PHKA1

PHKA1 encodes the muscle (αM) subunit of phosphorylase kinase, a multimeric enzyme complex that controls glycogen breakdown in muscle (PMID:2757032, PMID:33799212). The gene maps to Xq12-q13, distinct from a liver-specific α-subunit paralog localized to distal Xp, providing the molecular basis for tissue-specific X-linked phosphorylase kinase deficiencies (PMID:2757032, PMID:1505214). Loss-of-function mutations in PHKA1 directly abolish muscle PHK enzymatic activity: a frameshift mutation produced completely absent muscle PHK activity, and a missense mutation (D299V) was established as causative for muscle-specific PHK deficiency, defining PHKA1 as the gene underlying X-linked muscle glycogenosis with myopathy and exercise intolerance (PMID:12825073, PMID:15637709). Beyond its established role in muscle glycogenolysis, the biochemical mechanism by which the αM subunit assembles into and regulates the PHK holoenzyme has not been characterized in the available corpus.

1. 1989 High

   Pinpointing the chromosomal location of the muscle PHK alpha subunit was needed to identify the gene responsible for X-linked muscle PHK deficiency; mapping placed PHKA1 at Xq12-q13.

   Evidence Southern blot of rodent-human somatic cell hybrid panels and in situ chromosomal hybridization

   PMID:2757032

   Open questions at the time

   • Mapping alone did not demonstrate causative mutations
   • No protein-level characterization of the subunit
2. 1992 Medium

   Distinguishing the muscle from the liver alpha-subunit locus clarified why PHK deficiencies are tissue-specific; the liver-specific PHKA gene was localized to distal Xp, separate from PHKA1 at Xq.

   Evidence In situ hybridization with cohybridization of an X-centromere probe

   PMID:1505214

   Open questions at the time

   • Functional candidacy inferred from localization, not from mutation in patients
   • Single lab
3. 2003 Medium

   Direct genetic proof was needed that PHKA1 mutations cause muscle PHK deficiency; a D299V missense mutation in a patient with enzymatic deficiency established causation, while broader sequencing showed known PHK subunit genes explain only a minority of low-PHK-activity muscle glycogenosis.

   Evidence Sequencing of PHKA1, PHKB, PHKG1, CALM1/2/3, PYGM, PRKAG3 in six patients

   PMID:12825073

   Open questions at the time

   • Single mutant patient for PHKA1
   • Genetic cause unexplained in most low-PHK-activity cases
   • No in vitro reconstitution of mutant subunit
4. 2005 Medium

   Whether PHKA1 loss-of-function fully abolishes enzyme activity was unresolved; a frameshift mutation correlated with completely absent muscle PHK activity, confirming the αM subunit is required for holoenzyme function.

   Evidence Mutation sequencing correlated with PHK enzyme activity assay in patient muscle

   PMID:15637709

   Open questions at the time

   • Single patient
   • Mechanism of subunit contribution to catalysis not defined
5. 2021 Low

   The phenotypic spectrum of PHKA1 loss was extended beyond myopathy; a novel mutation in siblings was associated with progressive myopathy, exercise intolerance, and cognitive impairment.

   Evidence Clinical and genetic characterization of siblings with biochemically confirmed PHK deficiency

   PMID:33799212

   Open questions at the time

   • Single family without in vitro mechanistic follow-up
   • CNS involvement causation not established
   • No molecular mechanism for cognitive phenotype
6. 2025 Low

   A possible role outside muscle glycogenolysis was explored; PHKA1 knockdown in breast cancer cells reduced proliferation, invasion, migration, stem-like properties, glycolysis, and mitochondrial function, implicating it in glycolytic metabolic reprogramming.

   Evidence siRNA knockdown in MDA-MB-231 and MCF-7 cells with glycolysis and mitochondrial stress assays and marker analysis

   PMID:40882873

   Open questions at the time

   • Single lab with phenotypic readouts only
   • No direct pathway placement or mechanistic reconstitution
   • Relationship to PHK holoenzyme function in cancer unknown

• How the αM subunit assembles into and regulates the phosphorylase kinase holoenzyme at the structural and biochemical level, and whether the cancer-associated metabolic role is mechanistically connected to its glycogenolytic function, remain unresolved.
• No structural model of the αM subunit within PHK
• Mechanism linking PHKA1 to cancer glycolysis undefined
• Most low-PHK-activity muscle glycogenosis cases remain genetically unexplained