J Hematol

Journal of Hematology, ISSN 1927-1212 print, 1927-1220 online, Open Access

Article copyright, the authors; Journal compilation copyright, J Hematol and Elmer Press Inc

Journal website https://www.thejh.org

Case Report

Volume 13, Number 3, June 2024, pages 128-136

Unraveling the Rare Entity of KIT D816V-Negative Systemic Mastocytosis

Figure 1. Case 2: skin and hair color pre (left) and post (right) avapritinib treatment. Wild-type KIT signaling is involved in melanogenesis and hair pigmentation, and inhibition can result in hair depigmentation and lightning of the skin through a temporary melanocyte dysfunction. Avapritinib skin and hair changes were reported in 6% to 21% of treated patients.

**Table 1.** The WHO Fifth Edition (WHO 5th) and the ICC Classifications of Mastocytosis
Cutaneous mastocytosis	ICC	WHO 5th
^aIn the WHO classification, well-differentiated systemic mastocytosis (WDSM) is a morphologic variant that can occur in any SM subtype, including mast cell leukemia. ^bIn the ICC, those variants of SM must be correlated with B- and C-findings. ^cIn the ICC, mast cell leukemia must meet the diagnostic criteria of SM and have ≥ 20% atypical immature mast cells. ^dBone marrow mastocytosis (BMM) typically lacks the B-findings and with normal/near normal serum tryptase. The WHO classifies BMM as an SM subcategory, while in ICC, it is considered a variant of indolent SM. SM: systemic mastocytosis; WHO: World Health Organization; ICC: International Consensus Classification.
	1. Urticaria pigmentosa/maculopapular cutaneous mastocytosis	1. Urticaria
	2. Diffuse cutaneous mastocytosis	Pigmentosa/maculopapular cutaneous mastocytosis
	3. Mastocytoma of skin	Monomorphic
		Polymorphic
		2. Diffuse cutaneous mastocytosis
		3. Cutaneous mastocytoma
		Isolated mastocytoma
		Multilocal mastocytoma
SM	ICC	WHO 5th^a
	1. Indolent SM (includes bone marrow mastocytosis)^b	1. Bone marrow mastocytosis^d
	2. Smoldering SM^b	2. Indolent SM
	3. Aggressive SM^b	3. Smoldering SM
	4. SM with an associated myeloid neoplasm	4. Aggressive SM
	5. Mast cell leukemia^c	5. SM with an associated hematologic neoplasm
		6. Mast cell leukemia
Mast cell sarcoma	ICC	WHO 5th^a

Table 1. The WHO Fifth Edition (WHO 5th) and the ICC Classifications of Mastocytosis

Cutaneous mastocytosis

ICC

WHO 5th

^aIn the WHO classification, well-differentiated systemic mastocytosis (WDSM) is a morphologic variant that can occur in any SM subtype, including mast cell leukemia. ^bIn the ICC, those variants of SM must be correlated with B- and C-findings. ^cIn the ICC, mast cell leukemia must meet the diagnostic criteria of SM and have ≥ 20% atypical immature mast cells. ^dBone marrow mastocytosis (BMM) typically lacks the B-findings and with normal/near normal serum tryptase. The WHO classifies BMM as an SM subcategory, while in ICC, it is considered a variant of indolent SM. SM: systemic mastocytosis; WHO: World Health Organization; ICC: International Consensus Classification.

1. Urticaria pigmentosa/maculopapular cutaneous mastocytosis

1. Urticaria

2. Diffuse cutaneous mastocytosis

Pigmentosa/maculopapular cutaneous mastocytosis

3. Mastocytoma of skin

Monomorphic

Polymorphic

2. Diffuse cutaneous mastocytosis

3. Cutaneous mastocytoma

Isolated mastocytoma

Multilocal mastocytoma

ICC

WHO 5th^a

1. Indolent SM (includes bone marrow mastocytosis)^b

1. Bone marrow mastocytosis^d

2. Smoldering SM^b

2. Indolent SM

3. Aggressive SM^b

3. Smoldering SM

4. SM with an associated myeloid neoplasm

4. Aggressive SM

5. Mast cell leukemia^c

5. SM with an associated hematologic neoplasm

6. Mast cell leukemia

Mast cell sarcoma

ICC

WHO 5th^a

**Table 2.** The WHO Fifth Edition (WHO 5th) and the ICC Diagnostic Criteria for SM
WHO 5th^a	ICC^a
^aFor SM diagnosis at least one major and one minor or three minor criteria must be met. ^bSpindle-shaped mast cells do not count as an SM criterion when they line vascular cells, fat cells, nerve cells, or the endosteal-lining cell layer. Valent et al. describe detailed morphological criteria [11]. ^cFor KIT mutation, high-sensitivity PCR is recommended to avoid false negatives. In the absence of a KIT D816V mutation, exclusion of KIT mutation variants is strongly recommended (this includes codons 417, 501 - 509, 522, 557 - 560, 642, 654, 799, 816, 820, 822, the list of variants provided in the appendix of WHO proposal [1]. In cases of negative KIT, particularly those with eosinophilia, tyrosine kinase gene fusions associated with myeloid/lymphoid neoplasms with eosinophilia and tyrosine kinase gene fusions must be excluded. ^dBy flow cytometry or immunohistochemistry or by both techniques. ^eIn the WHO classification, patients with hereditary alpha-tryptasemia (HαT) need adjustment for tryptase level (although not standardized). The suggested correction is to divide the basal tryptase level by (1 + the extra copy numbers) of the alpha tryptase gene. For example, when the tryptase level is 60 and three extra copies of the alpha tryptase gene are present, the corrected tryptase level is 15 (60/4 = 15) and thus it does not meet this minor SM criterion. ^fPer ICC, elevated tryptase does not count as a minor SM criterion in SM with an associated myeloid neoplasm. SM: systemic mastocytosis; WHO: World Health Organization; ICC: International Consensus Classification; BM: bone marrow.
Major criterion
Infiltrates of mast cells (≥ 15 mast cells in aggregates) in BM or extracutaneous organs^b	Infiltrates of tryptase- and/or CD117 positive mast cells (≥ 15 mast cells in aggregates) in the BM or extracutaneous organs^b
Minor criteria
≥ 25% of all mast cells are atypical cells (type I or type II) or are spindle-shaped	≥ 25% of all mast cells are atypical cells or are spindle-shaped
KIT-activating KIT point mutation(s) at codon 816 or in other critical regions of KIT^c	KIT-activating KIT point mutation(s) at codon 816 or in other critical regions of KIT^c
Mast cells express at least one of the following markers: CD2 and/or CD25 and/or CD30^d	Mast cells express at least one of the following markers: CD2 and/or CD25 and/or CD30^d
Baseline serum tryptase > 20 ng/mL^e	Baseline serum tryptase > 20 ng/mL^f

Table 2. The WHO Fifth Edition (WHO 5th) and the ICC Diagnostic Criteria for SM

WHO 5th^a

ICC^a

^aFor SM diagnosis at least one major and one minor or three minor criteria must be met. ^bSpindle-shaped mast cells do not count as an SM criterion when they line vascular cells, fat cells, nerve cells, or the endosteal-lining cell layer. Valent et al. describe detailed morphological criteria [11]. ^cFor KIT mutation, high-sensitivity PCR is recommended to avoid false negatives. In the absence of a KIT D816V mutation, exclusion of KIT mutation variants is strongly recommended (this includes codons 417, 501 - 509, 522, 557 - 560, 642, 654, 799, 816, 820, 822, the list of variants provided in the appendix of WHO proposal [1]. In cases of negative KIT, particularly those with eosinophilia, tyrosine kinase gene fusions associated with myeloid/lymphoid neoplasms with eosinophilia and tyrosine kinase gene fusions must be excluded. ^dBy flow cytometry or immunohistochemistry or by both techniques. ^eIn the WHO classification, patients with hereditary alpha-tryptasemia (HαT) need adjustment for tryptase level (although not standardized). The suggested correction is to divide the basal tryptase level by (1 + the extra copy numbers) of the alpha tryptase gene. For example, when the tryptase level is 60 and three extra copies of the alpha tryptase gene are present, the corrected tryptase level is 15 (60/4 = 15) and thus it does not meet this minor SM criterion. ^fPer ICC, elevated tryptase does not count as a minor SM criterion in SM with an associated myeloid neoplasm. SM: systemic mastocytosis; WHO: World Health Organization; ICC: International Consensus Classification; BM: bone marrow.

Major criterion

Infiltrates of mast cells (≥ 15 mast cells in aggregates) in BM or extracutaneous organs^b

Infiltrates of tryptase- and/or CD117 positive mast cells (≥ 15 mast cells in aggregates) in the BM or extracutaneous organs^b

Minor criteria

≥ 25% of all mast cells are atypical cells (type I or type II) or are spindle-shaped

≥ 25% of all mast cells are atypical cells or are spindle-shaped

KIT-activating KIT point mutation(s) at codon 816 or in other critical regions of KIT^c

Mast cells express at least one of the following markers: CD2 and/or CD25 and/or CD30^d

Baseline serum tryptase > 20 ng/mL^e

Baseline serum tryptase > 20 ng/mL^f

**Table 3.** The WHO Fifth Edition Summarizing the Clinical Findings of SM, Classified Into B-Findings and C-Findings (Adapted From Valent et al [1, 9])
B-findings	C-finding
WHO: World Health Organization; BM: bone marrow; VAF: variant allele frequency; PB: peripheral blood; MPN: myeloproliferative neoplasm; MDS: myelodysplastic syndrome; LN: lymph node; ANC: absolute neutrophil count; Plt: platelet; Hb: hemoglobin; HTN: hypertension; GIT: gastrointestinal tract.
Increased mast cell burden: ≥ 30% by histology on BM and/or serum tryptase ≥ 200 ng/mL and/or KIT D816V VAF ≥ 10% in BM or PB white blood cells	Cytopenia/s (one or more of the following): ANC < 1 × 10⁹/L, Hb < 10 g/dL, Plt < 100 × 10⁹/L
Features suggestive of MPN and/or MDS	Hepatopathy: hepatomegaly or cirrhosis ± ascites and elevated liver enzymes ± portal HTN
Organomegaly: palpable hepatomegaly or/and splenomegaly without systemic effects (i.e., no ascites or signs of organ damage and no hypersplenism, weight loss, and/or lymphadenopathy or visceral LN enlargement (> 2 cm) on imaging	Spleen: palpable splenomegaly with hypersplenism ± weight loss ± hypoalbuminemia
	GIT: malabsorption and hypoalbuminemia ± weight loss
	Bone: large-sized osteolysis (≥ 2 cm) with pathologic fracture ± bone pain

Table 3. The WHO Fifth Edition Summarizing the Clinical Findings of SM, Classified Into B-Findings and C-Findings (Adapted From Valent et al [1, 9])

B-findings

C-finding

WHO: World Health Organization; BM: bone marrow; VAF: variant allele frequency; PB: peripheral blood; MPN: myeloproliferative neoplasm; MDS: myelodysplastic syndrome; LN: lymph node; ANC: absolute neutrophil count; Plt: platelet; Hb: hemoglobin; HTN: hypertension; GIT: gastrointestinal tract.

Increased mast cell burden: ≥ 30% by histology on BM and/or serum tryptase ≥ 200 ng/mL and/or KIT D816V VAF ≥ 10% in BM or PB white blood cells

Cytopenia/s (one or more of the following): ANC < 1 × 10⁹/L, Hb < 10 g/dL, Plt < 100 × 10⁹/L

Features suggestive of MPN and/or MDS

Hepatopathy: hepatomegaly or cirrhosis ± ascites and elevated liver enzymes ± portal HTN

Organomegaly: palpable hepatomegaly or/and splenomegaly without systemic effects (i.e., no ascites or signs of organ damage and no hypersplenism, weight loss, and/or lymphadenopathy or visceral LN enlargement (> 2 cm) on imaging

Spleen: palpable splenomegaly with hypersplenism ± weight loss ± hypoalbuminemia

GIT: malabsorption and hypoalbuminemia ± weight loss

Bone: large-sized osteolysis (≥ 2 cm) with pathologic fracture ± bone pain

**Table 4.** Clinical and Pathologic Comparison of Our SM Cases and Cases in the Literature
	Sex	Age (years)	SM subtype	Presenting symptoms	Management	Response
^aThe patient died of hemorrhagic stroke and disseminated intravascular coagulation. ^bThe patient initially responded to interferon-α, however, it was changed due to myelosuppression. Cladribine resulted in improvement in eosinophilia. The patient died from sepsis. SM: systemic mastocytosis.
Case 1	F	49	Aggressive SM	Skin rashes, dizziness, nausea, vomiting	Imatinib 300 mg	Clinical response
Case 2	M	40	Indolent SM	Recurrent anaphylactic episodes	Avapritinib 150 mg/day	Clinical response
Azad et al, 2023 [25]	M	53	Aggressive SM	Headache, fatigue, weight loss, cognitive impairment, insomnia, exertional dyspnea, abdominal pain, bone and joint pain, diarrhea, bloody stools, and a pruritic rash localized in the torso and extremities	Failed imatinib, cladribine, nilotinib, hydroxyurea, and midostaurin	Clinical and biochemical response to avapritinib.
					Avapritinib 200 mg/day
Conde-Fernandes et al, 2017 [27]	M	15	Indolent SM	Recurrent flushing, hypotension, and syncope, preceded by skin lesions	Oral disodium cromoglycate	Clinical and biochemical response
Caceres-Nazario et al, 2016 [14]	M	78	Aggressive SM	Incidental discovery of normocytic anemia with thrombocytopenia	Imatinib to interferon-α	Death
					Prednisone
Savini et al, 2015 [28]^a	F	65	Mast cell leukemia	Flushing and hypotension	Antihistamine	Death
					Imatinib 400 mg to dasatinib 100 mg
Sakane-Ishikawa et al, 2013 [26]^b	F	43	Aggressive SM	Vertebral compression fracture, multiple lytic bone lesions with epidural mass, and eosinophils	Emergent laminectomy and subsequent irradiation of the tumor	Death
					Interferon-α to dasatinib to cladribine

Table 4. Clinical and Pathologic Comparison of Our SM Cases and Cases in the Literature

Sex

Age (years)

SM subtype

Presenting symptoms

Management

Response

^aThe patient died of hemorrhagic stroke and disseminated intravascular coagulation. ^bThe patient initially responded to interferon-α, however, it was changed due to myelosuppression. Cladribine resulted in improvement in eosinophilia. The patient died from sepsis. SM: systemic mastocytosis.

Case 1

Aggressive SM

Skin rashes, dizziness, nausea, vomiting

Imatinib 300 mg

Clinical response

Case 2

Indolent SM

Recurrent anaphylactic episodes

Avapritinib 150 mg/day

Clinical response

Azad et al, 2023 [25]

Aggressive SM

Headache, fatigue, weight loss, cognitive impairment, insomnia, exertional dyspnea, abdominal pain, bone and joint pain, diarrhea, bloody stools, and a pruritic rash localized in the torso and extremities

Failed imatinib, cladribine, nilotinib, hydroxyurea, and midostaurin

Clinical and biochemical response to avapritinib.

Avapritinib 200 mg/day

Conde-Fernandes et al, 2017 [27]

Indolent SM

Recurrent flushing, hypotension, and syncope, preceded by skin lesions

Oral disodium cromoglycate

Clinical and biochemical response

Caceres-Nazario et al, 2016 [14]

Aggressive SM

Incidental discovery of normocytic anemia with thrombocytopenia

Imatinib to interferon-α

Death

Prednisone

Savini et al, 2015 [28]^a

Mast cell leukemia

Flushing and hypotension

Antihistamine

Death

Imatinib 400 mg to dasatinib 100 mg

Sakane-Ishikawa et al, 2013 [26]^b

Aggressive SM

Vertebral compression fracture, multiple lytic bone lesions with epidural mass, and eosinophils

Emergent laminectomy and subsequent irradiation of the tumor

Death

Interferon-α to dasatinib to cladribine