. SPTCL was associated with autoimmune diseases in 25% of patients, and with the development of hemophagocytic syndrome in 18% of patients. Oral steroids alone or in combination with low-dose methotrexate or cyclosporine A were the most common initial treatment, achieving a complete response in 85% of the treated patients. The median follow-up time was 14 months. The 5-year disease-specific survival rate was 85.7%. LIMITATIONS: This was a retrospective study. CONCLUSIONS: SPTCL has an excellent prognosis. Immunosuppressive agents can be considered for first-line treatment.

PLCG1 mutations in cutaneous T-cell lymphomas.

Cutaneous T-cell lymphoma (CTCL) is a heterogeneous group of primary cutaneous T-cell lymphoproliferative processes, mainly composed of mycosis fungoides and Sézary syndrome, the aggressive forms of which lack an effective treatment. The molecular pathogenesis of CTCL is largely unknown, although neoplastic cells show increased signaling from T-cell receptors (TCRs). DNAs from 11 patients with CTCL, both normal and tumoral, were target-enriched and sequenced by massive parallel sequencing for a selection of 524 TCR-signaling-related genes. Identified variants were validated by capillary sequencing. Multiple mutations were found that affected several signaling pathways, such as TCRs, nuclear factor κB, or Janus kinase/signal transducer and activator of transcription, but PLCG1 was found to be mutated in 3 samples, 2 of which featured a redundant mutation (c.1034T>C, S345F) in exon 11 that affects the PLCx protein catalytic domain. This mutation was further analyzed by quantitative polymerase chain reaction genotyping in a new cohort of 42 patients with CTCL, where it was found in 19% of samples. Immunohistochemical analysis for nuclear factor of activated T cells (NFAT) showed that PLCG1-mutated cases exhibited strong NFAT nuclear immunostaining. Functional studies demonstrated that PLCG1 mutants elicited increased downstream signaling toward NFAT activation, and inhibition of this pathway resulted in reduced CTCL cell proliferation and cell viability. Thus, increased proliferative and survival mechanisms in CTCL may partially depend on the acquisition of somatic mutations in PLCG1 and other genes that are essential for normal T-cell differentiation.