MedGen

Spinocerebellar ataxia-49 (SCA49) is an autosomal dominant neurologic disorder characterized initially by gait abnormalities, gaze-evoked nystagmus, and hyperreflexia. The age at onset is highly variable, ranging from the second to seventh decades, even within the same family. The disorder is slowly progressive, and later features may include dysarthria, dysmetria, diplopia, pyramidal signs, and axonal peripheral neuropathy. Brain imaging shows cerebellar atrophy and myelination defects (Corral-Juan et al., 2022). [from OMIM]

Monosomy 7 myelodysplasia and leukemia syndrome-1 (M7MLS1) is an autosomal dominant hematologic disorder with highly variable manifestations. Most patients present in early childhood with pancytopenia and dyspoietic or dysplastic changes in the bone marrow. These abnormalities are almost always associated with monosomy 7 in the bone marrow. In severely affected individuals, the phenotype progresses to frank myelodysplastic syndrome (MDS) or acute myelogenous leukemia (AML). Less severely affected individuals may have transient thrombocytopenia or anemia, or have normal peripheral blood counts with transient bone marrow abnormalities or transient monosomy 7. Germline mutations in the SAMD9L gene, located on chromosome 7q, have a gain-of-function suppressive effect on the cell cycle, resulting in decreased cellular proliferation. It is hypothesized that this germline defect leads to selective pressure favoring somatic loss of the chromosome 7 harboring the mutant allele (adaptation by aneuploidy) (summary by Wong et al., 2018). Monosomy 7 or partial deletion of the long arm of chromosome 7 (7q-) is a frequent cytogenetic finding in the bone marrow of patients with myelodysplasia and acute myelogenous leukemia. Furthermore, monosomy 7 or 7q- is the most frequent abnormality of karyotype in cases of AML that occur after cytotoxic cancer therapy or occupational exposure to mutagens. The age distribution of de novo cases shows peaks in the first and fifth decades. Monosomy 7 is found in about 5% of de novo and 40% of secondary cases of AML. These findings suggest that loss of certain genes at this region is an important event in the development of myelodysplasia (summary by Shannon et al., 1989). Genetic Heterogeneity of Monosomy 7 Myelodysplastic and Leukemia Syndrome See also M7MLS2 (619041), caused by germline mutation in the SAMD9 gene (610457) on chromosome 7q21. [from OMIM]

SAMD9L ataxia-pancytopenia (ATXPC) syndrome is characterized by cerebellar ataxia, variable hematologic cytopenias, and predisposition to marrow failure, myelodysplasia, and myeloid leukemia, sometimes associated with monosomy 7. The onset of hematologic abnormalities has been reported as early as age three months. The cytopenias in all cell lineages range from mild to very severe. Onset of neurologic impairment is variable. Nystagmus, dysmetria, increased deep tendon reflexes, and clonus are common. Gait impairment and other neurologic abnormalities are slowly progressive. [from GeneReviews]