NRAS^Q61K mutated primary leptomeningeal melanoma in a child: case presentation and discussion on clinical and diagnostic implications

We describe the case of a child with primary leptomeningeal melanoma (LMM) that was initially misdiagnosed with tuberculous meningitis. We review clinical and molecular aspects of LMM and discuss clinical and diagnostic implications.

Primary leptomeningeal melanocytic neoplasms (LMN) can be focal (melanomas) or diffuse (melanomatosis) [8]. Since the first description by Virchow in 1859 [9], primary LMM has been reported in few hundreds of patients, mainly adults, with peak incidence in the fourth decade of life [2, 10, 11]. Pediatric experience is extremely limited, accounting for about 0.1 % of central nervous system tumors, this affecting the diagnostic approach and clinical management of patients. During embryogenesis, melanocytic precursors spread from the neural crest to the skin. Few cells can also be found in mucosae, eyes and leptomeninges, explaining primary extracutaneous localizations of melanomas.

Isolated LMNs are a challenging diagnosis in children because they are usually found in association to cutaneous melanomatosis. As previously reported, neurologic signs and symptoms of primary LMM are nonspecific, including seizures, psychiatric disturbances, and signs and symptoms of raised intracranial pressure, often with rapid evolution and fatal course [2]. Unlike most other cerebral tumors, the classic MRI appearance of LMM consists in high signal intensity on T1-weighted images and low signal intensity on T2-weighted images, depending on the presence of free paramagnetic radicals from melanin. Nonetheless, different signal patterns may be observed because of intratumoral hemorrhage [11]. A milestone in the characterization of cutaneous melanoma is the finding of the BRAF^V600E mutation in over 50 % of cases. Few molecular data are available about LMN making diagnosis challenging both on the clinical and pathological side [12].

Recent data suggest the presence of specific mutations in diffuse melanomatosis. Interestingly, different mutations have been found in adults (GNAQ and GNA11 mutations) and children (NRAS^Q61K), with BRAF^V600E mutation being observed in only 2 % of adult cases. In the largest series of children with cutaneous melanomatosis, 51 out of 66 were found to have the NRAS^Q61K mutation in their lesions [13]. Notably, in children with the neurocutaneous form (12 out of 16), the same mutation was found in leptomeningeal lesions suggesting a common origin of neoplastic precursors [14–16]. It has been suggested that a post-zygotic NRAS mutation of neural crest cells during embryogenesis, before migration to skin and leptomeninges, might condition a NRAS mosaicism in the same organism [17]. Nonetheless, mutations occurring before commitment to the neural crest lineage might explain the detection of the same NRAS mutation in tumors other than melanocytic. Interestingly, a NRAS^Q61R mutation has been reported in a spinal neurocristic hamartoma associated to NCM and leptomeningeal melanocytosis by Kinsler et al. Other observed tumors include meningioma and choroid plexus papilloma. Shih et al. reported a NRAS^G13R mutation in a primary mesenchimal brain neoplasm [18]. Invariably, the same mutation was documented in associated CMN but the observation of a germline single-nucleotide polymorphism of the MET gene suggests the possibility of a second hit to condition the clinical picture. In fact, NRAS mutations do not result in melanoma according to in vitro and in vivo preclinical models and to the evidence of mutated cells in CMN [19]. Possible co-operators in melanoma development include MET and CDKN2A [15, 20, 21].

Our case adds some information to the limited experience of the literature, confirming the presence of the NRAS^Q61 mutation in children with meningeal melanomatosis. To our knowledge, this is the first case of LMN without associated skin lesions to harbor this mutation. Despite thorough clinical examination we cannot exclude the possibility of cutaneous melanoma having been overlooked in our patient. Nonetheless, cutaneous melanomas have also been described to regress spontaneously. Our child was initially treated for tuberculous meningitis based on MRI picture. LMNs have typically been described to show T1 hyperintensity and T2 hypointensity on baseline MRI [22]. Our patient did not show these features, making the diagnosis of infective meningitis more appealing in the first instance, even in presence of a negative work-up for TB. We would recommend reconsideration of diagnosis in case of suspect TB showing clinical-radiological progression during anti-tubercular treatment. Biopsy should be considered, not only to confirm diagnosis of LMN but also to study molecular profile and guide target therapy. NRAS inhibitors are not currently available but downstream pathways, such as MAPK and PI3K/AKT/mTOR constitute possible targets. In fact, promising results have been reported both in vitro and in vivo [16, 23–27]. Multi-target combinational approaches might help overcome resistance to treatment, however their clinical significance remains to be further determined [23, 24, 27–29].

Primary LMNs constitute a wide family of rare tumors. Peculiar pathogenetic mutations have been described in the adult (GNAQ and GNA11) and pediatric (NRAS) population. Most LMN present in association to cutaneous melanosis (NCM, CMN) and, in fact, a common molecular signature has been demonstrated in these cell populations. Our case is, to our knowledge, the first report of a LMN not associated to cutaneous findings but sharing the same NRAS^Q61 mutation widely reported in the literature. Isolated LMN presentation might be insidious, mimicking TB meningitis, and should be suspected if no definite diagnosis is possible or if anti-TB treatment does not result in dramatic clinical improvement. Further molecular profiling and preclinical models will be pivotal in testing combination of target therapy to treat this challenging disease.

CSF, cerebrospinal fluid; LMM, leptomeningeal melanoma; LMN, leptomeningeal melanocytic neoplasms; MRI, magnetic resonance imaging; TB, tuberculosis